Physical and chemical properties of elements - Practice Questions & MCQ

Atomic Volume

In moving from left to right in a period the atomic volume first decreases and then increases because of the following reasons:

• As we move in a period from left to right, first the increase in nuclear charge overcomes the increase in the number of electrons. Thus, the atomic volume decreases.

• But on moving further in a period, newly added electrons enter into the p orbitals and because of the high shielding power of s and p orbitals, atomic volume increases.

 

Li	Be	B	C	N	O	F	Ne
13	5	5	5	14	11	15	17
Na	Mg	Al	Si	P	S	Cl	Ar
24	14	10	12	17	16	19	24

 

In moving from top to bottom in a group, the atomic volume increases due to the increase of the number of shells.

 

Density

The density of elements also follows almost the same order as the atomic volume. In moving from left to right in a period, the density increases first and becomes maximum till some middle elements thereafter it starts decreasing gradually.

 

Melting Points and Boiling Points

The melting point of elements show a general trend with their atomic numbers. The elements with low atomic numbers have high melting points while the elements with higher atomic numbers have low melting points. In a period from left to right, the melting points of elements first increase and become maximum till some middle elements and then it decreases gradually.  

 

Li	Be	B	C	N	O	F	Ne
454	1550	2303	4000	63	54	53	24
Na	Mg	Al	Si	P	S	Cl	Ar
370.8	924	933	1693	317	392	172	83.6

 

The boiling points of elements also show similar trends as the melting points but the trend is not that much regular. 

In moving down the group, melting and boiling points follow the regular trend but the trend is different for different groups. For example for alkali group, the melting and boiling points decrease in moving from top to bottom but for halogens, the melting and boiling points increase in moving down the group.

Valency

The number of electrons that an atom donates or takes from other atom to form a compound is known as valency. In case of representative elements, the valency is equal to the number of valence electrons or eight minus the number of electrons. In case of transition elements, they show variable valency involving the valence electrons and d electrons of the penultimate shell. In moving from left to right in period, the valency first increases till 4 and then decreases to 0. 

 

Group	1	2	13	14	15	16	17	18
Number of valence electrons	1	2	3	4	5	6	7	8
Valency	1	2	3	4	3	2	1	0

 

But in group, all the elements have the same number of valence electrons thus all the elements have the same valency. 
 

Nature of Oxides

When the oxygen atom combines with any other atom and forms a compound, then it is known as as oxide. In moving from left to right in a period, the nature of oxide changes from basic to acidic while in moving down the group, the basic nature of oxides increases and acidic nature decreases. The oxides of metals are basic in nature, oxides of non-metals are acidic in nature and oxides of metalloids are amphoteric in nature.

 

Na2O	MgO	Al2O3	SiO2	P2O5	SO3	Cl2O7
Strongly basic	Basic	Amphoteric	Weakly acidic	Acidic	Strongly acidic	Strongly acidic

 

If an element forms a number of oxides, then acidic nature of oxides increases with the increase oxygen atoms.

 

MnO	Mn2O3	MnO2	MnO3	Mn2O7
Basic	Basic	Neutral	Acidic	Acidic

Nature of Oxy-acids

• The strength of oxyacids formed by non-metals increases in moving from left to right in a period. Thus, oxyacids strength increases as shown below:

HNO₃ > H₂CO₃ > H₃BO₃

• In moving from top to bottom in a group, the strength of oxyacids decreases. Thus, the order is followed as given below:

HNO₃ > H₃PO₄ > H₃AsO₄

• If any non-metal forms a number of oxyacids, then the strength of oxyacids increases with the increase of oxygen atoms. Thus, chlorine forms a number of oxyacids and strength follows the given order:

HClO₄ > HClO₃ > HClO₂ > HClO

 

Nature of Hydrides

Hydrides are basically the compounds with hydrogen atom or atoms and some other element.

In a period, the nature of hydrides changes from basic to acidic in moving from left to right.

 

NH3	H2O	HF
Weak base	Neutral	Weak acid

 

In a group, the acidic nature of hydrides increases in moving from top to bottom. Thus,

HI > HBr > HCl > HF

 

Anomalous Behaviour of Elements of Second Period 

The elements of the second period or the first elements in each group of the representative elements show anomalous behaviour in many aspects from the other elements in their respective groups. This anomalous behaviour of these elements is due to the following reasons:

• Small atomic size of the atom.

• Higher electronegativity

• The d-orbitals in their valence shell is absent.

• High charge to radius ratio.

• Capability to form multiple bonds by carbon, nitrogen and oxygen.