# Periodic Table

## NCERT Exercise

### Part 2

Question 21: Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.

Answer: When an electron is added to O atom to from O- ion, energy is released. So, first electron gain enthalpy is negative. But when an electron is added to O- energy is required to overcome the strong force of repulsion. So, second electron gain enthalpy is positive.

Question 22: What is the basic difference between the terms electron gain enthalpy and electronegativity?

Answer: Electron gain enthalpy gives the enthalpy involved in adding an electron to an atom or ion. But electronegativity is about the ability of an atom to attract shared electrons.

Question 23: How would you react to the statement that the electronegativity of N on Pauling scale is 3.0 in all the nitrogen compounds?

Answer: Electronegativity of an element is not constant, rather it depends on the element with which it is making a particular compound. So, electronegativity of N on Pauling scale varies, depending on another constituent of the compound. Hence, this statement is wrong.

Question 24: Describe the theory associated with the radius of an atom as it (a) gains an electron (b) loses an electron.

Answer: When an atom gains an electron, force of repulsion comes into effect because of extra electron. Due to this, atomic radius of an anion is more than that of its parent atom. When an atom loses an electron, the force of attraction (between nucleus and electrons) increases. Due to this, atomic radius of a cation is less than that of its parent atom.

Question 25: Would you expect the first ionization enthalpies for two isotopes of the same element to be the same or different? Justify your answer.

Answer: Isotopes have same atomic number and hence same electronic configuration. So, first ionization enthalpies of two isotopes of same element would be same.

Question 26: What are the major differences between metals and non-metals?

Answer: Metals: 78% of all known elements are metals and they appear on the left side of the Periodic Table. Metals are generally solid at room temperature, have high melting and boiling points, are good conductors of heat and electricity, and are ductile and malleable.

Non-metals: Non-metals are located at the top right hand side of the Periodic Table. Non-metals are generally solids or gases at room temperature, have low melting and boiling points, are poor conductors of heat and electricity and solids are brittle.

Question 27: Use the periodic table to answer the following questions:

(a) Identify an element with five electrons in the outer subshell.

Answer: Elements belonging to halogen group (Group 17), e.g. Chlorine

(b) Identify an element that would tend to lose two electrons.

Answer: Magnesium (elements of group 2)

(c) Identify an element that would tend to gain two electrons.

(d) Identify the group having metal, non-metal, liquid as well as gas at the room temperature.

Question 28: The increasing order of reactivity among group 1 elements is Li < Na < K < Rb < Cs whereas that among group 17 elements is F > Cl > Br > I. Explain.

Answer: Group 1 elements have 1 electron in their outermost orbital. So, they have a strong tendency to lose 1 electron to attain a stable configuration. When we move down the group, ionization enthalpy increases and reactivity increases as a consequence.

Group 17 elements can easily gain an electron to attain stable configuration. So, their reactivity depends on electron gain enthalpy. Electron gain enthalpy becomes less negative when we move down a group. Due to this, reactivity decreases in case of halogens.

Question 29: Write the general outer electronic configuration of s-, p-, d- and f-block elements.

Answer: General outer electronic configuration of s, p, d, and f-block elements are as follows:

1. s-block Elements: ns1, ns2
2. p-block Elements: ns2 np2 to ns2 np6
3. d-block Elements: (n-1)d1-10 ns0-2
4. f-block Elements: (n-2)f1-14 (n-1)d0-1 ns2

Question 30: Assign the position of the element having outer electronic configuration (a) ns2 np4 for n = 3 (b) (n-1)d2 ns2 for n = 4, and (c) (n-2) f7 (n-1)d1 ns2 for n = 6, in the periodic table.

1. Period = 3, p-block, Group = 10 + no. of electrons in valence shell = 10 + 6 = 16
2. Period = 4, d-block, Group = 4 (no. of electrons in valence shell and d)
3. Period = 6, f-block, Group = 3 (because all f-block elements belong to group 3

Question 31: The first (ΔiH1) and the second (ΔiH2) ionization enthalpies (in kJ mol-1 and the (ΔegH) electron gain enthalpy (in kJ mol-1) of a few elements are given below:

ElementsiH1)iH1)egH)
I5207300-60
II4193051-48
III16813374-328
IV10081846-295
V23725251+48
VI7381451-40

Which of the above elements is likely to be:

(a) The least reactive elements

Answer: V is the least reactive element because it has highest first ionization enthalpy and a positive electron gain enthalpy

(b) The most reactive metal

Answer: II is most reactive because of its lowest first ionization enthalpy and low negative electron gain enthalpy

(c) The most reactive non-metal

Answer: III is most reactive non-metal because of its high first ionization enthalpy and high negative electron gain enthalpy

(d) The least reactive non-metal

Answer: V can be the least reactive non-metal because of its high first ionization enthalpy and a positive electron gain enthalpy

(e) The metal which can form a stable binary halide of the formula MX2 (X = Halogen)

Answer: Element VI has low negative electron gain enthalpy, so it is a metal. It has the lowest second ionization enthalpy. So, it can form a stable binary halide of the formula MX2

(f) The metal which can form a predominantly stable covalent halide of the formula MX.

Answer: Element I has low first ionization energy and highest second ionization enthalpy. So, it can form a predominantly stable covalent halide of the formula MX.

Question 32: Predict the formulae of stable binary compounds that would be formed by combination of following pairs of elements

1. Lithium and oxygen
2. Magnesium and nitrogen
3. Aluminium and iodine
4. Silicon and oxygen
5. Phosphorus and fluorine
6. Element 71 and fluorine

Answer: (a) Li2O, (b) Mg3N2, (c) AlI3, (d) SiO2, (e) PF3 or PF5, (f) LuF3

Question 33: In the modern periodic table, the period indicates the value of

1. Atomic number
2. Atomic mass
3. Principal quantum number
4. Azimuthal quantum number

Question 34: Which of the following statements related to the modern periodic table is incorrect?

1. The p-block has 6 columns, because a maximum of 6 electrons can occupy all the orbitals in a p-shell.
2. The d-block has 6 columns because a maximum of 8 electrons can occupy all the orbitals in a d-subshell.
3. Each block contains a number of columns equal to the number of electrons that can occupy a subshell.
4. The block indicates the value of azimuthal quantum number (l) for the last subshell that received electrons in building up the electronic configuration.

Answer: (c) Each block contains a number of columns equal to the number of electrons that can occupy a subshell.

Question 35: Anything that influences the valence electrons will affect the chemisty of the element. Which one of the following factors does not affect the valence shell?

1. Valence principal quantum number (n)
2. Nuclear charge (Z)
3. Nuclear mass
4. Number of core electrons

Question 36: The size of isoelectronic species – F-, Ne and Na+ is affected by

1. Nuclear charge
2. Valence principal quantum number (n)
3. Electron-electron interaction in the outer orbitals
4. None of the factors because their size is the same

Question 37: Which one of the following statements is incorrect in relation to ionization enthalpy?

1. Ionization enthalpy increases for each successive electron.
2. The greatest increase in ionization enthalpy is experienced on removal of electron from core noble gas configuration.
3. End of valence electrons is marked by a big jump in ionization enthalpy.
4. Removal of electron from orbitals bearing lower n value is easier than from orbital having higher n value.

Answer: (d) Removal of electron from orbitals bearing lower n value is easier than from orbital having higher n value.

Question 38: Considering the elements B, Al, Mg and K, the correct order of their metallic character is

1. B > Al > Mg > K
2. Al > Mg > B > K
3. Mg > Al > K > B
4. K > Mg > Al > B

Answer: (d) K > Mg > Al > B

Question 39: Considering the elements B, C, N, F and Si, the correct order of their non-metallic character is

1. B > C > Si > N > F
2. Si > C > B > N > F
3. F > N > C > B > Si
4. F > N > C > Si > B

Answer: (c) F > N > C > B > Si

Question 40: Considering the elements F, Cl, O and N, the correct order of their chemical reactivity in terms of oxidising property is:

1. F > Cl > O > N
2. F > O > Cl > N
3. Cl > F > O > N
4. O > F > N > Cl

Answer: (b) F > O > Cl > N