Periodic Table

Imagine a library with 63 books thrown on the floor and no catalogue. You would soon sort them by something: size, subject, author. In the early 1800s chemists faced exactly this problem. Elements were being discovered every few years, each with its own colour, weight and temper, and nobody could see a pattern. Classification simply means grouping elements so that the ones that behave alike sit together. Every great name in this topic, Döbereiner, Newlands, Mendeleev, Moseley, tried a better way of sorting, and each fixed the weakness of the one before.

Mendeleev's real genius was not the table but his courage. He left gaps for elements not yet discovered and even predicted their properties. When gallium (1875), scandium (1879) and germanium (1886) were found with almost exactly the predicted properties, the world believed in his table. He used the Sanskrit word 'Eka' (meaning 'one') for the names: Eka-boron means 'one place below boron'.

Mendeleev's table had one deep flaw: it trusted atomic mass, but atomic mass is not the real identity card of an atom. In 1913 Henry Moseley bombarded elements with electrons and measured the X-rays they emitted. The frequency of the X-rays rose in a neat step-by-step way with the number of protons in the nucleus, not with the mass. So the true order is by atomic number. Overnight the problems of Mendeleev's table (tellurium before iodine, cobalt before nickel) vanished, because in each such pair the element placed first really does have the smaller atomic number.

The Modern Periodic Table arranges all known elements in order of increasing atomic number (Moseley's idea). It has 18 vertical columns called groups and 7 horizontal rows called periods. Think of a cinema hall: the row tells you how far from the screen you are, and the seat column tells you which neighbours you resemble. In the table, the period tells you how many electron shells an atom has, and the group tells you how many electrons sit in its outermost shell, so elements in one group behave alike. That is the whole logic of the table; everything else is detail.

Period number = number of electron shells. Group tells valence electrons. Sodium (Na, Z = 11) has configuration 2, 8, 1: three shells so it is in period 3, one valence electron so it is in group 1. For groups 13 to 18, group number = 10 + number of valence electrons (so 2 valence electrons for group 2, but 6 valence electrons for group 16). Radhika's practice question: an element with configuration 2, 8, 6 sits in period 3 and group 16; it is sulphur.

Now look at blocks. Elements are also sorted by the sub-shell in which their last electron enters. This gives four blocks, and each block has a personality. The s-block and p-block together are the main group (representative / normal) elements, the real stars of GS questions. The d-block is the bridge between them (the transition elements), and the f-block hides at the bottom as the inner transition elements.

Why do properties repeat? Because they depend on the electrons in the outer shell, and the outer shell pattern repeats after every period. Two simple forces decide almost every trend. The nucleus pulls the outer electrons inwards (more protons, stronger pull). The distance and the inner electrons weaken that pull (more shells, weaker grip). Walk left to right in a period: protons increase but the shell stays the same, so the pull wins and the atom shrinks and holds its electrons tightly. Walk down a group: a whole new shell is added, so distance wins and the atom grows and lets go of its electrons easily. Learn these two sentences and you can derive every trend yourself; no memorising needed.

Effective nuclear charge (Zeff) is the pull actually felt by an outer electron after the inner electrons have shielded some of the nuclear charge. Across a period, shielding stays the same but protons increase, so Zeff rises; that single fact explains why the atom shrinks and why IE and electronegativity rise. Among B, C, Li and F, fluorine is farthest right, so it has the highest Zeff.

Some groups are so well known that they carry family names. Meet the family: Group 1 (alkali metals) are the wild youngsters with one loose outer electron, ready to give it away at the slightest provocation. Group 2 (alkaline earth metals) are their calmer cousins with two outer electrons. Group 17 (halogens) are the electron-hungry ones, only one electron short of a full shell. Group 18 (noble gases) are the contented aristocrats whose outer shell is already full, so they want nothing from anyone. Chemistry is simply the story of who gives, who takes and who refuses to trade.

The word halogen means 'salt-former': a halogen reacting with a metal gives a salt (sodium + chlorine gives sodium chloride). All halogens exist as diatomic molecules (F₂, Cl₂, Br₂, I₂), have valency 1 and form negative ions called halides. A more reactive halogen pushes a less reactive one out of its salt: chlorine gas passed through potassium bromide solution turns it brown because bromine is set free.

Noble gases are 'inert' because their outer shell is already full: 8 electrons (the octet), and 2 electrons (the duplet) in the case of helium. They have no reason to gain, lose or share electrons, so their valency is zero. They are monoatomic gases. Every other element is trying to reach this stable arrangement, which is the whole reason chemical bonds exist.

Every element has a short name tag called its symbol. The modern system of one or two letters was introduced by the Swedish chemist Berzelius (about 1813-14); before that, Dalton drew circles and pictures. Today the names and symbols are approved by IUPAC (International Union of Pure and Applied Chemistry). The idea is simple: the first letter is always capital and the second, if any, is small (Co is cobalt, but CO is carbon monoxide). Most symbols come from the English name (O for oxygen, Ca for calcium), but some old metals keep the symbol of their Latin or German name. Those are the ones examiners love.

The table is still growing. Elements up to uranium (Z = 92) occur in nature (plutonium and neptunium only in traces), while those beyond it, the transuranium elements, are made in laboratories by smashing nuclei together. Technetium (Z = 43) was the first element to be made artificially (1937). In 2016 IUPAC approved the names of the four newest elements: nihonium (Nh, 113), moscovium (Mc, 115), tennessine (Ts, 117) and oganesson (Og, 118), which completed the seventh period. Several are named after great scientists: curium (Cm), einsteinium (Es), fermium (Fm), mendelevium (Md), rutherfordium (Rf), bohrium (Bh), meitnerium (Mt), seaborgium (Sg) and copernicium (Cn).