Elements in the Periodic table are arranged in order of Atomic Number (or Proton Number) - the number of protons in the nucleus of the atom.

They are not always arranged by mass - though the lightest elements are at the top and the heaviest are at the bottom

But the table is also carefully arranged in columns which we call Groups

And also in rows that we call Periods

Elements in each group share similar chemical properties (react in similar ways).

Elements in the same Period can be very different to each other - the metals are on the left, the non-metals on the right

The Groups shown above are the only ones we need to know about for GCSE.

From left-to-right they are Group 1,2, Transition metals, Groups 3,4,5,6,7 and 0 - although Group 0 is sometimes known as Group 8, or just the Noble Gases.

Elements in Group 1 (Alkali metals) should all be similar to each other because they all have 1 electron in their outer shell. ₃Li (2,1), ₁₁Na (2,8,1), ₁₉K (2,8,8,1) etc

In this case Group 1 metals all react vigorously with water. We wouldn't usually think of Hydrogen being at the top of Group 1 as it is shown here, because Hydrogen is nothing like other Group 1 elements but it does have 1 electron in its outer shell)

Elements in Group 7 (Halogens) are colourful, toxic and react vigorously with Alkali metals. All Halogens have 7 electrons in their outer shell eg ₉F (2,7), ₁₇Cl (2,8,7)

The horizontal periods are shown in this Periodic Table.

Notice that Periods 6 and 7 include the two rows usually shown at the bottom.

Elements in the same period have the same number of shells but that is their only similarity.

There are only two elements in Period 1 because the first shell is full when it contains two electrons.

The next two periods contain eight elements because shells 2 and 3 can take eight electrons.

Elements in the same period are not like each other at all. Each Period (except Period 1) starts with Metals on the left and ends with Non-metals on the right, with Semi-metals in between.