# 5.19: Valence Electrons


### What makes a particular element very reactive and another element non-reactive?

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.

## Valence Electrons

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. Lithium has a single electron in the second principal energy level, and so we say that lithium has one valence electron. Beryllium has two 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. 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.

## Summary

• Valence electrons are the outer-shell electrons of an atom.
• Valence electrons determine the reactivity of an atom.

## Review

1. Define valence electron.
2. Define inner shell electron.
3. How many valence electrons are there in fluorine?
4. What are the 2s electrons in nitrogen?
5. How many inner shell electrons are there in beryllium?

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