A chemical reaction involves either electron removal, electron addition, or electron sharing. The path that a specific element will take in a reaction depends on where the electrons are in the atom and how many there are.
In the study of chemical reactivity, electrons in the outermost principal energy level are very important and so are given a special name. Valence electrons are the electrons in the highest occupied principal energy level of an atom. In the second period elements, the two electrons in the \(1s\) sublevel are called inner-shell electrons and are not involved directly in the element's reactivity, or in the formation of compounds.
Valence electrons for representative elements
Lithium has a single electron in the second principal energy level, and so we say that lithium has one valence electron.
Li: 1s22s1 (the electron in the 2s energy level is the valence electron)
Beryllium has two valence electrons:
Be: 1s2 2s2 (the two electrons in the 2s energy level are the valence electrons)
How many valence electrons does boron have? Recognize that the second principal energy level consists of both the \(2s\) and the \(2p\) sublevels, and so the answer is three.
B: 1s2 2s2 2p1 (there are three electrons on the highest occupied energy level n=2)
In fact, the number of valence electrons goes up by one for each step across a period, until the last element is reached. Neon, with its configuration ending in \(2s^2 2p^6\), has eight valence electrons.
Valence electrons for transition elements.
Transition elements are a bit trickier. In this case, we also need to consider the electrons in the highest occupied energy level (n) plus the electrons in the (n-1) d orbital. For example, the electron configuration of iron is Fe is 1s2 2s2 2p6 3s2 3p6 4s2 3d6. The total number of valence electrons for iron is 8: 2 electrons in the highest occupied energy level (n=4) plus 6 electrons in the (n-1) d orbital, that is, 3d.
Using abbreviated electron configuration (or noble gas configuration) to identify valence electrons
The best approach to identify the number of valence electrons is to use the abbreviated notation for electron configuration. In this notation, the inner-shell electrons are abbreviated by indicating the noble gas that corresponds to that same number of electrons
For example, the abbreviated electron configuration for Lithium is [He] 2s1, so the number of valence electrons is 1 (2s1)
In the case of Boron, the abbreviated electron configuration is [He] 2s2 2p1, so the number of valence electrons is 1 (2s2 2p1)
In the case of Iron, the abbreviated electron configuration is [Ar] 4s2 3d6, so the number of valence electrons is 8 (4s2 3d6)
- Valence electrons are the outer-shell electrons of an atom.
- Valence electrons determine the reactivity of an atom.