Valence electrons are electrons in the outer shell of an atom. They are the electrons which form bonds with other atoms in chemical reactions.
Quick definition of valence electrons:
Valence electrons are electrons in the outer electron shell of an atom.
Explanation of electrons and reactivity
This answer seems quite lengthy but it is actually reasonably simple, with a lot of examples. Most of it is building up foundations to the final explanation, so read the end first and then read the rest of the answer if you want to know more about the concepts.
Note: This explanation will only go into the first 20 elements on the periodic table (up to Calcium) as the transition metals can be a bit tricky.
First, a little explanation of electron shells :
Electrons orbit the electron in "shells" (energy levels), and only a certain number of electrons can exist in each shell.
The first shell can hold 2 electrons, the second can hold 8, the third can also hold 8 and the fourth can hold 18 (There are more shells but they are not particularly relevant to the first 20 elements).
The first shell will always be filled before the second, which is always filled before the third, etc. The configuration of electrons around the atoms can be written as
e.g. Sodium (11 electrons) has an electron configuration of
Neon (10 electrons) has an electron configuration of
and Chlorine (17 electrons) has an electron configuration of
Now for the explanation of valence electrons
Valence electrons are electrons in the outer shell of the atom. For example Sodium has
The number of valence electrons an atom has determines its reactivity.
An atom "wants" to have a full valence shell and it will exchange electrons with other atoms to achieve this.
For example Chlorine has 7 valence electrons so it receives 1 in a reaction with another atom. It now has 1 more electron so it has 18 electrons and an electron configuration of
Sodium has 1 valence electron which it loses in a reaction giving it an electron configuration of
Neon has 8 valence electrons. It already has a full valence shell so it does not react easily and does not gain or lose any electrons.
How valence electrons influence reactivity
Atoms which are "closer" to a full valence shell will more readily react and gain or lose electrons.
For example Sodium (1 valence electron) is more reactive than Magnesium (2 valence electrons), and Chlorine (7 valence electrons; 1 electron short of full valence shell) is more reactive than Sulfur (6 valence electrons; 2 electrons short of full valence shell)
Neon, Argon, Helium and the other noble gases have a full valence shell so they do not react readily and can be considered inert.