Mass is neither created nor destroyed in a chemical reaction
Lavoisier
Constant (definite) proportions
A compound always has the same elements in the same mass ratio (water: H and O in 1 : 8)
Proust
Law of multiple proportions
Different compounds of the same elements combine in simple whole-number ratios
Dalton
Dalton's atomic theory
All matter is made of atoms, which are indivisible.
Atoms of one element are identical; those of different elements differ.
Atoms combine in simple whole-number ratios to form compounds.
Atoms are neither created nor destroyed in a reaction.
Limitation: atoms are divisible (into electrons, protons, neutrons) and isotopes of an element have different masses.
Atoms, molecules and ions
Atom: the smallest particle of an element that takes part in a reaction. Molecule: a group of atoms bonded together (O2, H2O, CO2).
Atomicity is the number of atoms in a molecule: monatomic (He, Ne, Ar), diatomic (H2, N2, O2, Cl2), triatomic (O3 ozone, CO2), tetratomic (P4), octatomic (S8).
Ion: a charged particle. Cation is positive (lost electrons, e.g. Na+); anion is negative (gained electrons, e.g. Cl-).
Symbols: often the first letter or two (H, He, Ca), or from Latin: Na (natrium, sodium), K (kalium, potassium), Fe (ferrum, iron), Cu (cuprum, copper), Ag (argentum, silver), Au (aurum, gold), Pb (plumbum, lead), Hg (hydrargyrum, mercury), Sn (stannum, tin), Sb (stibium, antimony), W (wolfram, tungsten).
Most abundant elements: in the Earth's crust: oxygen, then silicon, aluminium, iron; in the universe: hydrogen; in the human body: oxygen; in the atmosphere: nitrogen. The most abundant metal in the crust is aluminium.
Atomic mass unit (u): 1/12 of the mass of a carbon-12 atom.
Mole concept
Mole: the amount of substance containing 6.022 x 10^23 particles (Avogadro's number).
Molar mass is the mass of one mole in grams. Moles = given mass / molar mass.
1 mole of any gas at STP occupies 22.4 litres.
Example: 1 mole of water = 18 g = 6.022 x 10^23 molecules. Molecular mass of CO2 = 12 + 32 = 44 u.
Discovery of subatomic particles
Particle
Charge
Mass
Discoverer
Location
Electron (e-)
Negative (-1)
Very small (1/1836 of a proton)
J. J. Thomson (1897)
Outside the nucleus
Proton (p+)
Positive (+1)
1 u
Goldstein (canal rays), named by Rutherford
Nucleus
Neutron (n)
Neutral
1 u
James Chadwick (1932)
Nucleus
Atomic models
Model
Idea
Limitation
Thomson (plum pudding)
Positive sphere with electrons embedded
Could not explain scattering results
Rutherford (nuclear model)
Alpha-scattering experiment on gold foil: a small, dense, positively charged nucleus with electrons around it; most of the atom is empty space
Did not explain stability of the atom
Bohr
Electrons revolve in fixed orbits (shells) without radiating energy; energy is quantised
Fails for larger atoms
Shells: K, L, M, N (n = 1, 2, 3, 4). Maximum electrons in a shell = 2n^2: K = 2, L = 8, M = 18, N = 32. The outermost shell holds at most 8 electrons (the octet rule; duplet for first shell).
Valence electrons are those in the outermost shell. Valency is the combining capacity: it equals the number of valence electrons (up to 4), otherwise 8 minus valence electrons.
Examples: Na (2,8,1) valency 1; Mg (2,8,2) valency 2; Al (2,8,3) valency 3; Cl (2,8,7) valency 1; Ne (2,8) zero (inert).
Atomic number, mass number, isotopes
Atomic number (Z) = number of protons (= electrons in a neutral atom).
Mass number (A) = protons + neutrons. Neutrons = A - Z.
Isotopes: atoms of the same element with the same atomic number but different mass numbers. Same chemical properties. Examples: protium, deuterium, tritium (hydrogen); carbon-12 and carbon-14; chlorine-35 and 37; uranium-235.
Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.