Describe how the Periodic Table is arranged into groups and periods and relate an element's position to its electron arrangement

What this dot point is asking

The syllabus wants you to describe how the Periodic Table is arranged, in order of increasing proton number, into vertical groups and horizontal periods, and to link an element's position to its electron arrangement. The big idea is that the table is not a random list: an element's group tells you its number of outer electrons, and its period tells you how many shells it has. This makes the table a map of chemical behaviour.

The answer

Order of the table

The elements are arranged in order of increasing proton number, starting at hydrogen. Each element has one more proton, and one more electron, than the one before. This steady increase creates a repeating pattern of properties, which is why the table is laid out in rows and columns.

Groups

A group is a vertical column in the table, numbered I to VIII (or 0). Elements in the same group have the same number of electrons in their outer shell, which is why they have similar chemical properties:

• the group number equals the number of outer-shell electrons (for the main groups).

So all Group I elements have one outer electron, and all Group VII elements have seven.

Periods

A period is a horizontal row in the table. Going across a period, each element gains one more electron in the same outer shell. The period number equals the number of occupied shells. So an element in Period 3 has three shells holding electrons.

Reading position from electron arrangement

You can find an element's place from its electron arrangement:

• count the outer electrons to get the group, and
• count the number of shells to get the period.

For example, the arrangement $2, 8, 1$ has one outer electron and three shells, so the element is in Group I, Period 3, which is sodium.

Examples in context

Example 1. Predicting an unknown element's reactions. If you are told an element sits in Group I, you immediately know it has one outer electron, is a reactive metal, and forms a $1+$ ion, without needing any other information. This predictive power is the whole point of arranging elements by their electron patterns.

Example 2. Why the table has gaps and blocks. The transition metals form a wide block in the middle because their electron shells fill in a more complicated way. Knowing that position reflects electron arrangement explains why the table is shaped the way it is rather than being a simple grid.

Try this

Q1. State the group and period of an element with the electron arrangement $2, 8, 8, 1$. [2 marks]

• Cue. One outer electron means Group I; four occupied shells mean Period 4 (this is potassium).

Q2. Explain why all the elements in Group VII have similar chemical properties. [2 marks]

• Cue. They all have seven electrons in their outer shell, and chemical properties depend on the outer electrons, so they react in similar ways.

Q3. State the principle on which the order of the Periodic Table is based. [1 mark]

• Cue. The elements are arranged in order of increasing proton number (atomic number).