Valence electrons: the outer electrons that bond

0:00When atoms bond, which of their electrons take part?

0:04A Bohr diagram shows electrons in rings called shells, around the nucleus.

0:11Sodium has 11 electrons. They fill the shells 2, then 8, then 1.

0:17Electrons in the outer shell are called valence electrons: the ones lost, gained or shared in bonding.

0:25A column of the periodic table is a group, and a row is a period. Sodium is in group 1, with 1 valence electron.

0:34Magnesium has 2, 8, 2: 2 valence electrons, in group 2.

0:41Carbon has 2, 4: 4 valence electrons, in group 14.

0:47Nitrogen has 2, 5: 5 valence electrons, in group 15.

0:53Oxygen has 2, 6: 6 valence electrons, in group 16.

0:59Chlorine has 2, 8, 7: 7 valence electrons, in group 17.

1:06Groups 1 and 2 have 1 and 2 valence electrons, and groups 13 to 18 have 3 to 8.

1:14Helium is the exception: in group 18, but with only 2 electrons in its one shell.

1:21A Lewis dot symbol shows just the valence electrons, as dots around the symbol.

1:27From lithium to neon, the dots go 1, 2, 3, 4, 5, 6, 7, 8. We place one on each side before any pair up.

1:39Apart from helium, elements in a group have the same number of valence electrons: sodium has 1, like lithium, and chlorine 7, like fluorine.

1:50So they bond in similar ways: lithium and sodium tend to lose 1 electron, fluorine and chlorine to gain or share 1.

1:59Valence electrons are the outer electrons that bond, and the same number means similar bonding.