Across a period

Take any period and read the ionization energies from left to right. Two things stand out at once.

The two ends of every period

The lowest first ionization energy in a period belongs to the element in group I.

The highest belongs to the element in group VIII.

Check it in period 3. Sodium in group I is 495; argon in group VIII is 1521 — the largest in that row. Period 2 does the same thing: lithium 519 at one end, neon 2080 at the other.

Between those ends the value varies in a regular pattern as you move from left to right.

Figure to be added

Figure 3.8: a graph of first ionization energy (kJ mol⁻¹) on the vertical axis against atomic number 1 to 20 on the horizontal axis, with each point labelled by element symbol — showing the sawtooth pattern, peaking at He, Ne and Ar and dropping to Li, Na and K.

Try this

Using graph paper, plot the first ionization energy values from the previous page against atomic number. Then describe, from your graph, how the first ionization energy varies from left to right along a period and from top to bottom down a group.