The Periodic Table and Its Groups
The Development of the Periodic Table
Before the discovery of protons and neutrons, scientists attempted to classify elements by arranging them in order of atomic weight. Dmitri Mendeleev (1869) arranged elements in order of atomic weight but left gaps for undiscovered elements and swapped some elements so that those with similar properties lined up in the same columns. When elements such as gallium and germanium were later discovered and matched his predictions, his table was widely accepted.
The modern periodic table arranges elements in order of atomic number (not atomic weight). Elements in the same group (column) have the same number of electrons in their outer shell, giving them similar chemical properties. Elements in the same period (row) have the same number of electron shells.
Elements are arranged by atomic number. Groups (columns) share the same number of outer electrons and similar properties. Periods (rows) share the same number of electron shells. Metals are on the left; non-metals are on the right.
Group 1 — Alkali Metals
The alkali metals (Li, Na, K, Rb, Cs, Fr) all have one electron in their outer shell. They are soft, have low density and react vigorously with water, producing a metal hydroxide and hydrogen gas:
$$2Na + 2H_2O \rightarrow 2NaOH + H_2$$
Going down Group 1, reactivity increases because the outer electron is further from the nucleus and is more easily lost (less electrostatic attraction). The elements also have lower melting points and higher densities going down the group.
Group 7 — Halogens
The halogens (F, Cl, Br, I, At) have seven electrons in their outer shell. They exist as diatomic molecules (\(Cl_2\), \(Br_2\), etc.) and react with metals to form ionic compounds called halides (e.g. NaCl).
Going down Group 7, reactivity decreases because the outer shell is further from the nucleus, making it harder to gain an extra electron. A more reactive halogen can displace a less reactive halogen from its compound, e.g. chlorine displaces bromine from potassium bromide.
Group 1: reactivity increases down the group (easier to lose outer electron). Group 7: reactivity decreases down the group (harder to gain an electron). These trends are opposite because metals lose electrons while non-metals gain them.
The noble gases (He, Ne, Ar, Kr, Xe, Rn) have full outer electron shells, making them very stable and unreactive. They are used where inert gases are needed: helium in balloons and airships, neon in advertising signs, argon in light bulbs and welding. Boiling point increases down the group.
Transition metals are found in the central block of the periodic table. They have typical metallic properties: high melting points, high density, good conductors. Unlike Group 1 metals, transition metals are less reactive, harder and denser. They form coloured compounds and can act as catalysts (e.g. iron in the Haber process).