Electronic configuration

1. The electrons in an atom occupy electron shells around the nucleus, and each shell can hold only a limited number of electrons.
2. Electrons fill the shell closest to the nucleus first, and a new shell only starts once the shell inside it is full.
3. For the elements with proton numbers 1 to 20, the shells fill in a fixed pattern.
   1. The first shell holds a maximum of 2 electrons.
   2. The second shell holds a maximum of 8 electrons.
   3. The third shell holds 8 electrons before any electrons go into the fourth shell.
4. The electronic configuration is written as the number of electrons in each shell, starting with the shell nearest the nucleus and separating the numbers with commas.
   1. Aluminium, with 13 electrons, has the configuration $2,8,3$.
5. In a neutral atom the number of electrons equals the proton number, so the numbers in the configuration must add up to the proton number.
6. Potassium, with 19 electrons, is $2,8,8,1$ and calcium, with 20 electrons, is $2,8,8,2$, because the third shell takes 8 electrons before the fourth shell begins.
7. In a shell diagram, the nucleus is drawn at the centre, each shell is drawn as a circle around it, and the electrons are drawn as dots or crosses on the circles.

Electronic configuration of ions

1. An ion forms when an atom loses or gains electrons, while the protons and neutrons in its nucleus stay exactly the same.
2. A positive ion has fewer electrons than protons, so for each positive charge you subtract one electron from the proton number.
3. A negative ion has more electrons than protons, so for each negative charge you add one electron to the proton number.
4. Once you know how many electrons the ion has, place them into shells in the same order as for an atom.
5. The ions of the first 20 elements usually have the same configuration as a noble gas, with a full outer shell.
   1. The sodium ion, ${Na}^{+}$, has the configuration $2,8$, which is the same as a neon atom.
   2. The chloride ion, ${Cl}^{-}$, has the configuration $2,8,8$, which is the same as an argon atom.

Groups and outer-shell electrons

1. The Periodic Table arranges the elements in order of proton number, in vertical columns called groups and horizontal rows called periods.
2. The outer shell is the occupied shell furthest from the nucleus, and the electrons in it are called outer-shell electrons.
3. For Groups I to VII, the number of outer-shell electrons in an atom is equal to its group number.
   1. Lithium, $2,1$, and sodium, $2,8,1$, each have one outer-shell electron, so both are in Group I.
   2. Nitrogen, $2,5$, has five outer-shell electrons, so it is in Group V.
   3. Chlorine, $2,8,7$, has seven outer-shell electrons, so it is in Group VII.
4. The noble gases in Group VIII all have a full outer shell.
   1. Helium, $2$, has a full first shell even though it has only two electrons.
   2. Neon, $2,8$, and argon, $2,8,8$, each have eight electrons in a full outer shell.
5. Elements in the same group have the same number of outer-shell electrons, which is why the Periodic Table places them in the same column.

Periods and occupied shells

1. A shell is occupied if it contains at least one electron.
2. The period number of an element is equal to the number of occupied shells in its atom.
   1. Lithium, $2,1$, has two occupied shells, so it is in Period 2.
   2. Sodium, $2,8,1$, has three occupied shells, so it is in Period 3.
   3. Potassium, $2,8,8,1$, has four occupied shells, so it is in Period 4.
3. Moving one place to the right along a period adds one electron to the outer shell, while moving down one place in a group adds one more occupied shell.
4. The electronic configuration therefore tells you exactly where an element sits in the Periodic Table.