Chemistry PDF: Classification of Elements

## Classification of Elements You have studied in the previous class that materials, made up of same kind of particles, are known as pure substances. A sample, of a given element, or a given compound, is a 'pure substance'. This is because a given element (compound) is made up of atoms (molecules)...

## Classification of Elements You have studied in the previous class that materials, made up of same kind of particles, are known as pure substances. A sample, of a given element, or a given compound, is a 'pure substance'. This is because a given element (compound) is made up of atoms (molecules) of the same kind. Different elements/compounds differ from each other in their physical and chemical properties as they are made up of atoms/molecules of different kind. ### Classification of Elements There are about 118 elements known at present. It is difficult to study and describe all the properties of all these elements separately. Hence, elements, showing similar properties, are grouped together and their general characteristics are studied. Such 'grouping' of elements is called classification of elements. The elements have been broadly classified into two categories: metals and non-metals; this classification is based on the differences in their physical and chemical properties. ### Occurrence of Elements * **Metals** exist in abundance in the earth's crust. Aluminium is the most abundant metal; it is followed by iron. Highly reactive metals, like sodium, potassium, aluminium and magnesium, are highly reactive metals, form of their compounds (like oxides, sulphides and carbonates). Less reactive metals, like gold, silver and platinum, occur in nature in their elemental, or native state. * **Many non-metals** are found in their free state in the atmosphere. Oxygen and nitrogen are two well-known examples of such non-metals. Several non-metals exist in nature in the form of their compounds (like oxides and nitrates). Sulphur exists in the free state as well as in the combined state (as sulphides and sulphates). Carbon exists in its native state as diamond, graphite and fullerene; it also exists in the form of its compounds like carbon dioxide, carbonates and bicarbonates. ### Minerals and Ores * **A mineral** is a naturally occurring inorganic substance found deep under the surface of the earth. * **An ore** is a mineral from which one or more metals can be extracted profitably. For example, aluminium is extracted from its bauxite ore and iron from its haematite ore. The sequence of processes, used to extract a metal, in its pure form from its ore, is called metallurgy. The extraction of a metal, from its ore, generally involves the following steps: - **Concentration of ore**: It is the process of removal of impurities from the ore. - **Reduction**: It is the process of 'treating the metal ore' to get the metal in its free state. - **Refining of metal**: The metal, obtained by reduction, is generally impure. Refining is the process of purification of this impure metal. Let us now study and compare the physical and chemical properties of metals and non-metals. ### Physical Properties 1. **Physical state**: All metals (except mercury, which is a liquid) are solid at room temperature. Iron, copper, aluminium, gold and silver are some of the examples of metals. 2. **Melting point and boiling point**: Metals generally have high melting as well as high boiling points. For example, melting point of iron is 1536°C and its boiling point is 3000°C. Some metals, however, have an exceptionally low melting point. For example, the melting point of caesium metal is only 28.7°C. Non-metals generally have low melting as well as low boiling points. For example, the melting point of sulphur is 119°C. However, there are exceptions in non-metals also. The melting point of diamond (form of carbon) is very high, i.e. 3723°C. 3. **Density**: Metals generally have a high density. Non-metals generally have a low density. However, there are some exceptions. Sodium and potassium, which are metals, have quite a low density; their density value is less than that of water. 4. **Hardness**: Most metals are very hard. They can withstand quite high pressures without getting distorted. Non-metals are generally not hard; they are brittle and easily break into pieces when hammered. Again there are some exceptions. Sodium and potassium are metals but they are so soft that they can be easily cut with a knife or a razor blade. On the other hand, diamond (a non-metal), which is a form of carbon, is the hardest substance known. 5. **Lustre**: Metals have a shining surface. This is known as metallic lustre. Non-metals generally have a dull appearance, that is, they are non-lustrous. Iodine is the only non-metal which has a natural lustre. 6. **Malleability**: Metals are malleable, that is, they can be hammered into thin sheets without breaking. Non-metals are non-malleable, that is, they cannot be hammered into sheets. They are brittle, and break into pieces on being hammered. 7. **Ductility**: Metals can be drawn into thin wires. This property of metals is known as ductility. Copper, aluminium and iron wires are commonly used for electrical fittings, making net doors and wire meshes, and so on Non-metals are non-ductile, that is, they cannot be drawn into wires. 8. **Tensile strength**: It is the property due to which a substance can bear a lot of strain without breaking. Metals have high tensile strength due to their ductility and malleability. Non-metals generally do not have high tensile strength (except carbon-fibre). 9. **Conductivity**: The ability of a material, to transfer heat energy, or electrical current, from one point to another, is taken as an indicator of its conductivity. We generally speak of two types of conductivities: thermal conductivity and electrical conductivity. - **Thermal Conductivity**: The thermal conductivity, of a material, is a measure of the ease with which heat energy can flow through it. - **Electrical Conductivity**: The electrical conductivity, of a material, is a measure of the ease with which electric current can flow through it. 10. **Sonorosity**: When a piece of metal is struck with something hard, a ringing sound is produced. This property of metals is known as sonorosity; metals are, therefore, said to be sonorous. It is due to this property that metals are used for making bells. Non-metals are non-sonorous. It is on the basis of these differences in their physical properties that one can distinguish metals from non-metals. These differences in their properties have been summarised in Table 1. | Property | Metals | Non-metals | |---|---|---| | Physical state | They are all solids, except mercury (which is a liquid). | They may be solid, liquid or gaseous. | | Melting and boiling point | They generally have high melting as well as high boiling points. | They generally have low melting as well as low boiling points. | | Density | They (generally) have a high density. | They (generally) have a low density. | | Hardness | They are quite hard (with exceptions of sodium and potassium). | They are not hard (except diamond). | | Lustre | They possess a natural shine. | They generally have a dull appearance. | | Malleability and ductility | They are malleable and ductile. | They are not malleable and ductile; they are brittle. | | Tensile strength | They have high tensile strength. | They do not have tensile strength (except carbon fibre). | | Thermal and electrical conductivity | They are good thermal as well as good electrical conductors. | They are non-conductors or insulators (except graphite). | | Sonorosity | They are sonorous. | They are not sonorous. | ### Chemical Properties Metals and non-metals differ from each other in their chemical properties also. Let us compare the chemical properties of metals with those of non-metals. 1. **Reaction with Oxygen** - Hold a magnesium ribbon with a pair of tongs and ignite it. Magnesium ribbon burns with a bright white light and forms a white powder. The white powder formed is magnesium oxide. * 2Mg(s) + O₂(g) → 2MgO(s) - Magnesium + Oxygen (from air) → Magnesium Oxide (white powder) - Collect this white powder and dissolve it in water. Dip a strip of red litmus paper in this solution. The red litmus turns blue, indicating that the solution is alkaline. This happens because magnesium oxide dissolves in water to form magnesium hydroxide (which is an alkali and (hence) turns red litmus blue). * MgO(s) + H₂O(l) → Mg(OH)₂(aq) - Magnesium Oxide + Water → Magnesium Hydroxide - This activity shows that metals react with oxygen to form metallic oxides which are basic in nature. Potassium (K), sodium (Na) and calcium (Ca) are highly reactive metals. They react with the oxygen present in the air, even at room temperature, to form their respective oxides. 2. **Reaction with Water** - Take a piece of magnesium ribbon. Clean its surface by rubbing it with a sand paper to remove the layer of its oxide. Put it in a boiling tube; half fill the tube with water. Invert another boiling tube over the mouth of the first boiling tube and gently warm the lower tube over the flame of a bunsen burner (as shown in the figure). You will observe the evolution of a gas in the form of bubbles. Remove the burner. Carefully bring a burning splinter near the mouth of the upper (inverted) boiling tube. It burns with a crackling sound. This happens because magnesium reacts with water on heating and liberates hydrogen gas which burns with a popping sound. * Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g) - Magnesium + Water → Magnesium Hydroxide + Hydrogen - This activity shows that some metals can react with water to liberate hydrogen gas. Metals differ in their reactivity towards water. For example, sodium and potassium metals reacts very vigorously with water; this reaction gives out so much heat that the hydrogen evolved catches fire. Therefore, these metals are stored under kerosene, or paraffin wax. Magnesium does not react with cold water but reacts on heating. Zinc reacts with boiling water and iron reacts with steam, indicating that it is very much less reactive. Metals like copper, silver, gold, platinum and mercury do not react with water at all. - Non-metals do not react with water. Therefore, some reactive non-metals are stored in water to prevent their reaction with air. For example, phosphorus is kept in water to prevent its contact with air; it catches fire on reaction with air. 3. **Reaction with Acids** - Take some iron nails and clean their surface by rubbing them with sand paper. Put them in a test tube and then add some dilute hydrochloric acid to the tube. Invert another test tube over the mouth of the first test tube. You will observe the evolution of a gas in the form of bubbles. Carefully bring a burning splinter near the mouth of the inverted test tube. It burns with a popping sound which shows the evolution of hydrogen. Repeat the activity by taking magnesium ribbon and zinc granules; allow them to react with other acids like dilute sulphuric acid. Compare the observations. - Hydrogen gas is released as per the following reactions: * Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g) - Iron + Hydrochloric acid → Iron chloride + Hydrogen * Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) - Zinc + Sulphuric acid → Zinc sulphate + Hydrogen - This activity shows that most metals react with dilute acids to liberate hydrogen gas and form metal salts. Some metals, like copper and mercury, do not react with dilute hydrochloric acid. They react with sulphuric acid and nitric acid but they do not liberate hydrogen gas. Metals, like gold and platinum, do not react with dilute acids. - Non-metals generally do not react with acids. Some non-metals, like sulphur and phosphorus, react with hot concentrated sulphuric acid, or nitric acid, but they do not liberate hydrogen gas. 4. **Reaction with Alkalies** - Most metals do not react with alkalis. Aluminium and zinc can, however, react with sodium hydroxide, or potassium hydroxide, to form their salts and liberate hydrogen gas. ### Reactivity of Metals The tendency of an element to react with other substances to form compounds is an indicator of its reactivity. The more is the tendency of an element to form compounds, the more is its reactivity. All metals do not have the same reactivity. Some are more reactive than the others, that is, they have a greater tendency to form compounds. Such metals occur in the form of their compounds in earth's crust. Lesser reactive metals occur in their native state, that is, elemental state. On the basis of experiments, involving the reaction of different metals with a particular substance, metals have been arranged in the decreasing order of their reactivity. The series of metals, arranged in the order of their decreasing reactivity, is called the reactivity series. Table 2 (given below) shows the reactivity series. | Symbol of the element | Name of the element | Symbol of the element | Name of the element | |---|---|---|---| | K | Potassium | Sn | Tin | | Na | Sodium | Pb | Lead | | Ca | Calcium | Cu | Copper | | Mg | Magnesium | Hg | Mercury | | Al | Aluminium | Ag | Silver | | Zn | Zinc | Au | Gold | | Fe | Iron | Pt | Platinum | Potassium is the most reactive metal while platinum is the least reactive. ### Displacement Reactions You have already learnt about displacement reactions. Reaction of a metal, with an acid, is an example of a displacement reaction in which the metal displaces hydrogen from the acid. A reaction, in which a more reactive metal displaces a lesser reactive metal from the aqueous solution of its salt, is another example of a displacement reaction. Let us perform an activity to understand this better. - Take 50 ml of water in a beaker and dissolve a few crystals of copper sulphate in it. A blue coloured solution is obtained. Dip a magnesium ribbon in this solution. You will observe that the blue colour of the solution fades and after sometime, the solution becomes colourless. Also, the magnesium ribbon gets coated with a brown layer. - This happens because magnesium, being more reactive than copper, displaces it from copper sulphate. The magnesium sulphate formed is colourless and the copper metal, that gets deposited on the magnesium ribbon, appears as a brown coating. This displacement reaction can be represented by the following equation: * Mg(s) + CuSO₄(aq) → MgSO₄(aq) + Cu(s) - Magnesium + Copper sulphate (blue) → Magnesium sulphate (colourless) + Copper - Repeat the activity by taking a solution of silver nitrate and dipping a copper wire in it. Report your observation to the teacher and write the equation for the chemical reaction involved. ### Noble Metals As seen from the reactivity series, platinum is the least reactive metal. It does not react with air, water, acids, bases and most other substances. Another metal, which shows a similar behaviour, is gold. Gold and platinum are called noble metals. Since noble metals are least reactive, they are not chemically affected by the substances around them. Hence, they do not get tarnished, and retain their lustre for a very long time. Both, these noble metals are also highly ductile and malleable; they can be drawn into extremely thin wires and can be beaten into very thin foils. It is because of these properties that gold and platinum are used for making jewellery. Gold can also be used for plating other metals, like copper and silver. Platinum is used in dentistry and in making scientific instruments. Pure gold is very soft. Therefore, it cannot be used for making jewellery in its pure form. Hence, for making jewellery, it is often mixed with silver or copper to make it appropriately hard. The purity of gold is expressed in terms of carats (or karats). The carat number gives the number of parts of gold present in 24 parts of a mixture of gold with the other metals. For example, 22 carat gold means that 22 parts of pure gold is present in 24 parts of a mixture of gold with copper (or silver). This implies that pure gold would be rated as: '24 carat gold'. ### Uses of Metals, Alloys and Non-Metals #### Uses of Common Metals - Metals have been an integral part of our daily life since ancient times. They have played an important part in the development of different civilisations. Even today, metals are used for a variety of purposes in our day-to-day life. The most commonly used metals, in every day life, are iron, copper, aluminium and silver. - **Iron** is the most widely used metal. It is used for making cooking vessels, water boilers, stoves, toys, tools, pipes, agricultural implements, chains, wires, nails, bolts, electromagnets, and so on. - **Aluminium**, being a very light metal, is used for making aircraft bodies. It is also used for making cooking vessels. Its thin foils are used for packaging of food stuffs and medicines. - **Copper** is the most widely used metal for making electrical cables and other electrical goods. It is also used for making cooking vessels. - **Silver** is used for making jewellery, decoration pieces, tableware, etc. Silver, being highly malleable, can be converted into very thin foils which are used for decorating food items. Silver is a very good conductor of electricity, but it is not commonly used for electrical fittings because it is very expensive. Silver and Gold wires are, however, used for high precision electrical contacts in computers. #### Uses of Alloys Besides being used in their pure form, metals are also often used in the form of their alloys. An alloy is a homogenous mixture of two or more metals, or a metal and a non-metal. By adding appropriate amount of other metals, or non-metals, to form the alloys, the properties of a given metal can be (significantly) modified. Alloys are generally stronger, harder and more resistant to corrosion than the (pure) metal itself. Uses of some common alloys are mentioned in Table 3 (given below). | Name of the Alloy | Made from | Used for making | |---|---|---| | Steel | Iron + carbon | Construction material, machine parts | | Stainless steel | Iron + chromium + nickel | Cooking utensils and cutlery, surgical implements, decorative statues, nuts and bolts | | Brass | Copper + zinc | Cooking utensils, decorative statues, nuts and bolts | | Bronze | Copper + tin | Cooking utensils, coins, medals, statues, decorative items | | Duralumin | Aluminium + copper + magnesium + manganese | Tableware, aircraft bodies, automobile parts, undersea vessels | | German Silver | Copper + zinc + nickel | Tableware | | Alnico | Aluminium + nickel + cobalt | Magnets | | Gun metal | Copper + tin + zinc | Gun-barrels | #### Uses of Non-Metals Some of the common uses, of some of the well-known non-metals, are given below: 1. Nitrogen, in the form of fertilisers, is essential for the growth and development of seeds and plants. 2. Phosphorus is used in matchbox industry and in fertilisers. 3. Iodine is used as an antiseptic. 4. Sulphur is used for making fire crackers, gun powder and sulphuric acid. 5. Oxygen is essential for survival of all living beings. 6. Diamond (a form of carbon) is used in making jewellery, in cutting glass and for grinding of tools. 7. Graphite (also a form of carbon) is used in batteries and in pencils. ### Keywords * **Alloy**: a homogenous mixture of two or more metals, or a metal and a non-metal. * **Conductivity**: the ability of a material to transfer heat energy, or electrical current, from one point to another. * **Ductility**: the property of metals due to which they can be drawn into thin wires. * **Malleability**: the property of metals due to which they can be hammered into thin sheets. * **Metalloid**: elements which show (some) properties of metals as well as non-metals. * **Metallurgy**: the sequence of processes used to extract a metal from its ore in its pure form. * **Mineral**: a natural occuring inorganic substance found deep under the surface of earth. * **Noble metals**: those metals which are least reactive and so are not chemically affected by the substances around them. * **Ore**: it is a mineral from which one, or more, metals can be extracted. * **Reactivity**: the tendency of an element to react with other substances to form compounds.