Trends in atomic size (radius)

• Definition of atomic radius:

Covalent radius:

The covalent bond length in H2 molecule is the distance between the two nuclei.

Assuming that the two hydrogen atoms are two balls in contact with each other, the radius of each ball would be then half the bond length.

Bond Length BL= rH+rHSimilarly for other elements, Cl2, Br2….

BL can be measured experimentally: BL → covalent atomic radiusII/

Factors affecting the atomic size:• n of the outermost electrons

• Attraction force exerted by nucleus on outermost electrons

Effective nuclear charge Zeff : charge of nucleus felt by outermost

electrons.

Electron doesn’t feel the total charge of nucleus because it is screened by inner shells electrons.

Thumb rule: Zeff= Z - no. of core electrons

3+

n=12e-

n=21e-

Valence electrons: electrons in outermost shell (last n)Core electrons: the remaining electrons.

In Li : Zeff=3-2=+1

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Zeff constant

n ↑

r ↑

n constant

Zeff ↑r ↓

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Ionic radius

Li : 1s2 2s1 Li+ : 1s2

Highest energy electronremoved

3+2e-

1e-

F : 1s2 2s2 2p5 F- : 1s2 2s2 2p69+

2e-7e-

Electron added in next vacant orbital

e-

Increased repulsion between electronsElectrons more apart from each otherIncreased volume

Metals tend to lose electrons forming cations.

Non-metals tend to gain electrons forming anions.

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Cations are smaller than corresponding atoms.

Anions are larger than corresponding atoms.II/

Due to increased attractionLarger nuclear charge

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Transition metal cations

• s-electrons removed before the d-electrons!!!!!!

Sc3+ :

Ti2+ :

V3+ :

Cr3+ :

Mn2+ :

Fe3+ :

Co3+ :

Ni2+ :

Cu+ :

Zn2+ :

Find the electron configuration for:Cu2+, Ti4+, Fe2+, V2+, Co2+, Cr2+, Mn3+.

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