Question

how many valence electrons are in an atom of gallium (ga), which has the atomic number 31?
1
3
14
31

Explanation:

Step1: Write electron - configuration

Gallium (Ga) with atomic number 31 has the electron - configuration $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^{10}4p^{1}$.

Step2: Identify valence electrons

Valence electrons are in the outermost shell. For Ga, the outermost shell is the 4th shell ($n = 4$). The electrons in the 4s and 4p sub - shells of the 4th shell are valence electrons. There are 2 electrons in 4s and 1 electron in 4p, so a total of $2 + 1=3$ valence electrons.

Answer:

B. 3