Chemistry, Third edition

12 · CHEMICAL FAMILIES

Variation of properties of elements

within groups and periods

Changes in properties within groups

By now, you will have noticed that, although elements in the same group have similar

properties, there are definite variations in the properties of the elements and their

compounds as any group in the Periodic Table is descended.

As any group in the Periodic Table is descended, the general trends are as follows:

1. The elements become more metallic

On descending a group, the atoms become larger and the outer electrons become fur-

ther away and more shielded from the positive charge of the nucleus that binds them

to the atom. The ionization energies of the outer electrons therefore decrease. Metals

react by losing electrons to form positive ions. The ‘ease’ of this reaction is related to

the energy required to take away the outer shell electrons. Therefore, as the ionization

energies of the elements decrease, the metallic character becomes more pronounced.

This trend may be difficult to see in Groups 1 or 2, because the elements are all metal-

lic, but the trend is very apparent in Group 14.

2. Atomic and ionic radii increase

Examples from Group 1 and 2 elements are shown back in Exercise 12F. Atomic and

ionic radii for the halogens are shown below:

Symbol Atomic radius/pm Symbol Ionic radius/pm

F 050 F 136

Cl 099 Cl 181

Br 114 Br 195

I 133 I 215

3. The chlorides and oxides of the elements become more ionic

These properties are related to the metallic nature of the element – metallic chlorides

tend to be ionic, whereas non-metallic chlorides are covalent. Also, metallic oxides

12.8

210

Atomic and ionic

radii

(i)iiExplain why the radius

of a sodium atom is

largerthan that of a

sodium ion, whereas

that of a chlorine atom

issmallerthan a

chloride ion.

(ii)iThe ions Naand Mg^2 

areisoelectronic

(contain the same

number of electrons),

but Mg^2 is a much

smaller ion. Explain why

this should be so.

(iii)N^3 and Fare also

isoelectronic, but N^3 

is the larger ion – why?

Exercise 12S

Polydentate ligands

The ethylenediaminetetraacetate ion (EDTA^4 ) is a polydentate ligand. Its structure is as follows:

The lone pairs that are available for donation to a metal ion are shown on the diagram. EDTA^4 

also coordinates with Ca^2 or Mg^2 and is used to remove these ions from water (to ‘soften’

water) and is therefore often an ingredient of shampoos.

(i)How many lone pairs can EDTA^4 coordinate to a metal ion?

(ii)One molecule of an hydrated metal ion reacts with EDTA^4 , so that all of the water

molecules are replaced by one molecule of EDTA^4 :

[M(H 2 O) 6 ]^2 EDTA^4 

What is the overall charge of the new complex ion formed?

C CH 2

N

O

O

• O
  CH 2 CH 2 N
  CH 2

CH 2

• O C CH 2

C

O

O

C

O–

O–

EDTA4–

Exercise 12R