Question

1. For each set of atoms/ions, write them in increasing order of size (smallest to largest) a. Mg2+, Se2- , S2- , K+ , Ca2+ b. Ga, Si, Rb, N, In c. A2+, B+ , C, D- , E2- if these elements are isoelectronic (same number of electrons) d. Put the following elements, S, Ba, Bi, Cl, Te, Ar in order of increasing ionization energy, and increasing electron affinity.

2. The following questions involve the following atoms: Ge, As, K, N, O a. Draw the Lewis symbol for each atom b. Put the atoms in order of increasing electronegativity c. If any two atoms in the above list could be paired to make a bond, which two would lead to the most polar bond?

3. Calculate the lattice energy of MgCl2: MgCl2(s)Mg2+ (g) + 2Cl- (g) given the following: The sum of the 1st and 2nd ionization energy required to make Mg2+ is 2189 kJ/mol The heat of formation of Cl(g) is 121 kJ/mol The heat of formation of MgCl2 is -641.6 kJ/mol The electron affinity of Cl(g) is -349 kJ/mol The heat of formation of Mg(g) is 147.1 kJ/mol

4. Out of the following ionic compounds LiCl, Li2O, Na2S, BeF2, Al2S3, CaS a. Put them in order of increasing bond distance between the cation/anion, lowest to highest (Li-O distance in the formula unit Li2O). Use Figure 7.8 from text. b. Put them in order of increasing (lowest to highest) lattice energy

5. Draw Lewis structures (octet rule) for the following molecules: HCN, SF3 + , CH2Cl2, ClF3, BO3 3- , CH2CCl2 (the two carbon atoms are bonded to each other), CH3OH. If a double or triple bond is required, state your reasoning as to why you came to that conclusion.

6. Two Lewis structures for BF3 are given below. Determine the formal charge for each atom (write the formal charge next to the atom) in both structures and identify the correct structure. Redraw these structures on your homework.

7. Incorrect Lewis dot structures for PCl3 and CCl2O are given below. Disregarding any resonance considerations, identify the mistakes (explicitly state the mistake) in the list of instructions that led to the drawing of these incorrect structures.

Answer 1

1.Sol:-

(a). The correct increasing order of size (smallest to largest) is : Mg²⁺ < Ca²⁺ < K⁺ < S^2- < Se^2-.

Explanation :-

#.Down the group of the periodic table ionic radii increases, therefore Ca²⁺ is larger in size than Mg²⁺. Both Ca²⁺ and Mg²⁺ lies on the same group i.e. group 2.

#. Ca²⁺, K⁺ and S^2- are the isoelectronic species i.e. having same number of electrons, which are 18 electrons. For isoelectronic species a cation having more charge (i.e. Ca²⁺) is always smaller than a cation having less charge (i.e. K⁺), because nuclear charge per electron increases when a cation has more charge, due to which ionic radii decreases.

Ca²⁺ (Z=20), Number of protons = 20 and Number of electrons = 18

K⁺ (Z=19), Number of protons = 19 and Number of electrons = 18

S^2- (Z=16), Number of protons = 16 and Number of electrons = 18

#. Similarly, anion i.e. S^2- is larger in size than K⁺, because in case of S^2- anion effective nuclear charge per electron decrease.

#. Down the group of the periodic table ionic radii increases, therefore Se^2- is larger in size than S^2-. Both Se^2- and S^2- lies on the same group i.e. group 16.

(b). The correct increasing order of size (smallest to largest) is : N < Si < Ga < In < Rb.

Explanation :-

#. Down the group of the periodic table atomic radii increases due to increase in number of shells and Left to right atomic radii decreases because effective nuclear charge increases .Rb and In both lies on the same period i.e. 5^th period therefore Rb is bigger in size than In . similarly In is larger in size than Ga because both lies on the same group i.e. group 13. Si lies on the 14 group while N lies on the 15 group, therefore Si is bigger in size than N.

(c). The correct increasing order of size (smallest to largest) is : A²⁺ < B⁺ < C < D^- < E^2-.

Explanation :-

For isoelectronic species a cation having more charge (i.e. A²⁺) is always smaller than a cation having less charge (i.e. B⁺) and a cation (say B⁺) is always smaller than its parent atom (say C), because nuclear charge per electron increases when a cation has more charge, due to which ionic radii decreases.

Similary a anion having more charge (i.e. E^2-) is always larger in size than a anoin having less charge (i.e. D^-) or parent atom (say C), because effective nuclear charge per electron decreases.

(d).

(i). Increasing order of ionisation enthalpy is : Ba < Bi < Te < S < Cl < Ar

Explanation :-

Ionisation enthalpy is the amount of energy required to remove the most loosely bound valence shell electron from an isolated gaseous atom.

Down the group ionisation enthalpy decreases because atomic radii increases and left to right ionisation enthalpy increases because atomic radii decreases.

Ba (group 2) and Bi (group15) lies on the same period i.e. 6^th period.

S (group 16),Cl (group 17) and Ar i.e. a noble gas (group 18) lies on the same period i.e. 3rd.

Te (group 16) lies on the 5th period.

(ii).

Increasing order of electron gain enthalpy is : Ar < Ba < Bi < Te < S < Cl

Explanation :-

Electron gain enthalpy is the amount of energy released when an electron is added to an isolated gaseous atom to form gaseous anion.

Noble gas i.e. Ar have very less tendency to accept the electrons because of its fully fiiled electronic configuration. left to right electron gain enthalpy increases because atomic radii decreases while down the group it decreases because atomic radii increases. Therefore Cl will have more first electron gain enthalpy.