Electronic Configuration of the First 20 Elements

1Why do sodium and potassium behave alike?

Drop a tiny piece of lithium, sodium or potassium into water (your teacher will do this behind a safety screen) and each one does the same thing: it floats, fizzes as hydrogen gas escapes, and leaves an alkaline solution. Potassium is the most violent; its hydrogen catches fire with a lilac flame. Put neon or argon in contact with water and nothing happens at all.

Lithium, sodium and potassium have 3, 11 and 19 electrons, very different numbers. So the total number of electrons cannot explain why they behave alike. The answer lies in how the electrons are arranged.

2Electrons live in shells

The electrons of an atom are arranged in shells (also called energy levels) at increasing distances from the nucleus. Electrons in the first shell, closest to the nucleus, have the lowest energy and are held most tightly. Electrons fill the shells from the inside outwards:

• the first shell holds at most 2 electrons;
• the second shell holds at most 8 electrons;
• for the first 20 elements, the third shell takes 8 electrons, and then the fourth shell begins (potassium and calcium).

The largest number of electrons a shell can ever hold is $2n^{2}2n^2$, where $nn$ is the shell number: $2\times 1^2=22\; \backslash times\; 1^2\; =\; 2$, $2\times 2^2=82\; \backslash times\; 2^2\; =\; 8$, $2\times 3^2=182\; \backslash times\; 3^2\; =\; 18$. The third shell can therefore hold up to 18, but it stops at 8 until the fourth shell has started; you will see why in S4.

Figure 3.1: A Bohr diagram of a sodium atom. The configuration is written from the inside out: 2,8,1.

Figure 3.2: Bohr diagrams of elements 1 to 10. The atomic number is in brackets; the configuration is under each name.

Figure 3.3: Bohr diagrams of elements 11 to 20. Each new shell starts only when the one inside it has taken its 2 or 8 electrons.

Table 3.1: Electronic configurations of the first 20 elements