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Chapter 3

Metals and Non-metals

Physical Properties of Metals

Elements are classified as metals and non-metals. Metals exhibit the following physical properties:

  1. Metallic Lustre: Metal in its pure state have a shining surface. This property is known as metallic lustre.

  2. Malleability: The ability of metals to be beaten into thin sheets is called malleability. Gold and Silver are the most malleable metals.

  3. Ductility: The ability of metals to be drawn into thin wires is known as ductility. Gold is the most ductile metal.

  4. Conduction: Metals are good conductors of heat and electricity as they have free electrons. Silver and copper are the best conductors of heat whereas lead and mercury are poor conductors of heat.

  5. Melting and Boiling point: Metals have high melting points.

  6. Hardness: All metals are generally hard. The hardness varies from metal to non-metal.

  7. Sonorous: Metals that produces a sound on striking a hard surface are said to be sonorous.

Physical Properties of Nonmetals

Non-metals are either solids or gases. Only bromine is a non-metal that exists in a liquid state. Some examples of the non-metals are carbon, sulphur, iodine, oxygen, hydrogen etc. The following are the physical properties exhibited by non-metals.

  1. Non-metals are dull and do not have shining surface as metal.

  2. Non-Malleable: Non-metals are brittle so they cannot be converted into thin sheets.

  3. Non ductile: Non-metals cannot be rolled into wires.

  4. Poor Conductor: They are poor conductor of heat and electricity.

  5. Non-metal do not produces sound on striking a hard surface.

  6. Low melting and boiling point as compared to metals.

Exceptional Cases in Physical Properties of Metals and Non Metals

There are many exceptions that exists when we group elements according to their physical properties. Some of them are mentioned below;

  1. All metals exist as solid at room temperature except mercury.

  2. Metals have high melting and boiling points but gallium and caesium have very low melting and boiling points. These metals will melt even if they are kept in palm.

  3. Non-metal are dull but Iodine is a lustrous non-metal.

  4. Carbon is a non-metal that exist in different forms. Each form is called allotrope.

  5. Alkali metals are soft and can be cut with a knife. They have low densities and low melting point. Eg: Lithium, sodium, potassium

Therefore to get more clarity, it becomes important to classify metals and nonmetals on the basis of chemical properties.

Chemical Properties of Metals

1) When metals are burnt in Air

All metals combine with oxygen to form metal oxides.

Metal + Oxygen → Metal Oxide

Example:

2Cu + O2 → 2CuO

(Copper) (Copper (II) Oxide)

4Al + 3O2 → 2Al2O3

(Aluminium) (Aluminium Oxide)

Metals such as such as potassium and sodium react so vigorously that they catch fire if kept in the open. They are kept immersed in kerosene oil to prevent them and protect from accidental fires.

Amphoteric oxides: Metals oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. Aluminium oxide and zinc oxide are examples of amphoteric oxide.

The reaction of aluminium oxide with acid and base is shown below;

Al2O3 + 6HCl → 2AlCl3 + 3H2O

Al2O3 + 2NaOH → 2NaAlO2 + H2O

Metal Oxide

Metal Oxide are insoluble in water but some of them dissolve in water to form alkalis.

Na2O(s) + H2O(l) → 2NaOH(aq)

K2O(s) + H2O(l) → 2KOH(aq)

2) When metals react with Water

Metal react with water and produce a metal oxide and hydrogen gas. But all the metals do not react with water.

Metal + Water → Metal oxide + Hydrogen

Metal Oxide + Water → Metal hydroxide

Examples:

a) Potassium and Sodium react violently with cold water as shown below;

2K(s) + 2H2O(l) → 2KOH(aq) + H2(g) + heat energy

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g) + heat energy

b) Calcium react with water in a less violently

2Ca(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)

c) Aluminium, iron and zinc do not react with cold or hot water. They react with steam.

2Al(s) + 3H2O(g) → Al2O3(s) + 3H2(g)

3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)

d) Metals such as lead, copper, silver and gold do not react with water.

3) When metals react with Acids

Metals react with acids to give salt and hydrogen gas.

Metal + Dilute acid → Salt + Hydrogen

All metals do not react with acid in a similar manner. Reactivity decreases in the order

Mg > Al > Zn > Fe

4) Metals react with Solutions of other Salts

Reactive metals displace less reactive metals from their compounds in solution or molten form. It is like a displacement reaction, if metal A displaces Metal B from its solution, then it is more reactive than B.

Metal A + Metal B → Salt solution of A + Metal B



The Reactivity Series

The reactivity or activity series is a list of metals arranged in the order of their decreasing activities. The most reactive metal is potassium and least reactive is gold as shown below.

K > Na > Ca > Mg > Al > Zn > Fe >Pb >H > Cu > Hg > Ag > Au

How do Metals and Non-metals react?

Reactivity of the elements is the tendency to attain a completely filled valence shell. Atoms of metals lose their valence shell to form cation whereas the atoms of the nonmetals gain electrons in the valence shell to form anions.

Formation of NaCl

Formation of MgCl2

The compounds formed by the transfer of electrons from metal to nonmetal are known as ionic compound or electrovalent compounds.

Properties of Ionic Compounds

  1. Physical nature: Ionic compound are solids and are hard due to the strong force of attraction between the positive and negative ions. These compounds are brittle and break into pieces when pressure is applied.

  2. Melting and boiling points: Ionic compounds have high melting and boiling points because a high amount of energy is required to break the strong inter ionic attraction force.

  3. Solubility: Electrovalent compounds are soluble in water and insoluble in solvents such as kerosene, petrol etc.

  4. Conduction of electricity: Ionic compounds in solid state do not conduct electricity as the movement of ions is not possible. But they conduct electricity in molten state since the electrostatic force of attraction between the oppositely charged ions are overcome due to heat.

Occurrence of Metals

The elements and compounds which occur naturally in the earth's crust are known as minerals. At some places these minerals have higher percentage of a particular metal which can be extracted from it. These minerals are known as ores.

1) Extraction of Minerals

On the basis of the reactivity metals are categorised into 3 parts:

i) Metal of low reactivity: These metals are mostly found in free states. Eg: Gold, silver, platinum, copper

ii) Metal of medium reactivity: These metals are found mainly as oxides, sulphides or carbonates. Ores of many metals exists in oxides form.

iii) Metal of high reactivity: These metals are highly reactive and thus are never found in nature as free element. Eg: K, Na, Ca, Mg, Al

The figure below shows the activity series and related metallurgy.

Different techniques are used for obtaining the metals falling in each category. The diagram below explains the extraction of pure metal from ores in several steps.

2) Enrichment of Ores

The ore mined from the earth contaminated with large amounts of impurities such as soil, sand etc; which is called as gangue. The process used for removing the gangue from the ore are based on the differences between the physical or chemical properties of the gangue and ore. Different separation techniques are used accordingly.

3) Extracting Metals Low in the Activity Series

Extraction of mercury from cinnabar (HgS) ore.

Extraction of copper from Cu2S ore.

4) Extracting Metals in the Middle of the Activity Series

The metals in the middle of the activity series are usually present as sulphide or carbonate in nature. It is easier to extract metal from its oxide form as compared to its sulphide and carbonate form. So, the metals in the middle of the activity series are first converted in oxide form.

The process of converting sulphide ores into oxides by heating strongly in the presence of excess air is known as roasting.

The process of converting carbonate ores into oxides by heating strongly in limited air is known as calcination.

Now, metal oxides are reduced to its corresponding metals by using suitable reducing agent such as carbon. The metal zinc is obtained from zinc oxide in the following way;

ZnO(s) + C(s) → Zn(s) + CO(g)

6) Reducing Agents

Apart from carbon (coke), displacement reaction is also used to obtain metal from metal oxide.

The highly reactive metals (such as Na, Ca, Al) are used as reducing agents because they can displace metal of lower reactivity from their compounds. These displacement reactions are highly exothermic.

3MnO2(s) + 4Al(s) → 3Mn(l) + 2Al2O3(s) + Heat

Reaction of iron(III) oxide with aluminium is known as thermite reaction. This reaction is used to join railway tracks or cracked machine parts.

Fe2O3(s) + 2Al(s) → 2Fe(l) + Al2O3(s) + Heat

7) Extracting Metals towards the Top of the Activity Series

The metals high in the reactivity series are very reactive. They have more affinity for oxygen than carbon that's why they cannot be obtained from their compounds by heating with carbon. These metals are obtained by electrolytic reduction. Sodium is obtained by electrolysis of molten chloride. Sodium is deposited at cathode, whereas chlorine is liberated at the anode. The reaction is given as;

At cathode Na+ + e– → Na

At anode 2Cl– → Cl2 + 2e–

8) Refining of Metals

The metals obtained through reduction process are not pure. They can impurities which must be removed to obtain pure metal. The method used for refining impure metal is electrolytic refining. In this process, the impure metal is used as anode and strip of pure metal is used as a cathode. The solution of metal salt is used as an electrolyte. On passing the current through the electrolyte, the pure metal from the anode dissolve into the electrolyte. An equivalent amount of pure metal from the electrolyte is deposited on the cathode. The soluble impurities go into the solution, whereas the insoluble impurities settle down at the bottom of the anode and are known as anode mud.

Corrosion

Iron when exposed to moist air for a long time acquires a coating of a brown flaky substance called rust. The process of rusting is known as corrosion.

Prevention of Corrosion

There are different methods through which rusting of iron can be prevented. These are:

  1. Painting

  2. Oiling

  3. Greasing

  4. Chrome plating

  5. Anodising or Alloying

  6. Galvanisation: The method of protecting steel and iron from rusting by coating them with a thin layer of zinc.

Alloy

An Alloy is a homogeneous mixture of two or more metals, or a metal and a nonmetal. It is prepared by first melting the primary metal, and then, dissolving the other elements in it in definite proportions. After that it is cooled at room temperature.

  • If one metal is mercury then the alloy is known as amalgam.

  • Solder is an alloy of lead and tin (Pb and Sn). It has low meting point and is used for welding electrical wires together.

Properties of Alloys

  1. Electric conducting of an alloy is less than the pure metals.

  2. Melting point of alloy is less as compared to pure metals.