1.4 The Periodic TableEdexcel IGCSE Chemistry: Revision notes

Elements are arranged in order of increasing atomic number, in horizontal rows called periods and vertical columns called groups.

• Elements in the same period have the same number of electron shells.
• Elements in the same group have the same number of electrons in their outer shell, giving them similar chemical properties.

For the first 20 elements, electrons fill shells in order, with a maximum of 2 in the first shell, 8 in the second shell, and 8 in the third shell (up to element 20).

1. The group number (for main groups) equals the number of outer-shell electrons.
2. The period number equals the number of electron shells used.
3. Electronic configurations are written as numbers separated by commas, e.g. sodium (11 electrons): 2,8,1.

Element's position in the table can be deduced from its configuration, and vice versa: an atom with configuration 2,8,7 is in Group 7, Period 3 (chlorine).

Metals and non-metals can be distinguished by physical and chemical properties:

Metal oxides typically react with acids to form salts and water (basic), while non-metal oxides typically react with alkalis or dissolve in water to form acidic solutions.

An element's position in the Periodic Table (left/centre vs right, separated by a 'staircase' line) can be used to predict whether it is a metal or non-metal.

Elements in the same group have the same number of outer-shell electrons, which determines chemical behaviour — this is why they react in similar ways (e.g. all Group 1 metals react vigorously with water).

Group 0 (noble gases) have a full outer shell of electrons (2 for helium, 8 for the others). A full outer shell is a very stable arrangement, so noble gases have little tendency to lose, gain or share electrons — this is why they are almost completely unreactive.