valence electron
nounA valence electron is an electron on the outside of an atom that can join with other atoms.
Sodium gives away its one valence electron easily, which is why it reacts so strongly with water.
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OTHER MEANINGS
2. An electron in the highest occupied energy level of an atom that determines its chemical properties.
The number of valence electrons in an element's atoms explains its position in a group of the periodic table.
3. An electron that takes part in forming chemical bonds, either by being shared or transferred.
Carbon has four valence electrons, so it can form four bonds at once.
4. The outermost electron of an atom, which is loosely held and can move to other atoms.
Metals conduct electricity because their valence electrons are free to move through the material.