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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.

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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.

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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).

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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.

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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. 

A compound is a substance that is created when two or more elements are bonded by a chemical reaction. It is difficult to split a compound back into its original elements. Compounds do not necessarily take on the characteristics of the elements that form them. For example, sodium is a metal and chlorine is a gas. Together they form a compound called sodium chloride, which is not like either of them. In fact, sodium chloride is the chemical name for the salt that we put on our food.

In chemistry, a compound is a substance that results from a combination of two or more different chemical elements, in such a way that the atom s of the different elements is held together by chemical bonds that are difficult to break. These bonds form as a result of the sharing or exchange of electron s among the atoms. The smallest unbreakable unit of a compound is called a molecule.

A compound differs from a mixture, in which bonding among the atoms of the constituent substances does not occur. In some situations, different elements react with each other when they are mixed, forming bonds among the atoms and thereby producing molecules of a compound. In other scenarios, different elements can be mixed and no reaction occurs, so the elements retain their individual identities. Sometimes, when elements are mixed, the reaction occurs slowly (as when iron is exposed to oxygen); in other cases it takes place rapidly (as when lithium is exposed to oxygen). Sometimes, when an element is exposed to a compound, a reaction occurs in which new compounds are formed (as when pure elemental sodium is immersed in liquid water).

Often, a compound looks and behaves nothing like any of the elements that comprise it. Consider, for example, hydrogen (H) and oxygen (O). Both of these elements are gases at room temperature and normal atmospheric pressure. But when they combine into the familiar compound known as water, each molecule of which contains two hydrogen atoms and one oxygen atom (H 2 O), the resulting substance is a liquid at room temperature and normal atmospheric pressure.

The atoms of a few elements do not readily bond with other elements to form compounds. These are called noble or inert gases: helium, neon, argon, krypton, xenon, and radon. Certain elements readily combine with other elements to form compounds. Examples are oxygen, chlorine, and fluorine.

1: Pure water is a compound made from two elements - hydrogen and oxygen. The ratio of hydrogen to oxygen in water is always. Each molecule of water contains two hydrogen atoms bonded to a single oxygen atom.

2: Pure table salt is a compound made from two elements - sodium and chlorine. The ratio of sodium ions to chloride ions in sodium chloride is always.

3: Pure methane is a compound made from two elements - carbon and hydrogen. The ratio of hydrogen to carbon in methane is always.

4: Pure glucose is a compound made from three elements - carbon, hydrogen, and oxygen. The ratio of hydrogen to carbon and oxygen in glucose is always.

Elements do not usually exist on their own. In the natural world, they are found in combination with other elements. By understanding how elements combine, scientists have been able to make new combinations, creating molecules that are not found in nature. These combinations are not made simply by mixing two or more substances together. Brown sugar and salt can be stirred together, for example, but this does not create a new substance. Each little particle is either a grain of sugar or a grain of salt — they have remained separate. Mixtures can usually be separated again, but when elements are chemically joined together, they are said to be bonded and have created a new substance.

Bonding is caused by a chemical reaction. Most chemical reactions need some form of energy to start them. Usually, this energy is supplied in the form of heat. Many compounds are made by heating two or more substances together until their molecules are moving so fast that they react with each other.

Energy plays a key role in chemical processes. According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved on making new bonds, and the net result is the absorption of energy. Such a reaction is said to be endothermic if the energy is in the form of heat. The opposite of endothermic is exothermic; in an exothermic reaction, energy as heat is evolved. The more general terms exoergic (energy evolved) and endoergic (energy required) are used when forms of energy other than heat are involved.

 A great many common reactions are exothermic. The formation of compounds from the constituent elements is almost always exothermic. Formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide (CaO) from calcium metal and oxygen gas are examples. Among widely recognizable exothermic reactions is the combustion of fuels (such as the reaction of methane with oxygen mentioned previously).

The formation of slaked lime (calcium hydroxide, Ca (OH)2) when water is added to lime (CaO) is exothermic. This reaction occurs when water is added to dry Portland cement to make concrete, and heat evolution of energy as heat is evident because the mixture becomes warm.

Not all reactions are exothermic (or exoergic). A few compounds, such as nitric oxide (NO) and hydrazine (N2H4), require energy input when they are formed from the elements. The decomposition of limestone (CaCO3) to make lime (CaO) is also an endothermic process; it is necessary to heat limestone to a high temperature for this reaction to occur. The decomposition of water into its elements by the process of electrolysis is another endoergic process. Electrical energy is used rather than heat energy to carry out this reaction.

Generally, evolution of heat in a reaction favours the conversion of reactants to products. However, entropy is important in determining the favorability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work.

A chemical reaction will favour the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive. An example is burning wood. Wood has low entropy. When wood burns, it produces ash as well as the high-entropy substances carbon dioxide gas and water vapour. The entropy of the reacting system increases during combustion. Just as important, the heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive, and the reaction is product-favoured.

When we cook food, chemical reactions take place as het energy is supplied to the ingredients. New compounds are formed, so that the cooked dish usually has a different appearance, texture and taste from the mixed raw ingredients.

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The amount of space that a substance takes up is called its volume. It is measured in cubic units. For example, a cube measuring one metre on each side has a volume of one cubic metre or 1m3. But a cubic metre of lead has a much greater mass than a cubic metre of wood. That is because the lead has a much higher density than the wood. Its particles are more tightly packed together. The density of an object is calculated by dividing its mass by its volume and is expressed as kilograms per cubic metre (kg/m3) or pounds per cubic foot (lb/ft3).

Density is a measure of how compact the mass in a substance or object is. The density of an object or substance can be calculated from this equation: density in kilograms per meter cubed is equal to mass in kilograms, divided by volume in meters cubed. Or in other words, density is mass spread out over a volume. Or in other, other words, it's the number of kilograms that 1 meter cubed of the substance weights. If each meter cubed weighs more, the substance is denser.

As we'll discuss in other lessons, density is super important because it relates to whether things rise or sink. Less dense materials tend to rise above more dense materials, particularly in the case of liquids and gases. So understanding density has major implications for the motions of materials and gases in the atmosphere and objects floating (or sinking) in water. Density is the reason some objects sink and other objects float. And it's the reason that some clouds are high in the sky, while others are low down.

Density means that if you take two cubes of the same size made out of different materials and weigh them, they usually won't weigh the same. It also means that a huge cube of Styrofoam can weigh the same as a tiny cube of lead.

Examples of dense materials include iron, lead, or platinum. Many kinds of metal and rock are highly dense. Dense materials are more likely to 'feel' heavy or hard. Although a sparse material (sparse is the opposite of dense) can feel heavy if it's really big. Examples of sparse materials would be Styrofoam, glass, soft woods like bamboo, or light metals like aluminum.

In general, gases are less dense than liquids and liquids are less dense than solids. This is because solids have densely-packed particles, whereas liquids are materials where particles can slide around one another, and gases have particles free to move all over the place.

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The mass of a substance is the amount of matter it contains. This is different from its weight, which is a measurement of the pull of gravity on this mass. For example, an astronaut would have the same mass on Earth as on the Moon, but his weight would be much less in the Moon’s gravity than in the Earth’s.

We use the word mass to talk about how much matter there is in something. (Matter is anything you can touch physically.) On Earth, we weigh things to figure out how much mass there is. The more matter there is, the more something will weigh. Often, the amount of mass something has is related to its size, but not always. A balloon blown up bigger than your head will still have less matter inside it than your head (for most people, anyhow) and therefore less mass.

The difference between mass and weight is that weight is determined by how much something is pulled by gravity. If we are comparing two different things to each other on Earth, they are pulled the same by gravity and so the one with more mass weighs more. But in space, where the pull of gravity is very small, something can have almost no weight. It still has matter in it, though, so it still has mass.

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As well as heating or cooling, changing the pressure acting on a substance can also cause it to change state. If the pressure on the molecules in a substance is increased, it becomes harder for them to move apart from each other, so the temperature at which they become a liquid is increased. Similarly, at low pressure, changes happen at lower temperatures. It is impossible to make a good cup of tea or coffee' at the top of Everest, for. example, because water boils at a temperature almost 30°C (50°F) less than at sea level.

All matter can move from one state to another. It may require extreme temperatures or extreme pressures, but it can be done. Sometimes a substance doesn't want to change states. You have to use all of your tricks when that happens. To create a solid, you might have to decrease the temperature by a huge amount and then add pressure. For example, oxygen (O2) will solidify at -361.8 degrees Fahrenheit (-218.8 degrees Celsius) at standard pressure. However, it will freeze at warmer temperatures when the pressure is increased.

Some of you know about liquid nitrogen (N2). It is nitrogen from the atmosphere in a liquid form and it has to be super cold to stay a liquid. What if you wanted to turn it into a solid but couldn't make it cold enough to solidify? You could increase the pressure in a sealed chamber. Eventually you would reach a point where the liquid became a solid. If you have liquid water (H2O) at room temperature and you wanted water vapor (gas), you could use a combination of high temperatures or low pressures to solve your problem.

One winter day, you sit by a window inside your warm home. You watch the snow pile up on the ground. You see small animals slide across a frozen pond in your backyard. You can see their hot breath as steam clouds in the cold air. You are drinking a cup of cocoa. You see steam rising from the mug, and you know it is too hot to drink. So you add an ice cube to the cup and wait for the melting ice to cool your cocoa. Solids, liquids, and gases are all around you. The solid ice in the pond, the liquid cocoa, and the steamy air are different states of matter. What is matter? How are solids, liquids, and gases different? Why did the solid ice cube melt into liquid when you put it into your cocoa?

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When molecules are heated, they gain heat energy in addition to the kinetic energy they already have. If the molecules in a solid gain enough energy, they can break free of each other and become liquid. This is called melting. If they gain even more heat energy, the liquid becomes a gas.

Water freezes, or becomes solid, at temperature of 0oC (32oF) or below. If the temperature outside drops to this level, the water on the surface of ponds and lakes will freeze, although the water below may hold enough heat to remain liquid.

When solid water (ice) is heated, it melts to become liquid. Generally speaking, we think of water as being liquid at a “room temperature” of 200C (68oF), or, in other words, under normal conditions, Copper, however, is a solid under such conditions, because it needs a temperature of 1083oC (1981oF) to melt into a liquid.

When water is heated and boils, it turns into a gas. We can see this when a kettle boils. In fact, it is not the billowing steam that is the gas – that is the water turning back in tiny droplets of liquid as it comes into contact with cool air. The real steam is invisible. It can be “seen” in the gap between the spout of the kettle and the visible vapour.

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In nature, it is rare to find one atom on its own. Atoms are usually grouped together in larger structures called molecules. A molecule is the smallest particle of a substance that can exist by itself. The atoms in a molecule are chemically bonded together. They may be atoms of the same element or they may be of different elements. A molecule of carbon dioxide, for example, has two atoms of oxygen and one of carbon.

For millennia, scientists have pondered the mystery of life – namely, what goes into making it? According to most ancient cultures, life and all existence was made up of the basic elements of nature – i.e. Earth, Air, Wind, Water, and Fire. However, in time, many philosophers began to put forth the notion that all things were composed of tiny, indivisible things that could neither be created nor destroyed (i.e. particles).

However, this was a largely philosophical notion, and it was not until the emergence of atomic theory and modern chemistry that scientists began to postulate that particles, when taken in combination, produced the basic building blocks of all things. Molecules, they called them, taken from the Latin “moles” (which means “mass” or “barrier”). But used in the context of modern particle theory, the term refers to small units of mass.

By its classical definition, a molecule is the smallest particle of a substance that retains the chemical and physical properties of that substance. They are composed of two or more atoms, a group of like or different atoms held together by chemical forces.

It may consist of atoms of a single chemical element, as with oxygen (O2), or of different elements, as with water (H2O). As components of matter, molecules are common in organic substances (and therefore biochemistry) and are what allow for life-giving elements, like liquid water and breathable atmospheres.

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