Chemistry / Unit 3: Electrons and the Periodic Table
3.4 Valence Electrons and Dot Diagrams
Notes
Valence electrons are the electrons in the outermost energy level. They take part in bonding.
- For main-group elements, the group number tells the valence count: group 1 has 1, group 2 has 2, groups 13 to 18 have 3 to 8.
- Example: phosphorus, $[\text{Ne}]3s^2 3p^3$, has 5 valence electrons.
- An electron dot (Lewis) diagram shows valence electrons as dots around the symbol.
- Octet rule: atoms tend to gain, lose or share electrons to get 8 valence electrons, like a noble gas.
- Metals lose electrons to form cations ($\text{Na}^+$, $\text{Mg}^{2+}$); nonmetals gain them to form anions ($\text{Cl}^-$, $\text{O}^{2-}$).
Practice
How many valence electrons does bromine have?
Bromine is in group 17 ($4s^2 4p^5$), so it has 7 valence electrons.
Which ion does aluminum usually form?
Aluminum has 3 valence electrons and loses them to reach the electron arrangement of neon.
An element's configuration ends in $3s^2 3p^4$. How many dots appear in its dot diagram?
Valence electrons are in level 3: $2 + 4 = 6$.
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