## Q. 7.PS.8

Valence Electrons

(a) Using the noble gas notation, write the electron configuration for bromine. Identify its core and valence electrons.
(b) Write the Lewis dot symbol for bromine

## Verified Solution

(a) $[Ar] 3d^{10}4s^24p^5$      Core electrons are $[Ar]3d^{10}$; valence electrons are $4s^24p^5$.

(b) $:\overset{..}{\underset{..}{Br}}⋅$

Strategy and Explanation Bromine is element 35, a Group 7A element. Its first 18 electrons are represented by [Ar], as shown in Table 7.6. These, along with the ten $3d$ electrons, make up the core electrons. Because it is in Group 7A, bromine has seven valence electrons, as given by $4s^24p^5$ and shown in the Lewis dot symbol.

 Table 7.6    Lewis Dot Symbols for Atoms 1A $ns^1$ 2A $ns^2$ 3A $ns^2np^1$ 4A $ns^2np^2$ 5A $ns^2np^3$ 6A $ns^2np^4$ 7A $ns^2np^5$ 8A $ns^2np^6$ Period 2 $Li⋅$ $⋅Be⋅$ $⋅\overset{.}{B}⋅$ $⋅\overset{.}{\underset{.}{C}}⋅$ $⋅\overset{..}{\underset{.}{N}}⋅$ $:\overset{..}{\underset{.}{O}}⋅$ $:\overset{..}{\underset{..}{F}}⋅$ $:\overset{..}{\underset{..}{Ne}}:$ Period 3 $Na⋅$ $⋅Mg⋅$ $⋅\overset{.}{Al}⋅$ $⋅\overset{.}{\underset{.}{Si}}⋅$ $⋅\overset{..}{\underset{.}{P}}⋅$ $:\overset{..}{\underset{.}{S}}⋅$ $:\overset{..}{\underset{..}{Cl}}⋅$ $:\overset{..}{\underset{..}{Ar}}:$