Periodic Trends

Question 1

What are the challenges faced when measuring radius of an atom

Answer 1

1. The size of the atom is very small ~ 1.2 A
2. The electron cloud surrounding the atom does not have a sharp boundary and hence atomic radius determination cannot be precise

Question 2

Covalent radius

Answer 2

Calculated by measuring the distance between nuclei of atoms of covalently bonded molecule and taking half of that value

Question 3

Metallic radius

Answer 3

Half the internuclear distance separating the cores in a metallic crystal

Question 4

Atomic Radius variation

Answer 4

Atomic radius increases down the group and decreases across the period except noble gases, which have a large radius

Question 5

Why does atomic radius increase down the group

Answer 5

As we go down the group, principal quantum number increases i.e., number of shells increase. These outer electrons are farther away from the nucleus due to good shielding effect of the inner electrons. Hence, radius keeps increasing

Question 6

Why does atomic radius decrease across the period

Answer 6

As we move from left to right, number of shells stay the same but nuclear charge increases which means the electrons are pulled towards the nucleus with more force, hence making it smaller.

Question 7

Discuss ionic radii variation

Answer 7

Ionic radii show the same variation as atomic radii
But, cations are smaller than the parent atom due to increased nuclear force due to lesser number of electrons
Anions are larger than parent atom due to increased repulsion caused by extra electron

Question 8

Isoelectronic species

Answer 8

Species with same number of atoms and valence electrons

O2-, F-, Na+, Mg2+ (all of them have 10 e-)

Question 9

Ionization enthalpy

Answer 9

Energy required to remove one electron from an isolated gaseous atom in its ground state
Always positive

Question 10

Variation between first, second, third IE

Answer 10

The second IE is always more than the first because it is harder to remove an electron from a positive atom than a neutral one
Similarly, the third IE > second IE

Question 11

Variation of IE in the periodic table

Answer 11

1. Increases as we go across the period

2. Decreases as we go down the group

Question 12

Explain variation of IE

Answer 12

1. Along a period, increase in nuclear charge outweighs the shielding effect and so IE keeps increasing due to the increase in nuclear charge
2. Down the group, increase in shielding effect outweighs the nuclear charge and so lesser energy is required to remove an electron as we go down the group

Question 13

Exceptions in IE trend

Answer 13

1. B(5) has a smaller enthalpy than Be(4) because s electron is more attached to the nucleus. In B, a p electron is removed while in Be, an s electron is removed (which is harder so higher IE)
2. O(p4) has a lower IE than N(p3) because of extra stability of N’s half filled shell

Question 14

Electron gain enthalpy

Answer 14

Energy required to convert isolated gaseous atom in ground state to an anion by adding an electron
Could be negative or positive depending on the element considered

Question 15

Electron gain enthalpy variation

Answer 15

EGE becomes more negative as we go across the period and less negative as we go down the group

Question 16

Explain EGE variation

Answer 16

As we go along a period, number of valence electrons keep increasing. Adding an electron would get this element loser to noble gas configuration and hence EGE gets more negative across the period
Down the group, the added electron would keep going farther from the nucleus and so, EGE becomes less negative

Question 17

EGE Exception

Answer 17

O or F is less negative than that of the succeeding element because the electron added goes to the n=2 shell for these atoms and faces a lot of repulsion.

Question 18

Electronegativity

Answer 18

Qualitative measure of the tendency to attract a shared pair of electrons towards itself

Question 19

Scales to measure electronegativity

Answer 19

Mulliken-Jaffe scale, Pauling scale, Allred-Rochow scale

Question 20

Pauling Scale

Answer 20

According to this scale, F (most EN element) has a value of 4
Every other value is relative to this

Question 21

Electronegativity variation

Answer 21

EN increases across the period and decreases down the group

Question 22

EN - metallic nature relation

Answer 22

EN can be seen as a measure of non metallic character

EN is directly proportional to non metallic character and inversely proportional to metallic character

Question 23

Valence

Answer 23

Usually equal to the number of electrons in the outermost shell and/or eight minus the number of electrons in the outermost shell

Question 24

Oxidation state

Answer 24

Charge acquired by its atom on the basis of electronegative consideration from other atoms in the molecule

Question 25

Diagonal relationship

Answer 25

Li and Be show more similarities with Mg and Al respectively, rather than other elements of the same group

Question 26

Why do 2 period elements not show similarities to other elements of the same group

Answer 26

1. Small size
2. High electronegativity
3. High IE
4. Absence of d orbitals

Question 27

Chemical reactivity in the periodic table

Answer 27

1. Elements on either extremes are incredibly reactive due to high IE or high EGE
2. Both sides react with oxygen readily to form oxides

Question 28

Nature of oxides formed

Answer 28

1. The left side of the table forms basic oxides (Na2O)
2. The right side forms acidic oxides (Cl2O7)
3. The center forms either amphoteric oxides (Al2O3) or neutral oxides (NO, CO)