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.

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.