Radii of the Elements

Radii of the Elements• The atomic radius is defined as half the distance between

two adjacent atoms in a solid crystal. • In a period, the atomic radius of each successive atom is

usually smaller than its preceding neighbor.

• In a group, the atomic radius of each successive atom is larger than its preceding neighbor.

Li Be B C N O F Ne

Be Mg Ca Sr

• The atomic radii decrease across a period because each successive atom has one additional proton in the nucleus and one additional electron in the valence (or in the case of the transition elements, the next to last valence).

• The extra proton and electron increase the force of attraction between the nucleus and the valence shell pulling the valence electrons closer to the nucleus. Thus the radii become smaller !

Radii of the Elements

3p e-e-

e-

Li

4p e-e-

e-

e-

Be

5p e-e-

e-

e-

e-

B

6p e-e-

e- e-

e-e-

C

• The atomic radii increase down a group because each successive atom has one additional shell (PEL) around its the nucleus.

• The extra shell increases the distance between the valence electrons and the nucleus decreasing the force of attraction between the nucleus and the valence electrons. Thus the radii become larger !

Radii of the Elements

3p

e-

Li

11p

e-

Na

e-

19p

K

e-

19p

Rb

Ex(1) As the atomic numbers of the elements in a period on the Periodic Table increases, the volume of the elements

(1) increases and both the electronegativity and ionization energy increases.

(2) decreases and both the electronegativity and ionization energy increases.

(3) increases and both the electronegativity and ionization energy decreases.

(4) decreases and both the electronegativity and ionization energy decreases.

(5) increases and the electronegativity decreases while the ionization energy increases.

Radii of the Elements

Ex(2) In each of the following pairs of atoms, circle which atom is the larger.

Pair #1: He or Ne Pair #3: Cl or Ar

Pair #2: Ca or Mg Pair #4: Sn or Pb

Ex(3) Draw a full Bohr model (use full circles) for a neutral atom of hydrogen in the ground state, neutral atom of helium in the ground state, and a neutral atom of lithium in the ground.

Radii of the Elements

3p e-e-

e-

Li

1pe-

H

2p e-e-

He

Ex(4) In terms of atomic structure, explain why the radius ofhelium is smaller than the atomic structure of hydrogen.

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Ex(5) In terms of atomic structure, explain why the radius of

lithium is larger than the atomic structure of hydrogen.

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Radii of the Elements

Ex(6) Which grouping of circles, when considered from top to bottom, best represent the relative size of the atoms of Li, Na, K, and Rb, respectively?

Radii of the Elements

(1) (2) (3) (4)