Question

Add charges, single electron dots, and/or pairs of dots as appropriate to show the Lewis symbols for the most stable ion of each element. Treat n- 4 as the valence shell in all cases. 

Answer 1

Stable here indicates attaining the nearest Noble gas configuration. For that molecules have to either gain or lose electron. When electrons are gained molecule gets a negative charge because number of electrons is increased. When electrons are lost molecule gets a positive charge because number of electrons is decreased.

1. As

Atomic number = 33

Atomic number of nearest Noble gas (krypton) = 36

Hence As will have to gain 3 electrons to become stable

As^-3

2. Se

Atomic number = 34

Atomic number of nearest Noble gas (krypton) = 36

Hence Se will have to gain 2 electrons to become stable

As^-2

3. Br

Atomic number = 35

Atomic number of nearest Noble gas (krypton) = 36

Hence Br will have to gain 1 electron to become stable

Br^-1

4. K

Atomic number = 19

Atomic number of nearest Noble gas (argon) = 18

Hence k will have to lose one electron to become stable

K⁺¹

5. Ca

Atomic number = 20

Atomic number of nearest Noble gas (Argon) = 18

Hence Ca will have to lose 2 electrons to become stable

Ca⁺²

6. Ga

Atomic number = 31

Atomic number of nearest Noble gas (krypton) = 36

Hence Ga will have to gain 5 electrons to become stable

Ga^-5