My Science School

International symbols warn people that the contents of containers are dangerous.  Hazard pictograms alert us to the presence of a hazardous chemical.  The pictograms help us to know that the chemicals we are using might cause harm to people or the environment.  The CLP hazard pictograms are very similar to those used in the old labelling system and appear in the shape of a diamond with a distinctive red border and white background.  One or more pictograms might appear on the labelling of a single chemical.

Hazard symbols have come a long way from the rudimentary drawings used to designate poison in the early 1800s. As a result of updated OSHA chemical labeling requirements, 2016 marks the first full year of adoption of the Globally Harmonized System of Classification and Labeling of Chemicals (GHS) in the U.S.

The GHS system, part of OSHA's Hazard Communication Standard (HCS), consists of nine symbols, or pictograms, providing recognition of the hazards associated with certain substances. Uses of eight of the nine are mandatory in the U.S., the exception being the environmental pictogram. Each pictogram covers a specific type of hazard and is designed to be immediately recognizable to anyone handling hazardous material.  In addition to pictograms, labels are required to include a signal word (“danger” or “warning”), a brief hazard statement and a precautionary statement outlining ways to prevent exposure. 

Picture Credit : Google

An indicator is a substance that changes colour when it comes into contact with something acid or alkali. Several materials occurring in nature will do this, including litmus, which comes from lichen and a substance in red cabbage. By using a range of different dyes, scientists make something called a universal indicator, which is able to show how acidic or alkali a substance is.

Indicator any substance that gives a visible sign, usually by a colour change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution. An example is the substance called methyl yellow, which imparts a yellow colour to an alkaline solution. If acid is slowly added, the solution remains yellow until all the alkali has been neutralized, whereupon the colour suddenly changes to red.

Like most indicators, methyl yellow is visible even if its concentration is as low as a few parts per million parts of solution. Used at such low concentrations, indicators do not have any influence on the conditions for which they are recommended. The common application of indicators is the detection of end points of titrations.

The colour of an indicator alters when the acidity or the oxidizing strength of the solution, or the concentration of a certain chemical species, reaches a critical range of values. Indicators are therefore classified as acid-base, oxidation-reduction, or specific-substance indicators, every indicator in each class having a characteristic transition range. Methyl yellow, an acid-base indicator, is yellow if the hydrogen ion (acid) concentration of the solution is less than 0.0001 moles per litre and is red if the concentration exceeds 0.0001. Ferrous 1,10-phenanthroline, an oxidation-reduction indicator, changes from red to pale blue when the oxidation potential of the solution is increased from 1.04 to 1.08 volts; and diphenylcarbazone, an indicator for mercuric ion, changes from yellow to violet when the mercuric ion concentration is increased from 0.000001 to 0.00001 mole per litre. Each of these indicators thus has a relatively narrow transition range, and each is capable of giving a sensitive, sharp indication of the completion of a reaction, that is, the end point.

Although the visible change of the indicator is usually a colour change, in some cases it is a formation or disappearance of turbidity. If, for example, a soluble silver salt is added to a solution of cyanide that contains a trace of iodide, the solution remains clear until all the cyanide has reacted to form the soluble silver cyanide complex ion. Upon the addition of more silver, the solution becomes turbid because insoluble silver iodide forms. Iodide is therefore an indicator for excess silver ion in this reaction.

Another kind of indicator is the adsorption indicator, the best-known representative of which is the dye fluorescein. Fluorescein is used to detect the completion of the reaction of silver ion with chloride ion, the colour change occurring in the following manner. After a quantity of silver large enough to precipitate all the chloride has been added, additional silver ion is partially adsorbed on the surface of the particles of silver chloride. Fluorescein also is adsorbed and, in combining with the adsorbed silver ion, changes from yellow-green to red.

Picture Credit : Google

The abbreviation pH stands for “power of hydrogen”. It describes how concentrated the hydrogen ions in a substance are. A pH value below seven shows that the substance is acid. Above seven, it is an alkali.

The pH value describes the activity of hydrogen ions in aqueous solutions typically on a scale of 0 to 14.  Based on this pH scale, liquids are characterized as being acidic, alkaline or neutral: a solution which is neither acidic nor alkaline is neutral.  This corresponds to a value of 7 on the pH scale.  Acidity indicates a higher activity of hydrogen ions and a pH measurement value lower than 7.  Alkaline solutions are characterized by a lower hydrogen ion activity or higher hydroxide ion activity.

The pH scale is logarithmic.  A difference of one pH measurement unit represents a tenfold, or ten times increase or reduction of hydrogen ion activity in the solution.  This explains how a solution's aggressiveness increases with the distance from the neutral point.

One of the keys to understanding pH measurements is the term "activity", because the activity is temperature dependent it is not the same as the solution's concentration.  Activity, a, is defined as the product of the activity coefficient, y, which is always smaller than 1, and the actual concentration, c, of the concerned compound (a=y * c).

Activity is the effective concentration of a chemical compound, or more precisely its particles in the solution.  In a real solution the activity is constantly smaller than the actual concentration.  This is true because only in an ideal (infinitely thinned) solution the saluted particles do not affect each other.  In this case they are spread apart because many molecules of the solvent are between them.  The difference between activity and concentration becomes apparent in real solutions of ions, because ions interact with each other as a result of their electric charge.  To describe or calculate the characteristics of a solution as exactly as possible the activity and not the concentration must be used in the mass action law.

Picture Credit : Google

A base is the opposite of an acid. Most soap and many household cleaners are bases. An alkali is a base that can be dissolved in water. People commonly use the term alkaline for basic solutions, but their meanings are not the same. All alkaline solutions are basic, but not all bases are alkaline. It's common to refer to the alkalinity of a substance, such as soil, when pH is the property you're really discussing.

In chemistry, a base is a water solution of any chemical compound that produces a solution with a hydrogen ion concentration lower than that of pure water. Sodium hydroxide and ammonia are two examples. Bases are the chemical opposites of acids. Bases reduce the hydrogen ion concentration in water whereas acids increase them. Acids and bases neutralize each other when they combine.

In chemistry, the term alkali refers to salts (ionic compounds) containing alkali and alkaline earth metal elements that accept a hydrogen ion in solution. Alkaline bases are best known as bases that dissolve in water. Alkali metals react vigorously with water, producing hydroxides and releasing hydrogen. The reaction with air covers the surface of the solution with oxides. In nature, ionic compounds (salts) contain alkali metals but never in a pure state.

Alkaline bases include a slimy or soapy feel to the touch because of saponification of fatty acids in human skin. Alkalis form hydroxide ions (OH-) when dissolved in water and all are Arrhenius bases. Normally water-soluble, some alkalis, such as barium carbonate, become soluble only when reacting with an acidic solution containing water. Moderately concentrated solutions (pH of 7.1 or greater) turn litmus paper blue and phenolphthalein from colorless to pink. Concentrated solutions cause chemical burns (caustic).

Picture Credit : Google

The word “acid” comes from a Latin word meaning sour. Acids contain hydrogen and, when dissolved in water, produce positively charged hydrogen ions. Our tongues are able to detect acidic flavours, such as those of vinegar or citrus fruits, but these are very weak in comparison to some acids used in industry, such as sulphuric acid, which burns badly if it comes into contact with skin.

Acids are chemical agents that release hydrogen ions when added to water. Their chemistry makes them one of the most important classes of molecules in nature and science. So many of us have heard of the term pH, which in general is the measure of the amount of acidity or alkalinity that is in a solution. More specifically, it is a measure of the amount of protons or hydrogen ions that are present in an aqueous solution. Acids are primary contributors to the measure of pH in a solution, and the presence of acids a key characteristic of almost all solutions.

The pH scale is a scale that is used to represent the level of acidity in a solution. A solution with a pH of 7 is neutral, while a solution with a pH below 7 is an acid, and a solution with a pH above 7 is a base. An acid dissociates, or breaks apart, and donates protons, or hydrogen ions, in an aqueous solution, while a base donates hydroxide ions in a solution. Water, for example, is neutral with a pH of 7. When acids are added, they release more hydrogen ions into the solution, and this causes the pH of the solution to drop. Let me repeat: more hydrogen ions equals a lower pH and a more acidic solution.

Picture Credit : Google

Poor metals have a low melting point and are quite soft, but they are still very useful. The seven metals that come to the right of the transition metals in the periodic table are known as poor metals. They are aluminium, gallium, indium, thallium, tin, lead and bismuth. Lead has a very high density, so radiation cannot easily pass through it. That is why radioactive materials are often carried in lead-lined containers and the operators of x-ray machines wear lead aprons. Poor metals may be combined with other metals to form useful alloys.

The name aluminum is derived from the ancient name for alum (potassium aluminum sulphate), which was albumen (Latin, meaning bitter salt). Aluminum was the original name given to the element by Humphry Davy but others called it aluminum and that became the accepted name in Europe. However, in the USA the preferred name was aluminum and when the American Chemical Society debated on the issue, in 1925, it decided to stick with aluminum.

Aluminum is a soft and lightweight metal. It has a dull silvery appearance, because of a thin layer of oxidation that forms quickly when it is exposed to air. Aluminum is nontoxic (as the metal) nonmagnetic and non-sparking.

A silvery and ductile member of the poor metal group of elements, aluminum is found primarily as the ore bauxite and is remarkable for its resistance to oxidation (aluminum is actually almost always already oxidized, but is usable in this form unlike most metals), its strength, and its light weight. Aluminum is used in many industries to make millions of different products and is very important to the world economy. Structural components made from aluminum are vital to the aerospace industry and very important in other areas of transportation and building in which light weight, durability, and strength are needed. The use of aluminum exceeds that of any other metal except iron. Pure aluminum easily forms alloys with many elements such as copper, zinc, magnesium, manganese and silicon.

Nearly all modern mirrors are made using a thin reflective coating of aluminum on the back surface of a sheet of float glass. Telescope mirrors are also coated with a thin layer of aluminum. Other applications are electrical transmission lines, and packaging (cans, foil, etc.).

Because of its high conductivity and relatively low price compared to copper, aluminum was introduced for household electrical wiring to a large degree in the US in the 1960s. Unfortunately problems on the functioning were caused by its greater coefficient of thermal expansion and its tendency to creep under steady sustained pressure, both eventually causing loosening the connection; galvanic corrosion increasing the electrical resistance.

The most recent development in aluminum technology is the production of aluminum foam by adding to the molten metal a compound (a metal hybrid), which releases hydrogen gas. The molten aluminum has to be thickened before this is done and this is achieved by adding aluminum oxide or silicon carbide fibers. The result is a solid foam which is used in traffic tunnels and in space shuttle.

Aluminum is an abundant element in Earth's crust: it is believed to be contained in a percentage from 7.5% to 8.1%. Aluminum is very rare in its free form. Aluminum contribute greatly to the properties of soil, where it is present mainly as insoluble aluminum hydroxide.

Aluminum is a reactive metal and it is hard to extract it from its ore, aluminum oxide (Al2O3). Aluminum is among the most difficult metals on earth to refine, the reason is that aluminum is oxidized very rapidly and that its oxide is an extremely stable compound that, unlike rust on iron, does not flake off. The very reason for which aluminum is used in many applications is why it is so hard to produce.

Several gemstones are made of the clear crystal form of aluminum oxide known as corundum. The presence of traces of other metals creates various colors: cobalt creates blues sapphires, and chromium makes red rubies. Both these are now easy and cheap to manufacture artificially. Topaz is aluminum silicate coloured yellow by traces of iron.

Recovery of this metal from scrap (via recycling) has become an important component of the aluminum industry. Industrial production world-wide of new metal is around 20 million tons per year, and a similar amount is recycled. Known reserves of ores are 6 billion tones.

Calcium is one of a group of metals called the alkaline-earth metals. They form alkaline solutions with water and are found in many natural substances. Calcium is an important constituent of bones, making them hard and stable. Magnesium is found in chlorophyll, the green part of plants that can make energy from sunlight by photosynthesis. Alkaline-earth metals form the second group of the periodic table.

The chemical element Calcium (Ca), atomic number 20, is the fifth element and the third most abundant metal in the earth’s crust. The metal is trimorphic, harder than sodium, but softer than aluminium. A well as beryllium and aluminium, and unlike the alkaline metals, it doesn’t cause skin-burns. It is less chemically reactive than alkaline metals and then the other alkaline-earth metals.

Calcium ions solved in water form deposits in pipes and boilers and when the water is hard, that is, when it contains too much calcium or magnesium. This can be avoided with the water softeners. In the industry, metallic calcium is separated from the melted calcium chloride by electrolysis. This is obtained by treatment of carbonated minerals with chlorhydric acid, or like a sub product of the carbonates Solvay process.

In contact with air, calcium develops an oxide and nitride coating, which protects it from further corrosion. It burns in the air at a high temperature to produce nitride. The commercially produced metal reacts easily with water and acids and it produces hydrogen which contains remarkable amounts of ammonia and hydrocarbides as impurities.

The metal is used in aluminium alloys for bearings, as a helper in the bismuth removal form lead, as well as in controlling graphitic carbon in melted iron. It is also used as a deoxidizer in the manufacture of many steels; as a reducing agent in the preparation of metals as chromium, thorium, zirconium and uranium, and as separating material for gaseous mixtures of nitrogen and argon. Calcium is an alloying used in the production of aluminium, beryllium, copper, lead and magnesium alloys. It is also used in making cements and mortar that are used in buildings.

The calcium oxide, CaO, is produced by thermal decomposition of carbonated minerals in furnaces, applying a continuous bed process. The oxide is used in high intensity light arcs (lime light) for its unusual spectral characteristics and as dehydrating industrial agent. The metallurgic industry extensively uses the oxide during the reduction of ferrous alloys.

The pure calcium carbonate occurs in two crystalline forms: calcite, hexagonal shaped, which possesses birrefringent properties, and aragonite, rhombohedric. The natural carbonates are the most abundant calcium minerals. The Iceland spar and the calcite are essentially pure carbonate forms, whilst the marble is impure and much more compact, reason why it can be polished. It’s very demanded as construction material. Although the calcium carbonate is very little soluble in water, it is quite soluble if the water contains dissolved carbon dioxide, for in these solutions it forms bicarbonate when dissolving. This fact explains the cave formation, where the lime stone deposits have been in contact with acid waters.

Calcium is the fifth element and the third most abundant metal in the earth’s crust. The calcium compounds account for 3.64% of the earth’s crust. The distribution of calcium is very wide; it is found in almost every terrestrial area in the world. This element is essential for the life of plants and an animal, for it is present in the animal’s skeleton, in tooth, in the egg’s shell, in the coral and in many soils. Seawater contains 0.15% of calcium chloride. Calcium cannot be found alone in nature. Calcium is found mostly as limestone, gypsum and fluorite. Stalagmites and stalactites contain calcium carbonate.

Calcium is always present in every plant, as it is essential for its growth. It is contained in the soft tissue, in fluids within the tissue and in the structure of every animal’s skeleton. The vertebrate’s bones contain calcium in the form of calcium fluoride, calcium carbonate and calcium phosphate.

Some metals corrode badly on contact with air and water. This means that the surface of the metal reacts with oxygen to form an oxide. The metal loses its shine as a layer of oxide covers it. This is sometimes known as tarnishing. When a bowl covered with silver is cleaned, for example, what is really happening is that the layer of tarnish is being rubbed away. Over a long period of time, all the silver may be rubbed off. Iron corrodes in air and water to produce rust. Non-reactive metals are less likely to corrode than reactive ones.

Corrosion is a natural process that involves the deterioration of metal components. According to NACE International, corrosion is “the destruction of a substance (usually a metal) or its properties because of a reaction with its environment.” This ultimately causes potentially severe damage to your building or application and can become very costly to repair.

Corrosion is an electrochemical reaction that appears in several forms, such as chemical corrosion and atmospheric corrosion, the latter of which is the most common form. When acidic substances (including water) come in contact with metals, such as iron and/or steel, rust begins to form. Rust is the result of corroding steel after the iron (Fe) particles have been exposed to oxygen and moisture (e.g., humidity, vapor, and immersion). When steel is exposed to water, the iron particles are lost to the water’s acidic electrolytes. The iron particles then become oxidized, which results in the formation of Fe??. When Fe?? is formed, two electrons are released and flow through the steel to another area of the steel known as the cathodic area.

Oxygen causes these electrons to rise up and form hydroxyl ions (OH). The hydroxyl ions react with the FE?? to form hydrous iron oxide (FeOH), better known as rust. Where the affected iron particles were, has now become a corrosion pit, and where they are now, is called the corrosion product (rust). Corrosion can happen at any rate, depending on the environment that the metal is in. However, since atmospheric corrosion is so widespread, it is recommended to take effective precautionary measures when it comes to corrosion prevention.

Depending on the situation and application, you may be able to treat the area that has corroded. If the affected area is small and treatable, you may require some tools and products to remove it. Begin by removing the rust from the metal using tools such as a grinding wheel or needle gun.  Be careful not to cause any additional damage to the metal. For large corroded areas, you may want to consider a permanent protective coating, such as CSL’s SI-COAT Anti-Corrosion Protective Coating. You will also want to take this time to look at the application as a whole for other premature signs of corrosion.

The Golden Gate Bridge in San Francisco needs to be painted regularly to stop it from corroding.

Picture Credit : Google

An alkali metal is one that reacts vigorously with water to make an alkaline solution. Lithium, sodium, potassium, rubidium, cesium and francium are all alkali metals. They appear in the first column, or group, of the periodic table. All alkali metals are soft enough to cut with an ordinary knife and are a whitish silvery colour.

Alkali metal, any of the six chemical elements that make up Group 1 (Ia) of the periodic table -namely, lithium (Li), sodium (Na), potassium(K), rubidium (Rb), cesium (Cs), and francium (Fr). The alkali metals are so called because reaction with water forms alkalies (i.e., strong bases capable of neutralizing acids). Sodium and potassium are the sixth and seventh most abundant of the elements, constituting, respectively, 2.6 and 2.4 percent of Earth’ crust. The other alkali metals are considerably rarer, with rubidium, lithium, and cesium, respectively, forming 0.03, 0.007, and 0.0007 percent of Earth’s crust. Francium, a natural radioactive isotope, is very rare and was not discovered until 1939.

The alkali metals have the silver-like lustre, high ductility, and excellent conductivity of electricity and heat generally associated with metals. Lithium is the lightest metallic element. The alkali metals have low melting points, ranging from a high of 179oC (354 °F) for lithium to a low of 28.5 °C (83.3 °F) for cesium. Alloys of alkali metals exist that melt as low as ?78 °C (?109 °F).

The alkali metals react readily with atmospheric oxygen and water vapour. (Lithium also reacts with nitrogen.) They react vigorously, and often violently, with water to release hydrogen and form strong caustic solutions. Most common nonmetallic substances such as halogens, halogen acids, sulfur, and phosphorus react with the alkali metals. The alkali metals themselves react with many organic compounds, particularly those containing a halogen or a readily replaceable hydrogen atom.

Sodium is by far the most important alkali metal in terms of industrial use. The metal is employed in the reduction of organic compounds and in the preparation of many commercial compounds. As a free metal, it is used as a heat-transfer fluid in some nuclear reactors. Hundreds of thousands of tons of commercial compounds that contain sodium are used annually, including common salt (NaCl), baking soda (NaHCO3), sodium carbonate (Na2CO3), and caustic soda (NaOH). Potassium has considerably less use than sodium as a free metal. Potassium salts, however, are consumed in considerable tonnages in the manufacture of fertilizers. Lithium metal is used in certain light-metal alloys and as a reactant in organic syntheses. An important use of lithium is in the construction of lightweight batteries. Primary lithium (not rechargeable) are widely used in many devices such as cameras, cellular telephones, and pacemakers. Rechargeable lithium storage batteries that could be suitable for vehicle propulsion or energy storage are the subject of intensive research. Rubidium and cesium and their compounds have limited use, but cesium metal vapour is used in atomic clocks, which are so accurate that they are used as time standards.

Picture Credit : Google

Noble metals are those that can be found in their pure state, not mixed with other substances. As might be expected, they are not very reactive, which is why they do not readily form compounds in their natural state. This also means that they do not corrode easily making them traditionally suitable to be formed into coins or jewellery. Noble metals include gold, silver, platinum and copper.

Here is a list of noble metals, which are metals that resist oxidation and corrosion. Ruthenium, Rhodium, Palladium, Silver, Osmium, Iridium, Platinum, Gold, some lists include mercury as a noble metal. Rhenium is also included as a noble metal by some scientists and engineers.

While noble metals retain their shiny color, base metals tend to oxidize in moist air. However, some metals that resist corrosion are not considered noble metals. These include titanium, niobium, and tantalum. In atomic physics, the noble metal group consists of copper, silver, and gold. Only these three elements have completely filled d-subshells. Most noble metals are valuable and rare, yet the noble metals are not exactly the same as the precious metals.

The ability of an element to take part in a chemical reaction is called reactivity. Metals vary in their reactivity. Some alkali metals, such as sodium and potassium, are so reactive that they have to be stored in oil. They would react strongly with the oxygen in air or water. The least reactive metal is gold.

The metal reactivity series is a commonly taught concept in chemistry, placing the metals, as its name suggests, in order of reactivity from most reactive to least reactive. It’s also a useful tool in predicting the products of simple displacement reactions involving two different metals, as well as providing an insight into why different metals are extracted from their ores in different manners. This graphic places a selection of common metals into order of reactivity, as well as showing their reactions with air, water and steam.

Metals have a range of relativities – to illustrate this, you have to look no further than the classic alkali metals in water demonstration commonly used in chemistry classes. In this demonstration, small pieces of three different metals from group 1 of the periodic table are dropped into a large bowl of water. Lithium fizzes gently, sodium fizzes vigorously, and potassium’s reaction is so energetic it bursts into a lilac flame as it zips across the water’s surface. Cesium, the most reactive metal in the periodic table, reacts extremely violently – hence why it can’t be demonstrated in a classroom! This can be compared to other common metals, such as iron and copper, which produce no reaction when dropped into water.

The reactivity series offers a ranking of the metals in order of their reactivity. Group 1 metals, the most reactive metals in the periodic table, head up the rankings. They’re closely followed by the marginally less reactive group two metals. The metals designated as the transition metals in the periodic table are much less reactive, and metals such as gold and platinum prop up the bottom of the series, exhibiting little in the way of chemical reaction with any everyday reagents.

Tin cans are made of steel, not tin, but they do have a coating of tin to stop food inside corroding the steel. Drinks cans are often made of aluminium.

Picture Credit : Google

There are over 80 different metals. They tend to conduct heat and electricity well, and many of them can be shaped by pulling, beating, or melting and pouring into a mould. Metals with similar properties are often grouped together, although a metal may sometimes appear in more than one group, as these pages show. Unlike most non-metals, metals are shiny when cut. Metals have played an enormous part in the history of human activity, which is why some periods, such as the Iron Age and the Bronze Age, are known by the names of metals. Some people say that our present period should be called the Silicon Age. But silicon is what is known as a semi-metal, having some but not all of the properties of metals.

Hard, shiny, and tough—metals are the macho poster boys of the material world. Learning how to extract these substances from the Earth and turn them into all kinds of useful materials was one of the most important developments in human civilization, spawning tools, jewelry, engines, machines, and giant static constructions like briges and skyscrapers. Having said that, "metal" is an almost impossibly broad term that takes in everything from lead (a super-heavy metal) and aluminum (a super-light one) to mercury (a metal that's normally a liquid) and sodium (a metal soft enough to cut like cheese that, fused with chlorine, you can sprinkle on your food—as salt!).

When we talk about nonmetals, it ought to mean everything else—although things are a bit more complex than that. Sometimes you'll hear people refer to semi-metals or metalloids, which are elements whose physical properties (whether they're hard and soft, how they carry electricity and heat) and chemical properties (how they behave when they meet other elements in chemical reactions) are somewhere in between those of metals and nonmetals. Semi-metals include such elements as silicon and germanium—semiconductors (materials that conduct electricity only under special conditions) used to make integrated circuits in computer chips and solar cells. Other semi-metals include arsenic, boron, and antimony (all of which have been used in the preparation—"doping"— of semiconductors).

Picture Credit : Google

Ionic bonds happen when one atom gains one or more electrons from another atom. The electrons in an atom have a negative charge and are equal in number to the positively charged protons in the nucleus. When an atom gains or loses electrons, the balance of charges is broken, so the atom becomes either positively or negatively charged. It is called an ion. An atom that has gained electrons has a negative charge and is called an anion. One that has lost electrons has a positive charge and is called a cation. As opposite charges attract each other, the two atoms that have gained and lost electrons are pulled together into a bond.

When two atoms combine, they form a compound or molecule in a chemical bond, which links them together. This bond can be ionic or covalent. In an ionic bond, one atom donates an electron to the other to stabilize it. In a covalent bond, the atoms are shared by the electrons.

In the chemistry world, an ionic bond is made from atoms with different electronegativity values. It is considered a polar bond if the attraction is between two oppositely charged ions. This works much in the same way as magnets that attract each other. If two atoms have different electronegativity values, they will make an ionic bond.

The combination of sodium (Na) and chloride (Cl) forms NaCl or common table salt, and this is an example of an ionic bond. Sulfuric acid is also an ionic bond, combining hydrogen and sulfur oxide, and it is written as H2SO4.

Ionic bonds take more energy to break than covalent bonds, so ionic bonds are stronger. The amount of energy needed to break a bond is known as bond dissociation energy, which is basically the force it takes to break bonds of any type.

Picture Credit : Google

It is useful to think of an atom as having electrons circling in layers around its nucleus. These layers are known as “shells”. Each layer can only have a certain number of electrons before a new shell must be started. Atoms that have as many electrons as possible in the outer shell (or some other particular numbers) are said to be stable. They do not easily form bonds with other atoms. Atoms that are not stable try to become so by sharing electrons with, or borrowing electrons from, or giving electrons to, another atom. The number of electrons that an atom needs to give or gain to achieve a stable outer shell is called its valency.

The combining capacity of an atom is called its valency. Actually it can be defined as the number of electrons that an atom may lose (or) gain during a chemical reaction (or) the number of electrons shared. The numbers of electrons in outermost shell (or) valency shell of an atom are called valency electrons. 

The valency of an element is the number of atoms lost or gained by the atom of an element. Valence electrons are the number of electrons present in the outermost shell of an atom valency of an element depends on the valence electrons valency of atoms having 1, 2, 3 valence electrons = number of valence electrons valency of atoms having 5, 6, 7 valence electrons

= 8 - number of valence electrons

Covalent bond the interatomic linkage that results from the sharing of an electron pair between two atoms. The binding arises from the electrostatic attraction of their nuclei for the same electrons. A covalent bond forms when the bonded atoms have a lower total energy than that of widely separated atoms.

Molecules that have covalent linkages include the inorganic substances hydrogen, nitrogen, chlorine, water, and ammonia (H2, N2, Cl2, H2O, NH3) together with all organic compounds. In structural representations of molecules, covalent bonds are indicated by solid lines connecting pairs of atoms; e.g.

A single line indicates a bond between two atoms (i.e., involving one electron pair), double lines (=) indicate a double bond between two atoms (i.e., involving two electron pairs), and triple lines (?) represent a triple bond, as found, for example, in carbon monoxide (C?O). Single bonds consist of one sigma (?) bond, double bonds have one ? and one pi (?) bond, and triple bonds have one ? and two ? bonds.

The idea that two electrons can be shared between two atoms and serve as the link between them was first introduced in 1916 by the American chemist G.N. Lewis, who described the formation of such bonds as resulting from the tendencies of certain atoms to combine with one another in order for both to have the electronic structure of a corresponding noble-gas atom.

Covalent bonds are directional, meaning that atoms so bonded prefer specific orientations relative to one another; this in turn gives molecules definite shapes, as in the angular (bent) structure of the H2O molecule. Covalent bonds between identical atoms (as in H2) are nonpolar—i.e., electrically uniform—while those between unlike atoms are polar—i.e., one atom is slightly negatively charged and the other is slightly positively charged. This partial ionic character of covalent bonds increases with the difference in the electronegativity’s of the two atoms.