Figure 21 The diagram consists of three columns of squares. The first column is labelled 'Group 1' and coloured mauve. The second column, which is just to the right of the first, is labelled 'Group 2' and coloured green. The third column, which is much further to the right, is labelled 'Group 13' and coloured mauve.
The square at the top of the first column is labelled Li, and the square at the top of the second column is labelled Be. There is then a horizontal arrow labelled 'no intervening subshells' points to the right, and the square at the top of the third column is labelled B.
The next square in the first column is Na, and in the second column Mg. Once again a horizontal arrow labelled 'no intervening subshells' points to the right, and the square in the third column is labelled Al.
The next square in the first column is K, and in the second column Ca. A horizontal arrow labelled '3d subshells filled' points to the right, and the square in the third column is labelled Ga.
The next square in the first column is Rb, and in the second column Sr. A horizontal arrow labelled '4d subshells filled' points to the right, and the square in the third column is labelled In.
The next square in the first column is Cs, and in the second column Ba. A horizontal arrow labelled '4f and 5d subshells filled' points to the right, and the square in the third column is labelled Tl.