Electron shells fill from the inside out

1. The electrons in an atom occupy shells at fixed distances from the nucleus.
2. The first shell holds at most $2$ electrons.
3. The second and third shells each hold at most $8$ electrons across the first $20$ elements.
4. The fourth shell begins to fill once the third holds $8$.
5. A neutral atom has as many electrons as its atomic number.
6. An electronic configuration lists those electrons shell by shell, separated by full stops, starting with the shell nearest the nucleus.
7. Sodium's configuration $\mathrm{2.8.1}$ means $2$ in the first shell, $8$ in the second and $1$ in the third.

The first twenty configurations, one electron at a time

1. Period 1: hydrogen $1$, helium $2$.
2. Period 2: lithium $2.1$, beryllium $2.2$, boron $2.3$, carbon $2.4$, nitrogen $2.5$, oxygen $2.6$, fluorine $2.7$, neon $2.8$.
3. Period 3: sodium $\mathrm{2.8.1}$, magnesium $\mathrm{2.8.2}$, aluminium $\mathrm{2.8.3}$, silicon $\mathrm{2.8.4}$, phosphorus $\mathrm{2.8.5}$, sulfur $\mathrm{2.8.6}$, chlorine $\mathrm{2.8.7}$, argon $\mathrm{2.8.8}$.
4. Period 4: potassium $\mathrm{2.8.8.1}$, calcium $\mathrm{2.8.8.2}$.
5. Each element adds one electron to the one before it, so any configuration can be built from its neighbour.
6. A shell diagram carries the same information: the nucleus at the centre, concentric circles around it, and the right number of electrons marked on each circle.

The period number counts the occupied shells

1. Count how many numbers appear in the configuration, because each one stands for an occupied shell.
2. Hydrogen and helium have one number each, so they lie in period $1$.
3. Lithium to neon have two numbers, so they lie in period $2$.
4. Sodium to argon have three numbers, so they lie in period $3$.
5. Potassium and calcium have four numbers, so they lie in period $4$.
6. A number written as $1$ still counts, because that shell does hold an electron.
7. The period changes at the point where a new shell starts, such as between argon $\mathrm{2.8.8}$ and potassium $\mathrm{2.8.8.1}$.

The group number comes from the outer-shell electrons

1. Read the last number in the configuration to find the outer-shell electron count.
2. One outer-shell electron places an element in group $1$, and two place it in group $2$.
3. Counts of $3$ to $7$ place an element in groups $3$ to $7$ respectively.
4. A full outer shell places an element in group $0$, which holds the noble gases.
5. Helium belongs to group $0$ because its single shell is complete at $2$ electrons.
6. Chlorine $\mathrm{2.8.7}$ therefore lies in group $7$, and magnesium $\mathrm{2.8.2}$ in group $2$.
7. Elements in one group behave similarly because the same outer-shell count takes part in their reactions.

Going both ways between configuration and position

1. Write the configuration from the atomic number, filling each shell before starting the next.
2. Count the numbers for the period and read the last number for the group.
3. Calcium $\mathrm{2.8.8.2}$ sits in period $4$, group $2$.
4. Argon $\mathrm{2.8.8}$ sits in period $3$, group $0$.
5. The same two counts work from a shell diagram: occupied circles give the period, and electrons on the outer circle give the group.
6. Working backwards, an element in period $3$ and group $6$ has three occupied shells and six outer-shell electrons, giving $\mathrm{2.8.6}$.