The Periodic Table · Grade 9

Why does the periodic table have exactly the rows it has?

Because electrons fill energy shells in a fixed order and each shell holds a maximum. The first holds two electrons and the second and third hold eight each. The moment a shell is full the next electron starts a new shell — and in the table, a new row begins. The rows are not a decision made by whoever drew it.

Learning objectives

A fixed filling order

Electrons are not scattered around the nucleus as they please. They fill energy shells from the inside outwards, and each shell is filled before the next one begins.

For the first twenty elements the capacities are simple: two in the first shell, eight in the second, eight in the third. Sodium, with eleven electrons, fills 2 then 8 then 1 — and that single outer electron is all that matters about its chemistry.

The outer-shell electrons are called valence electrons. They are the only ones that meet other atoms, which is why they decide nearly all of an element's chemical behaviour.

The period and the group

The period number — the row — equals the number of occupied shells in the atom. Sodium has three occupied shells and sits in period 3. This is not a convention; it is a count.

The group number tells you how many valence electrons there are. In the older eight-column numbering of the main groups, group 1 means one valence electron and group 7 means seven. In the modern eighteen-column numbering those are group 1 and group 17.

Both schemes describe the same table, and it is worth knowing each: different textbooks use different ones, and the question underneath is identical.

Why the noble gases close every row

At the end of each row sits an element whose outer shell is full: helium with two, neon and argon with eight. It has no reason to give an electron away and no room to take one in.

An atom reacts in order to reach a full outer shell. Something already there does not need to react, which is why the noble gases barely react at all — and why they mark exactly where the row ends.

Worked examples

  1. Write the electron arrangement of oxygen (atomic number 8) and give its period and group.

    1. 8 electrons in total
    2. 2 in the first shell, 6 left for the second
    3. Two occupied shells, six valence electrons

    Answer: 2,6 — period 2, group 16 in the modern numbering and 6 in the older one

  2. An element has the arrangement 2,8,2. Where is it in the table?

    1. Three occupied shells means period 3
    2. Two valence electrons means group 2
    3. 12 electrons in total, so atomic number 12

    Answer: Period 3, group 2. This is magnesium

  3. Why does argon almost never react?

    1. Argon has 18 electrons: 2,8,8
    2. Its outer shell is full
    3. It has nothing to give and no room to take

    Answer: Because it is already in the state other elements react in order to reach

Common mistakes

Filling an outer shell before the inner one is full
The order is fixed from the inside outwards. An arrangement like 2,7,2 does not exist, because the second shell holds eight and still had room.
Counting every electron as a valence electron
Only the outer shell counts. Sodium has eleven electrons and one valence electron, and that is the difference between sodium and a completely different element.
Mixing up the period number and the group number
The period is the number of shells and the group is the number of valence electrons. Row and column answer two different questions.

What to remember

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