Across a period the number of protons in the nucleus increases, as do the number of electrons. This also happens down a Group; but the key difference is that across a period the shell of electrons is not completed until the noble gas at the end of the period is reached. As we go across a period the efficiency of screening is not so great because the shell of electrons is not completed. As a result of this the effective nuclear pull increases and the electrons are held more tightly.
As a consequence, the elements become harder to ionise, and they also tend to attract electrons towards them. Thus we see a change from the metals (which give positive ions) on the left of a period to the non-metals on the right. With this change there is a change in the nature of the hydrides, oxides and chlorides of the elements. These changes are summarised in Fig. 18.1.
OXIDES OF CARBON
Carbon on heating in air or oxygen produces two oxides namely; carbon monoxide and carbon dioxide
2 C(g) + O2(g) à 2CO(g)
C(s) + O2(g) à CO2(g)
Carbon monoxide is formed by incomplete combustion of carbon or carbon containing compounds.
Carbon dioxide is obtained when carbon is heated in excess supply of oxygen or air.
Solid CO2 is called dry ice and readily sublimes.
CO is neutral in character whereas CO2 is acidic.
Selected physical properties of CO and CO2 are given in Table 18.1. The structure of CO and CO2 is explained below:
Table 18.1. Selected Properties of CO and CO2
OXIDES OF NITROGEN AND PHOSPHORUS
Some common oxides of nitrogen and phosphorus are given below:
Oxides of Nitrogen and Phosphorus
Nitrogen forms six different oxides by combination with oxygen. The oxidation state of nitrogen in these oxides ranges from + 1 to +5. The molecular formula of various oxides of nitrogen along with the oxidation number and some physical characteristics of nitrogen are given in Tables 18.2 and 18.3 respectively.
Table 18.2. Oxides of Nitrogen
Table 18.3. Selected data for the Oxides of Nitrogen
N2O and NO are commonly called nitrous oxide and nitric oxide, respectively.
NATURE OF OXIDES
All the oxides of nitrogen (except NO and N20) and phosphorus are strongly acidic.
Reason. The change in character from acidic to basic can be explained on the basis of the size of atoms. As the size of nitrogen atom is small and it has a strong positive field, it interacts with water more strongly ‘pulling the electron pair between 0-H bond and thus helps in release of a+ ions. However, this tendency diminishes with the increase in size and therefore decreases the acidic character or conversely increases the basic character.
OXIDES OF SULPHUR
Sulphur combines with oxygen to form monoxide, dioxide and trioxide
S + O2 à SO2(g)
2S + 3O3 à 2SO3 (g)
Both the oxides of sulphur are acidic in character.
OXIDES OF HALOGENS
Halogens do not combine with oxygen directly but their oxides with oxygen can be prepared indirectly. For example, oxygen difluoride, OF2 is prepared by the action of fluorine on 2% sodium hydroxide solution.
2F2 + 2NaOH à 2NaF + OF2 + H2O
Similarly, dichlorine oxide, C~O is obtained indirectly by passing chlorine over precipitated mercuric oxide.
Some oxides of halogens are given below in Table 18.4.
Table 18.4. Oxides of Halogens
All the oxides are powerful oxidising agents and decompose explosively when subjected to mechanical shock or heat.