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UNIT 2. Atoms, Ions, Electronic Structure, and Periodic Properties of Atoms and Ions. Effective Nuclear Charge: Z eff. The effective nuclear charge Z eff experienced by the valence electrons is helpful in explaining the periodic trends of the properties of the elements.

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UNIT 2

Atoms, Ions,

Electronic Structure, and Periodic Properties of Atoms and Ions


Effective Nuclear Charge: Zeff

The effective nuclear charge Zeff experienced by the valence electrons is helpful in explaining the periodic trends of the properties of the elements.

For the valence electrons, Zeff is approximated by taking the difference between the charge on the nucleus and the charge of the core electrons.


Effective Nuclear Charge (Zeff)

For the valence electrons, Zeff is approximated by taking the difference between the charge on the nucleus and the charge of the core electrons.

Zeff (Mg) = 12 – 10 = +2

for the two 3s valence electrons.


Periodic Trends – Size of Atoms

e- configuration Zeff atomic radius

Z Element (core notation) =(Z-core) (bonding)

_____ _______ ________________ _______ _______________

1 H 1s1 1-0 = +1 0.37Å

3 Li [He]2s13-2 = +1 1.34Å

11 Na [Ne]3s1 11-10 = +1 1.54Å

19 K [Ar]4s1 19-18 = +1 1.96Å

37 Rb [Kr]5s1 37-36 = +1 2.11Å

1 Å = 1 angstrom = 0.1 nm

Atomic sizeincreasesdown a group.


Periodic Trends – Size of Atoms

e- configuration Zeff atomic radius

Z Element (core notation) =(Z-core) (bonding)

_____ _______ ________________ _______ _______________

11 Na [Ne]3s1 11-10 = +1 1.54Å

12 Mg [Ne]3s2 12-10 = +2 1.30Å

13 Al [Ne]3s23p1 13-10 = +3 1.18Å

14 Si [Ne]3s23p2 14-10 = +4 1.11Å

15 P [Ne]3s23p3 15-10 = +5 1.06Å

16 S [Ne]3s23p4 16-10 = +6 1.02Å

17 Cl [Ne]3s23p5 17-10 = +7 0.99Å

Atomic size generally decreases across a period due to the increasing Zeff experienced by the valence electrons (while n doesn’t change).

Atomic size generally increases right to leftin a period.



Periodic Trends – Size of Atoms

increasing size

increasing size


Periodic Trends – Size of Cations

atomic radius ionic radius

Element e- config (Å) ion e- config (Å)

________ ________ _______ ___ _________ ___________

Li [He]2s1 1.34 Li+ [He] 0.68

Na [Ne]3s11.54 Na+ [Ne] 0.97

K [Ar]4s1 1.96 K+ [Ar] 1.33

Rb [Kr]5s1 2.11 Rb+ [Kr] 1.47

1. The cation is ALWAYS SMALLER than its atom.

2. Otherwise, the trends in cation size are the same as atomic size.

Ionic size increases down a group.


Periodic Trends – Size of Anions

atomic radius ionic radius

Element e- config (Å) ion e- config (Å)

________ ________ _______ ___ _________ ___________

F [He]2s22p5 0.71 F- [Ne] 1.33

Cl [Ne]3s23p5 0.99 Cl- [Ar] 1.81

Br [Ar]3d104s24p5 1.14 Br- [Kr] 1.96

I [Kr]4d105s25p51.33 I- [Xe] 2.20

1. The anion is ALWAYS LARGER than its atom.

2. Otherwise, the trends in anion size are the same as atomic size.

Ionic sizeincreasesdown a group.



Periodic Trends – Size of Ions in an Isoelectronic Series

atomic radius ionic radius

ion e- config (Å) ion e- config (Å)

________ ________ _______ ___ _________ ___________

N3- 1s22s22p6 1.71 Na+ 1s22s22p6 0.97

O2- 1s22s22p6 1.40 Mg2+ 1s22s22p6 0.66

F- 1s22s22p6 1.33 Al3+ 1s22s22p6 0.51

Ions in anisoelectronic seriesall haveexactly the same electronic configuration.

Ionic size in an isoelectronic seriesdecreaseswith increasing nuclear charge (Z).


Periodic Trends – Ionization Energy

Ionization energy (I.E.) is the energy needed to remove an electron from an atom in its gaseous state.

Na (g)  Na+ (g) + e-

e- configuration first ionization

Z Element (core notation) energy (kJ/mol)

_____ _______ ________________ ___________________

1 H 1s1 1312

3 Li [He]2s1 520

11 Na [Ne]3s1 496

19 K [Ar]4s1 419

37 Rb [Kr]5s1 403

Ionization energy generally increases up a group.


Periodic Trends – Ionization Energy

Ionization energy (I.E.) is the energy needed to remove an electron from an atom in its gaseous state.

Na (g)  Na+ (g) + e-

e- configuration first ionization

Z Element (core notation) energy (kJ/mol)

_____ _______ ________________ _________________

11 Na [Ne]3s1 496

12 Mg [Ne]3s2 738

13 Al [Ne]3s23p1 578

14 Si [Ne]3s23p2 786

15 P [Ne]3s23p3 1012

16 S [Ne]3s23p4 1000

17 Cl [Ne]3s23p5 1251

18 Ar [Ar] 1521

Ionization energy generally increases across a period.



Periodic Trends – Ionization Energy

increasing I.E.

increasing I.E.


Periodic Trends – Electron Affinity

Electron affinity (E.A.) is the energy released when an electron is added to a gaseous element.

F (g) + e- F- (g)

e- configuration electron affinity

Z Element (core notation) (kJ/mol)

_____ _______ ________________ _______________

9 F [He]2s22p5 -328

17 Cl [Ne]3s23p5-349

35 Br [Ar]3d104s24p5 -325

53 I [Kr]4d105s25p5 -295

Negative energies denote large electron affinities.

Electron affinity generally increases up a group.


Periodic Trends – Electron Affinity

Electron affinity (E.A.) is the energy released when an electron is added to a gaseous element.

F (g) + e- F- (g)

e- configuration electron affinity

Z Element (core notation) energy (kJ/mol)

_____ _______ ________________ _______________

11 Na [Ne]3s1 -53

12 Mg [Ne]3s2 >0

13 Al [Ne]3s23p1-43

14 Si [Ne]3s23p2 -134

15 P [Ne]3s23p3-72

16 S [Ne]3s23p4 -200

17 Cl [Ne]3s23p5-349

18 Ar [Ar] >0

Negative energies denote large electron affinities.

Electron affinity generally increases across a period.


Periodic Trends – Electron Affinity

increasing E.A.

increasing E. A.


Periodic Trends – Metallic Character

  • Metallic character is

    • luster

    • malleability

    • ductility

    • good conductivity

    • the tendency to form ionic oxides that are basic.

  • Metallic character is closely related to how easily electrons can be removed from an element. The lower the I.E. of an element, the higher its metallic character.


Periodic Trends – Metallic Character

increasing metallic character

increasing metallic character


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