Matter, Atomic Structure and the Periodic Table

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States of matter

Matter has mass and occupies space; it is made of tiny particles with spaces between them that keep moving and attract each other. Solids have a fixed shape and volume, liquids a fixed volume only, and gases neither, so gases are highly compressible. Plasma, an ionised gas found in the Sun, the stars, fluorescent tubes and neon signs, is the fourth state; the Bose-Einstein condensate, based on the work of Satyendra Nath Bose and Albert Einstein and first made in a laboratory in 1995, is the fifth, formed close to absolute zero. In sublimation a solid such as camphor, naphthalene, iodine, ammonium chloride or dry ice turns directly into vapour.

Elements, compounds and mixtures

An element is made of one kind of atom and cannot be split into simpler substances by chemical means; elements are metals, non-metals or metalloids such as boron, silicon and germanium. A compound forms when elements combine chemically in a fixed ratio by mass, and its properties differ from theirs: hydrogen burns and oxygen helps burning, but water puts out fire. A mixture has components in any ratio that keep their own properties and can be separated physically. Solutions such as salt water, air and alloys are homogeneous mixtures; sand in water is heterogeneous. Colloids such as milk, fog and smoke look uniform but scatter a beam of light (Tyndall effect).

Separation methods

Salt is recovered from sea water by evaporation, cream from milk by centrifugation, the dyes in ink by chromatography, oil from water with a separating funnel, camphor or ammonium chloride from salt by sublimation, and pure copper sulphate by crystallisation. Distillation separates miscible liquids with widely different boiling points; fractional distillation is used when the boiling points differ by less than 25 K, as in refining petroleum and in getting nitrogen, oxygen and argon from liquid air. Alum (fitkari) makes the mud in water settle.

Inside the atom

John Dalton (1808) described matter as made of indivisible atoms. J. J. Thomson discovered the electron in 1897 from cathode rays and proposed the plum pudding model. E. Goldstein's canal rays (1886) gave the first clue to positive particles, and Ernest Rutherford identified and named the proton. Rutherford's scattering of alpha particles by thin gold foil led him in 1911 to the nuclear model: the atom is mostly empty space with a tiny, dense, positive nucleus. Niels Bohr (1913) placed electrons in fixed shells (K, L, M, N), and James Chadwick discovered the neutron, which has no charge and about the mass of a proton, in 1932. A shell holds at most 2n^2 electrons (K 2, L 8, M 18), and the outermost shell holds at most 8.

Atomic number, mass number, isotopes and isobars

The atomic number (Z) is the number of protons, equal to the number of electrons in a neutral atom, and fixes the identity of the element; the mass number (A) is protons plus neutrons. Sodium has Z = 11 and A = 23, so it has 12 neutrons and the electronic arrangement 2, 8, 1. Isotopes have the same atomic number but different mass numbers and the same chemical properties: hydrogen has protium (no neutron), deuterium (one neutron; heavy water, D2O, slows neutrons in nuclear reactors) and radioactive tritium (two neutrons). Carbon-14 dates fossils, cobalt-60 treats cancer, iodine-131 is used to treat thyroid disorders such as an overactive thyroid and thyroid cancer (NCERT words this as the treatment of goitre) and uranium-235 fuels nuclear reactors. Isobars have the same mass number but different atomic numbers, such as calcium-40 and argon-40.

Valency

Valency is the combining capacity of an element, the number of electrons an atom loses, gains or shares to complete its outer octet (a duplet for hydrogen and helium). Sodium and chlorine have valency 1, magnesium and oxygen 2, aluminium and nitrogen 3, carbon 4, and the noble gases 0. Crossing over the valencies gives formulae such as Al2O3 and CaCl2.

Towards the periodic table

Döbereiner (1817) grouped similar elements in triads, such as lithium, sodium and potassium, in which the middle atomic mass is about the average of the other two. Newlands (1866) found every eighth element resembling the first, like the notes of music (law of octaves), but this held only up to calcium. Dmitri Mendeleev (1869) arranged the 63 elements then known by increasing atomic mass and similar properties, left gaps and predicted eka-aluminium and eka-silicon, later found as gallium and germanium. Henry Moseley showed that atomic number is the more basic property, and the modern periodic law says the properties of elements are a periodic function of their atomic numbers.

Groups and periods

The modern table has 18 groups (columns) and 7 periods (rows). Elements in a group have the same number of valence electrons and similar properties, and the period number equals the number of shells. Period 1 has 2 elements, periods 2 and 3 have 8 each, periods 4 and 5 have 18 each, and periods 6 and 7 have 32 each. Group 1 holds the alkali metals, group 2 the alkaline earth metals, groups 3 to 12 the transition metals, group 17 the halogens (fluorine is the most reactive non-metal) and group 18 the noble gases. Across a period atomic size and metallic character decrease; down a group they increase.

Key facts

Plasma (Sun, stars, neon signs) is the fourth state of matter; the Bose-Einstein condensate (from the work of S. N. Bose and Einstein, first made in 1995) is the fifth.
Sublimation: camphor, naphthalene, iodine, ammonium chloride, dry ice. The Tyndall effect shows in colloids (milk, fog, smoke), not in true solutions.
Separation: cream from milk by centrifugation; ink dyes by chromatography; petroleum and liquid air by fractional distillation; oil and water by separating funnel; pure copper sulphate by crystallisation.
Discoverers: electron - J. J. Thomson (1897); canal rays - Goldstein (1886); proton - Rutherford; nucleus - Rutherford (1911); shells - Bohr (1913); neutron - Chadwick (1932).
Sodium: Z = 11, A = 23, so 12 neutrons; electronic configuration 2, 8, 1; valency 1.
Hydrogen isotopes: protium, deuterium (heavy water D2O, a moderator) and radioactive tritium; C-14 dating, Co-60 cancer treatment, I-131 thyroid disorders (NCERT: goitre), U-235 nuclear fuel.
Isotopes: same atomic number, different mass number; isobars: same mass number, different atomic number (calcium-40 and argon-40).
Döbereiner's triads (1817), Newlands' octaves (1866), Mendeleev (1869: atomic mass, 63 elements, gaps for gallium and germanium), Moseley (atomic number).
Modern table: 18 groups, 7 periods; group 1 alkali metals, 2 alkaline earth metals, 3 to 12 transition metals, 17 halogens, 18 noble gases; atomic size falls across a period and grows down a group.

Quick revision

Plasma is the 4th state, Bose-Einstein condensate the 5th; camphor sublimes; compound has a fixed ratio, mixture any ratio; centrifugation for cream, chromatography for dyes, fractional distillation for petroleum; electron Thomson 1897, nucleus Rutherford 1911, Bohr shells 1913, neutron Chadwick 1932; Z = protons, A = protons + neutrons; isotopes same Z, isobars same A; heavy water moderator; Mendeleev 1869 by atomic mass; modern law by atomic number; 18 groups, 7 periods; 17 halogens, 18 noble gases.

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