Electron arrangement in shells for the first 20 elements, the link between outer electrons, group number and reactivity, and the modern organisation of the Periodic Table by atomic number into periods and groups with metals and non-metals.

What this dot point is asking

CCEA Double Award wants electron arrangements for the first 20 elements, the link between outer-shell electrons, group number and reactivity, and how the Periodic Table is organised by atomic number into periods and groups, with metals and non-metals. Electron arrangement is the key skill, because it explains the patterns in the table.

Electron arrangement

To write an electron arrangement, fill the shells in order until you have placed all the electrons. For example, sodium has 11 electrons: 2 in the first shell, 8 in the second, 1 in the third, written 2, 8, 1. Calcium has 20 electrons: 2, 8, 8, 2.

Outer electrons, group and reactivity

The electrons in the outer shell control how an atom reacts. The number of outer electrons equals the group number (for the main groups). For example, sodium has 1 outer electron and is in Group 1; oxygen has 6 outer electrons and is in Group 6.

Atoms react to gain a full outer shell. This is why reactivity follows patterns down and across the table, and why elements in the same group react in similar ways.

Organisation of the Periodic Table

Metals are on the left and centre of the table; non-metals are on the right. The position of an element predicts its properties: metals tend to lose electrons and form positive ions, while non-metals tend to gain electrons and form negative ions. Going down a group, atoms gain shells; going across a period, the outer shell fills up.

The modern table is the work of Mendeleev, who arranged the known elements by their properties and even left gaps for elements not yet discovered, correctly predicting their properties. Today the elements are ordered by atomic number rather than mass, which fixed the few places where ordering by mass put elements in the wrong group. This shows why atomic number, not mass, is the true basis of the table.

Examples in context

Example 1. Why Group 1 metals all react similarly. Lithium, sodium and potassium each have 1 outer electron, so they all react by losing that electron to form a +1 ion. This shared outer-electron count is why they behave so alike, just with different reactivity down the group.

Example 2. Why noble gases are unreactive. Group 0 elements have full outer shells (helium 2, neon 2, 8). With no need to gain or lose electrons, they are very unreactive. Electron arrangement explains their lack of reactivity directly.

Try this

Q1. Write the electron arrangement of sodium (atomic number 11). [1 mark]

• Cue. 2, 8, 1.

Q2. What does the group number tell you about an atom? [1 mark]

• Cue. The number of electrons in its outer shell.