Bohr Diagrams: Electrons in Shells and Valence Electrons

0:00How are an atom's electrons arranged, and why does it matter?

0:05Take a sodium atom. Its nucleus, in the centre, holds 11 protons.

0:10An atom has as many electrons as protons, so sodium has 11 electrons.

0:17A Bohr diagram draws them in shells, rings around the nucleus that fill from the inside out.

0:25The first shell holds up to 2 electrons, and the second holds up to 8.

0:32The third shell holds up to 8 for the first 20 elements. Sodium's last electron goes there.

0:39So sodium's electron arrangement is 2, 8, 1.

0:45Electrons in the outer shell are called valence electrons. Sodium has 1.

0:52Valence electrons are the ones used in bonding, so they decide which compounds an atom forms.

0:59Chlorine has 17 electrons, arranged 2, 8, 7, so it has 7 valence electrons.

1:07So sodium tends to lose its 1 valence electron, chlorine tends to gain 1, and they form sodium chloride.

1:15Argon has 18 electrons, arranged 2, 8, 8, so its outer shell is full.

1:23With a full outer shell, argon hardly ever reacts.

1:28The number of shells gives the period, the row of the periodic table: argon has 3 shells, so it is in period 3.

1:37This is only a model: electrons do not really travel in fixed circles. Each shell stands for an energy level.

1:45In short, the valence electrons in the outer shell decide how an atom bonds.