Explain your answer.

7. which atom or ion is given with an excited outer electron configuration?

(a) na: 3s^1 (b) b: 2p^3 (c) he: 1s^2 (d) h^-1: 1s^2

9. how many unpaired electrons are there in the ti^3+ ion?

(a) 0 (b) 2 (c) 1

13. which set contains three isoelectronic species?

(a) zn, cd, hg (b) br^+, kr, rb^- (c) p^3-, se^2-, i^- (d) f, na^+, mg2^+

7. (B) B; 9. (C) 1; 13. None of the above

Explanation:

7. Excited outer electron configuration

(A) Na: 3s. No.

(B) B: 2p³. Yes.

The ground state electron configuration is 2s²2p, so the two 2s electrons have been excited to the 2p level.

(C) He: 1s². No.

(D) H⁻: 1s². No.

9. Unpaired electrons

Tl: [Ar]4s²3d²

Ti³⁺[Ar]3d

There is one unpaired electron.

13. Isoelectronic species

(A) Zn, Cd, Hg

Zn: [Ar]4s²3d¹⁰

Cd: [Kr]5s²4d¹⁰

Hg: [Xe]6s²4f¹⁴5d¹⁰

Not isoelectronic.

(B) Br⁺, Kr, Rb⁻

Br⁺: [Ar]4s²3d¹⁰4p⁴

 Kr: [Kr]

Rb⁻: [Kr]5s²

Not isoelectronic.

(C) P³⁻, Se²⁻, I⁻

 P³⁻: [Ar]

Se²⁻: [Kr]

    I⁻: [Xe]

Not isoelectronic

(D) F, Na⁺, Mg²⁺

     F: [He]2s²2p⁵

 Na⁺: [Ne]

Mg²⁺: [Ne]

Two isoelectronic species,

There  would be three isoelectronic species if you had F⁻ instead of F.