Building atoms, one electron at a time

The idea

The distribution of electrons among an atom's shells is its electronic configuration; it is built up by adding one electron to the appropriate shell each time the atomic number increases by 1.

From one atom to the next

Go from one element to the next: the atomic number goes up by 1, so the atom gets one more electron.

Sodium has 11 electrons. How are they arranged in the K, L and M shells?

• 2, 8, 1
• 11, 0, 0
• 2, 2, 7

Build the first eighteen atoms

Change the atomic number Z. Each step adds one electron to the right shell.

Electronic configuration

How the electrons of an atom are shared among its shells is called its electronic configuration. Each time the atomic number goes up by 1, one electron is added to the right shell.

The distribution of electrons among the shells of an atom is its electronic configuration. It is built up by adding one electron to the appropriate shell each time the atomic number increases by 1.

Notes

Electronic configuration is the distribution of electrons among the shells, built up by adding one electron to the right shell for each increase of 1 in the atomic number.

Check yourself

What is the electronic configuration of an atom?

Oxygen has 8 electrons. What is its electronic configuration (K, L)?

Magnesium has 12 electrons. How many are in its M shell?

Answer: 2

The configuration of magnesium is 2, 8, 2, so the M shell has 2 electrons.

Chlorine has 17 electrons. How many are in its M shell?

Answer: 7

The configuration of chlorine is 2, 8, 7, so the M shell has 7 electrons.

• The number of protons in its nucleus. That is the atomic number.
• The distribution of its electrons among the shells — correct. Yes!
• The total number of protons and neutrons. That is the mass number.
• 6, 2. The K shell can hold at most 2 electrons.
• 4, 4. The K shell is filled first, with 2 electrons, so it cannot have 4.
• 2, 6 — correct. Yes!