The Periodic TableEdexcel GCSE Chemistry: Revision notes

Dmitri Mendeleev arranged the elements known at the time into a table using the properties of the elements and their compounds, rather than strictly by atomic mass alone. He left gaps for elements not yet discovered and used his table to predict the existence and properties of these missing elements — predictions that were later confirmed, giving strong support to his arrangement.

Mendeleev generally ordered elements by increasing relative atomic mass, but this was not always followed strictly — he sometimes swapped the order of two elements to keep them in the correct group, because of the effect of relative isotope abundance on some elements' atomic masses.

The atomic number of an element is the number of protons in its nucleus, and also determines its position in the periodic table.

In the modern periodic table:

• Elements are arranged in order of increasing atomic number
• Horizontal rows are called periods
• Vertical columns are called groups, and elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell

Elements can be identified as metals or non-metals by their position in the periodic table: metals are found to the left and towards the bottom, non-metals to the right and top (roughly a diagonal 'staircase' divides them).

This division relates to atomic structure: metal atoms tend to have few electrons in their outer shell and readily lose them to form positive ions; non-metal atoms tend to have more electrons in their outer shell and tend to gain electrons to form negative ions, or share electrons covalently.

The electronic configuration of the first 20 elements can be predicted and written as a series of numbers, e.g. sodium is 2.8.1 (2 electrons in the first shell, 8 in the second, 1 in the third/outer shell).

The electronic configuration links directly to the periodic table:

• The period number equals the number of occupied electron shells
• The group number equals the number of electrons in the outer shell (for main groups)

This is why elements in the same group react in similar ways — they have the same number of outer-shell electrons.