Chemistry, Third edition

VARIATION OF PROPERTIES OF ELEMENTS WITHIN GROUPS

tend to be ionic and basic. Highly basic oxides such as Na 2 O form hydroxideswhen

dissolved in water:

Na 2 O(s)H 2 O(l)2NaOH(aq)

Non-metallic oxides tend to form covalent, acidic oxides.These oxides produce acids

when dissolved in water, for example:

SO 2 (g)H 2 O(l)H 2 SO 3 (aq)

4. The elements become less electronegative

The ability of an atom in a molecule to attract electron density to itself is related to

the size of the atom. The further away the electrons in the bond are from the nucleus,

the weaker they are attracted to the atom. In Group 17, therefore, fluorine is the

most electronegative element and iodine is the least electronegative.

Changes in properties across a period

Consider the elements of the second period, excluding Ne:

Element Li Be B C N O F

Electronic structure 2.1 2.2 2.3 2.4 2.5 2.6 2.7

s,p,d,f structure 1s^2 2s^1 1s^2 2s^2 1s^2 2s^2 2p^1 1s^2 2s^2 2p^2 1s^2 2s^2 2p^3 1s^2 2s^2 2p^4 1s^2 2s^2 2p^5

As we cross the period, from Li to F, the second shell of electrons is being filled.

Each addition of one electron is accompanied by the addition of one proton into the

nucleus, since the atoms in the elements are electrically neutral. Electrons in the

same shellhave little effect in shielding each other from nuclear charge, so that the

effective nuclear chargefelt by the outer electrons increases from left to right across the

period. As a result of this increased ‘pull’ on the outer electrons by the nucleus:

1. The atomic radii of the elements decreases

In period 2, for example, the atomic radii decrease from Li at 145 pm to F with an

atomic radius of 50 pm.

2. The first ionization energy increases

The energy required to remove one electron (the first ionization energy) increases as

the effective nuclear charge felt by the electron increases. The variation of first ion-

ization energy across the second period is shown in Figure 12.4.

Notice there is a general increasein ionization energy from left to right in the period.

The ‘saw-tooth’ appearance of the plot is due to the slight differences in energies

involved in removing electrons from filled or exactly half-filled subshells. A similar pat-

tern is seen if the first ionization energies are plotted for the third period (Na to Ar). Note

that sodium atoms have the lowest first ionization energy and argon atoms the highest.

3. The elements become less metallic

The tendency of atoms of the elements to lose electrons decreases from left to right

across a period, so the elements gradually become less metallic. On the extreme

right-hand side the halogens react, as typical non-metals, by gaining electrons. For

example, for chlorine in the reaction of sodium metal with chlorine gas,

Cl 2 (g)2e(g)2Cl(s)

211

Fig. 12.4First ionization

energies of the elements of

period 2.