Describe the arrangement of electrons into shells for the first twenty elements and relate electronic structure to group and period in the Periodic Table.

What this topic is asking

WJEC topic 1.2 wants you to know how electrons are arranged in shells (energy levels) for the first twenty elements, to write their electronic structures, and to link the structure to the element's group (number of outer electrons) and period (number of shells) in the Periodic Table.

How electrons fill shells

Electrons are arranged in shells (also called energy levels) around the nucleus. They fill from the inside out, lowest energy first:

The electronic structure is written as the number of electrons in each shell, separated by commas. For example, oxygen has 8 electrons: 2 in the first shell and 6 in the second, written $2,6$.

The first twenty elements

Filling the shells in order gives these structures:

• Hydrogen (1): $1$
• Helium (2): $2$
• Lithium (3): $2,1$
• Carbon (6): $2,4$
• Oxygen (8): $2,6$
• Neon (10): $2,8$
• Sodium (11): $2,8,1$
• Magnesium (12): $2,8,2$
• Chlorine (17): $2,8,7$
• Argon (18): $2,8,8$
• Potassium (19): $2,8,8,1$
• Calcium (20): $2,8,8,2$

For potassium and calcium, the third shell stays at 8 and the next electrons go into the fourth shell.

You can also draw the structure as a shell diagram: a central dot or symbol for the nucleus, then circles for each shell with the right number of crosses (electrons) on each. Electrons are usually drawn in pairs spaced around each shell. For example, a magnesium atom is drawn with 2 electrons on the inner circle, 8 on the middle circle and 2 on the outer circle, matching the structure $2,8,2$.

Linking structure to the Periodic Table

The electronic structure tells you exactly where an element sits:

This is why elements in the same group have similar chemistry: they have the same number of outer electrons, and it is the outer electrons that take part in bonding and reactions. For example, lithium $2,1$, sodium $2,8,1$ and potassium $2,8,8,1$ all have one outer electron, so they are all in Group 1 and all react in similar ways (each loses that one outer electron when it reacts).

Electronic structure and ions

When an atom reacts, it loses, gains or shares its outer electrons to reach a full, stable outer shell like the nearest noble gas. The number of outer electrons therefore predicts how an atom will bond:

• Metals in Groups 1, 2 and 3 have few outer electrons (1, 2 or 3) and tend to lose them to form positive ions. Sodium $2,8,1$ loses one electron to become $\text{Na}^+$ with the stable structure $2,8$.
• Non-metals in Groups 6 and 7 have nearly full outer shells and tend to gain electrons to form negative ions. Chlorine $2,8,7$ gains one electron to become $\text{Cl}^-$ with the stable structure $2,8,8$.

In both cases the atom ends up with a full outer shell, which explains why these reactions happen so readily.

Why a full outer shell is special

Atoms react to gain a full outer shell, which is a very stable arrangement (the same as a noble gas). Atoms with nearly full or nearly empty outer shells are reactive because they only need to gain or lose a few electrons. The noble gases (Group 0) already have full outer shells, so they are very unreactive.