Valence electrons

The electrons in any atom can be divided into two types. These are:

core electrons in filled shells. These electrons are not involved in bonding
valence (outer) electrons in unfilled shells. These are the bonding electrons.

1	2
							1H										2He
3Li	4Be											5B	6C	7N	8O	9F	10Ne
11Na	12Mg											13Al	14Si	15P	16S	17Cl	18Ar
19K	20Ca	21Sc	22Ti	23V	24Cr	25Mn	26Fe	27Co	28Ni	29Cu	30Zn	31Ga	32Ge	33As	34Se	35Br	36Kr

The first four periods (rows) of the periodic table are shown.
These contain many important elements.

Mg (12 electrons) has
2 electrons in shell 1 (a filled shell)
8 electrons in shell 2 (a filled shell)
2 electrons in shell 3 (incomplete).

Hence Mg has 10 core electrons and 2 valence electrons.

The number of core electrons depends on the period (row) in which the element is found.

Elements in period 2 have 2 core electrons
(equal to the number of elements in period 1).
Elements in period 3 have 10 core electrons (equal to the number of elements in periods 1 and 2).
Elements in period 4 have 18 core electrons
(equal to the number of elements in periods 1, 2 and 3).

The Groups in the periodic table are the columns with numbers at the top.
The number of valence (outer) electrons on an atom can be deduced from its Group number.

For main group* elements the number of valence electrons equals the last digit of the Group Number.
*Main group elements are the elements in Groups 1 and 2 as well as the elements in Groups