All pure substances are either elements or compounds. Water, common salt, sugar, oxygen, iron and gold – each one is either a single kind of atom (an element) or two or more elements joined chemically in a fixed ratio (a compound). Questions on symbols, properties of metals, the periodic table and common chemical names of compounds appear in almost every competitive exam.
What is an Element?
An element is a pure substance made of only one kind of atom, which cannot be broken down into simpler substances by chemical means.
- The word "element" in its modern sense was first used by Robert Boyle in 1661.
- Antoine Lavoisier, the father of modern chemistry, gave the first useful definition of an element.
- At present, 118 elements are known. About 94 occur naturally; the rest have been made in laboratories.
- Hydrogen (H) is the lightest and the most abundant element in the universe.
Classification of Elements
Elements are classified into metals, non-metals and metalloids.
Metals vs Non-metals
| Property | Metals | Non-metals |
|---|---|---|
| Physical state | Solid at room temperature (except mercury) | Solid, liquid (bromine) or gas |
| Lustre | Shiny (lustrous) | Dull (except iodine and graphite) |
| Malleability and ductility | Can be beaten into sheets and drawn into wires | Brittle, if solid |
| Conductivity | Good conductors of heat and electricity | Poor conductors (except graphite) |
| Sonority | Produce a ringing sound | Not sonorous |
| Oxides | Basic oxides | Acidic or neutral oxides |
| Ions formed | Lose electrons – positive ions | Gain electrons – negative ions |
| Examples | Iron, copper, aluminium, gold, silver, sodium | Oxygen, nitrogen, carbon, sulphur, chlorine, phosphorus |
Important Exceptions (Frequently Asked)
- Mercury is the only metal that is liquid at room temperature; bromine is the only non-metal that is liquid.
- Gallium and caesium melt at a very low temperature – gallium even melts on the palm of the hand.
- Sodium and potassium are so soft they can be cut with a knife; they are stored under kerosene because they react violently with air and water.
- Graphite, a non-metal, is a good conductor of electricity.
- Diamond, a form of carbon, is the hardest natural substance.
- Iodine is a non-metal with lustre.
- Silver is the best conductor of electricity; copper is next and is the most used for wiring.
- Gold is the most malleable and ductile metal; tungsten has the highest melting point among metals (about 3422°C) and is used in bulb filaments.
- Lithium is the lightest metal; osmium is among the densest.
- White phosphorus is stored under water because it catches fire in air.
Metalloids
Metalloids show properties of both metals and non-metals. Examples: boron, silicon, germanium, arsenic, antimony and tellurium. Silicon and germanium are used as semiconductors in computer chips and solar cells.
Noble Gases
Helium, neon, argon, krypton, xenon and radon are called noble (inert) gases because they hardly react. Helium is used in balloons and in breathing mixtures for deep-sea divers; neon in advertisement signs; argon in electric bulbs. Radon is radioactive.
Symbols of Elements
The modern system of symbols was given by the Swedish chemist J.J. Berzelius. Most symbols come from English names (H for hydrogen, O for oxygen), but several come from Latin names:
| Element | Symbol | Latin name |
|---|---|---|
| Sodium | Na | Natrium |
| Potassium | K | Kalium |
| Iron | Fe | Ferrum |
| Copper | Cu | Cuprum |
| Silver | Ag | Argentum |
| Gold | Au | Aurum |
| Lead | Pb | Plumbum |
| Mercury | Hg | Hydrargyrum |
| Tin | Sn | Stannum |
| Antimony | Sb | Stibium |
| Tungsten | W | Wolfram (German) |
Abundance of Elements
| Most abundant element in the universe | Hydrogen |
|---|---|
| Most abundant elements in the Earth's crust | Oxygen > Silicon > Aluminium > Iron |
| Most abundant metal in the Earth's crust | Aluminium |
| Most abundant gas in the atmosphere | Nitrogen (about 78%), then oxygen (about 21%) and argon (about 0.93%) |
| Most abundant element in the human body (by mass) | Oxygen, followed by carbon, hydrogen and nitrogen |
| Most abundant mineral element in the human body | Calcium |
The Periodic Table: How Elements Were Arranged
| Scientist | Contribution |
|---|---|
| J.W. Döbereiner (1817–1829) | Law of Triads – groups of three elements with similar properties |
| John Newlands (1865) | Law of Octaves – every eighth element shows similar properties |
| Dmitri Mendeleev (1869) | Arranged 63 known elements by increasing atomic mass; left gaps and correctly predicted undiscovered elements (gallium, scandium, germanium). Called the father of the periodic table. |
| Henry Moseley (1913) | Showed that atomic number is the basis; this gave the Modern Periodic Law |
- The modern periodic table has 18 vertical groups and 7 horizontal periods.
- Group 1 – alkali metals (Li, Na, K…); Group 2 – alkaline earth metals (Be, Mg, Ca…).
- Group 17 – halogens (F, Cl, Br, I); fluorine is the most electronegative and most reactive non-metal.
- Group 18 – noble gases.
- Element 101 is named mendelevium in honour of Mendeleev.
Molecules of Elements (Atomicity)
| Atomicity | Examples |
|---|---|
| Monoatomic | Helium (He), Neon (Ne), Argon (Ar) – noble gases |
| Diatomic | H2, O2, N2, Cl2 |
| Triatomic | Ozone (O3) |
| Tetratomic | Phosphorus (P4) |
| Polyatomic | Sulphur (S8) |
Allotropes of Carbon
Different physical forms of the same element are called allotropes. Carbon has several:
- Diamond – each carbon atom is bonded to four others in a rigid 3-D network; hardest natural substance, bad conductor of electricity.
- Graphite – carbon atoms in layers; soft, slippery, good conductor; used in pencils, electrodes and as a lubricant.
- Fullerene (C60) – football-shaped molecule called "buckminsterfullerene", discovered in 1985.
- Graphene – a single layer of graphite; first isolated in 2004 (Nobel Prize in Physics, 2010).
What is a Compound?
A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass. For example, in water, hydrogen and oxygen always combine in the ratio 1 : 8 by mass.
- This rule is the Law of Constant (Definite) Proportions, given by Joseph Proust.
- A compound's properties are completely different from those of its elements. Sodium is a reactive metal and chlorine is a poisonous gas, but their compound, sodium chloride, is the common salt we eat.
- Compounds can be broken into their elements only by chemical or electrochemical methods (e.g., electrolysis of water).
Types of Compounds by Bonding
- Ionic (electrovalent) compounds – formed by transfer of electrons from a metal to a non-metal. Usually solid, high melting point, conduct electricity in molten or dissolved state. Example: NaCl, MgO.
- Covalent compounds – formed by sharing of electrons between non-metals. Usually low melting point, poor conductors. Example: H2O, CH4, CO2.
Compound vs Mixture
| Compound | Mixture |
|---|---|
| Elements combine chemically | Substances are only mixed physically |
| Fixed composition | Variable composition |
| Properties differ from its elements | Each component keeps its own properties |
| Separated only by chemical methods | Separated by physical methods |
| Energy is usually given out or taken in during formation | No energy change during mixing |
| Example: water, salt, carbon dioxide | Example: air, salt solution, brass |
A popular classroom example: a mixture of iron filings and sulphur powder can be separated with a magnet, but when heated they form iron sulphide (FeS), a compound that is no longer attracted by a magnet.
Common Names and Formulas of Important Compounds
| Common name | Chemical name | Formula |
|---|---|---|
| Water | Dihydrogen monoxide | H2O |
| Heavy water | Deuterium oxide | D2O |
| Common salt | Sodium chloride | NaCl |
| Baking soda | Sodium hydrogen carbonate (sodium bicarbonate) | NaHCO3 |
| Washing soda | Sodium carbonate decahydrate | Na2CO3·10H2O |
| Caustic soda | Sodium hydroxide | NaOH |
| Caustic potash | Potassium hydroxide | KOH |
| Bleaching powder | Calcium oxychloride (calcium hypochlorite) | CaOCl2 |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO4·½H2O |
| Gypsum | Calcium sulphate dihydrate | CaSO4·2H2O |
| Quicklime | Calcium oxide | CaO |
| Slaked lime / Lime water | Calcium hydroxide | Ca(OH)2 |
| Limestone, marble, chalk | Calcium carbonate | CaCO3 |
| Blue vitriol | Copper sulphate pentahydrate | CuSO4·5H2O |
| Green vitriol | Ferrous sulphate heptahydrate | FeSO4·7H2O |
| White vitriol | Zinc sulphate heptahydrate | ZnSO4·7H2O |
| Oil of vitriol | Sulphuric acid | H2SO4 |
| Muriatic acid | Hydrochloric acid | HCl |
| Aqua regia | Conc. HCl and conc. HNO3 in 3 : 1 ratio | Dissolves gold and platinum |
| Vinegar | Dilute acetic (ethanoic) acid | CH3COOH |
| Potash alum (Phitkari) | Potassium aluminium sulphate | K2SO4·Al2(SO4)3·24H2O |
| Saltpetre | Potassium nitrate | KNO3 |
| Chile saltpetre | Sodium nitrate | NaNO3 |
| Laughing gas | Nitrous oxide | N2O |
| Marsh gas | Methane | CH4 |
| Dry ice | Solid carbon dioxide | CO2 |
| Sand / Quartz | Silicon dioxide | SiO2 |
| Rust | Hydrated ferric oxide | Fe2O3·xH2O |
| Ammonia | Ammonia | NH3 |
| Sal ammoniac (Nausadar) | Ammonium chloride | NH4Cl |
| Hydrogen peroxide | Hydrogen peroxide | H2O2 |
| Galena | Lead sulphide (ore of lead) | PbS |
| Haematite | Ferric oxide (ore of iron) | Fe2O3 |
| Magnetite | Ferroso-ferric oxide (ore of iron) | Fe3O4 |
| Bauxite | Hydrated aluminium oxide (ore of aluminium) | Al2O3·2H2O |
Uses of Some Common Compounds
- Baking soda – making cakes fluffy, antacid for acidity, in soda-acid fire extinguishers.
- Washing soda – removing permanent hardness of water, cleaning, making glass and soap.
- Bleaching powder – disinfecting drinking water, bleaching cotton and paper.
- Plaster of Paris – supporting fractured bones, making toys, statues and false ceilings. It is made by heating gypsum.
- Heavy water – used as a moderator in nuclear reactors.
- Potash alum – purifying water and as an antiseptic after shaving.
- Dry ice – cooling and preserving food, creating fog effects on stage.
Quick Revision Points
- 118 elements are known; hydrogen is the lightest and most abundant in the universe.
- Liquid metal: mercury; liquid non-metal: bromine.
- Best conductor: silver; non-metal conductor: graphite; hardest natural substance: diamond.
- Most abundant metal in the Earth's crust: aluminium; most abundant element: oxygen.
- Mendeleev arranged elements by atomic mass (1869); Moseley by atomic number (1913).
- Water has hydrogen and oxygen in a 1 : 8 ratio by mass – Law of Constant Proportions.
- Aqua regia = 3 parts HCl + 1 part HNO3; dissolves gold.
Read also: Matter and its Components
Practice MCQs
1. Which metal is liquid at room temperature?
(a) Gallium
(b) Mercury
(c) Sodium
(d) Caesium
2. The symbol of potassium, "K", comes from the Latin word:
(a) Kalium
(b) Kryptos
(c) Kalcium
(d) Kupfer
3. Which is the most abundant metal in the Earth's crust?
(a) Iron
(b) Aluminium
(c) Calcium
(d) Copper
4. Which non-metal is a good conductor of electricity?
(a) Sulphur
(b) Diamond
(c) Graphite
(d) Phosphorus
5. Mendeleev arranged elements in his periodic table according to:
(a) Atomic number
(b) Atomic mass
(c) Valency
(d) Number of neutrons
6. The chemical formula of baking soda is:
(a) Na2CO3
(b) NaHCO3
(c) NaOH
(d) NaCl
7. Plaster of Paris is obtained by heating:
(a) Limestone
(b) Gypsum
(c) Bauxite
(d) Washing soda
8. Aqua regia is a mixture of:
(a) 1 part HCl and 3 parts HNO3
(b) 3 parts HCl and 1 part HNO3
(c) HCl and H2SO4
(d) HNO3 and H2SO4
9. Sodium is stored in:
(a) Water
(b) Alcohol
(c) Kerosene
(d) Open air
10. "Laughing gas" is the common name of:
(a) Nitric oxide
(b) Nitrogen dioxide
(c) Nitrous oxide
(d) Ammonia
Answers: 1 – (b) | 2 – (a) | 3 – (b) | 4 – (c) | 5 – (b) | 6 – (b) | 7 – (b) | 8 – (b) | 9 – (c) | 10 – (c)
Reference: NCERT Science Class IX (Is Matter Around Us Pure; Atoms and Molecules) and Class X (Acids, Bases and Salts; Metals and Non-metals; Carbon and its Compounds); NCERT Chemistry Class XI (Classification of Elements).