Valence Electrons

Learning Objectives

  • Define valence electron.
  • Be able to indicate valence electrons when given the electron configuration for an atom.

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 a specific element will take depends on where the electrons are in the atom and how many there are.

Chemical reaction of iodine
Electron Configurations of Second-Period Elements
Element Name Symbol Atomic Number Electron Configuration
Lithium Li 3 1s22s1
Beryllium Be 4 1s22s2
Boron B 5 1s22s22p1
Carbon C 6 1s22s22p2
Nitrogen N 7 1s22s22p3
Oxygen O 8 1s22s22p4
Fluorine F 9 1s22s22p5
Neon Ne 10 1s22s22p6

In the study of chemical reactivity, we will find that the electrons in the outermost principal energy level are very important and so they 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 listed above, the two electrons in the 1 s 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? You must recognize that the second principal energy level consists of both the 2 s and the 2 p 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 s2p6, has eight valence electrons.


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


Use the link below to answer questions about valence electrons:


  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?


  • inner-shell electrons: Those electrons that are not in the outer shell and are not involved in the reactivity of the element.
  • valence electrons: The electrons in the highest occupied principal energy level of an atom.

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