Periodic table

The periodic table is the arrangement of all the elements in order of increasing proton number, set out in rows and columns so that elements with similar chemical properties fall together. Malay: jadual berkala · Chinese: 周期表.

The whole chapter rests on one idea: an element’s place in the table is fixed by its proton number, and that place tells you almost everything else about it. Read across a row and the proton number rises by one at each step; the position then reveals the electron arrangement, the number of valence electrons, and whether the element behaves as a metal or a non-metal. A diagonal staircase on the right separates the metals on the left from the non-metals on the right, with the metalloids straddling the line, so even the broad character of an element can be read from where it lands.

Example. Sodium has proton number 11 and the arrangement 2.8.1, so it sits in Period 3 (three occupied shells) and Group 1 (one valence electron). Chlorine, proton number 17, arrangement 2.8.7, sits in the same period but in Group 17 because it has seven valence electrons. Their very different chemistry is read straight off their positions.

Confusion to avoid. The modern table is ordered by proton number, not by relative atomic mass. Mendeleev’s early table used atomic mass and left gaps for undiscovered elements; the switch to proton number fixed the few places where mass order gave the wrong neighbours. In the exam, always say the elements are arranged “in order of increasing proton number”, not “atomic mass”.

In the Periodic Table of Elements chapter this is the framework that every later idea hangs on: groups, periods, the alkali metals, the halogens, the noble gases and the transition elements are all just descriptions of where an element sits in this single, ordered arrangement.