Covid Assignment 1: Ionisation Energies Answers

1. Explain why helium’s first ionisation energy is much higher than hydrogen’s.

2. Explain why lithium has lower ionisation energy than helium even though lithium has 1 proton more.

He has an electronic configuration of 1s², Li has an electronic configuration of 1s²2s¹.

Shielding in He is lower than that of Li since Li will start filling a new shell.

Since the electrons are further away from the nucleus the pull on the outermost electrons will be less.

The Ionisation Energy for Li will be lower than that of He.

3. Why does the main trend of first ionisation energy increase across a period?

Shielding along a period remains more or less similar.

Proton number increases.

Effective nuclear charge on the outermost electrons increases along a period. This means that electrons in the outer shell are held more tightly by the nucleus.

The Ionisation Energy increases along a period.

4. Explain why the ionisation energy of aluminium is lower than that of magnesium.

Mg has an electronic configuration of 1s²2s²2s⁶3s², Al has an electronic configuration of 1s²2s²2s⁶3s²3p¹.

Shielding in Al is higher than that of Mg since the 3s orbital (a spherical orbital) in Mg is full.

Effective nuclear charge on the outermost electrons in Al decreases. (less tightly held by the nucleus)

The Ionisation Energy decreases for Al.

5. Explain why the first ionisation of magnesium is 736 kJ mol^-1 whilst that of beryllium is 900 kJ mol^-1.

Mg has an electronic configuration of 1s²2s²2s⁶3s², Be has an electronic configuration of 1s²2s².

Magnesium has a shell more than Be.

The electrons of Magnesium are further away than those of Be.

The Ionisation Energy decreases for Mg.

6. Explain why the second ionisation energy of sodium is much higher than the second ionisation energy of magnesium.

Mg⁺ has an electronic configuration of 1s²2s²2s⁶3s¹, Na⁺ has an electronic configuration of 1s²2s²2s⁶.

Mg⁺ has its last electron in a different  shell when compared to Na⁺.

The electrons of Mg⁺ are further away than those of Na⁺.

The Ionisation Energy is higher for Na⁺.

7. The four successive ionisation energies of beryllium are: 900 1760 14800 21000 (kJ mol^-1). Explain why there is a very big increase in ionisation energies between the second and third ionisation energies.

The biggest energy jump for successive ionisation energies takes place when there is a change in shell. For Be, this happens when the first 2 electrons are lost from the 2nd shell whereas any other electrons will be closer to the nucleus.

8. Give the group number for the following elements having these successive first ionisation energies.
   1. 799 2420 3660 25040 32850
      The biggest jump in energy happens after the third electron, therefore this would be in group 3.
   2. 1000 2260 3390 4540 6990 8790 27100 31700
      The biggest jump in energy happens after the sixth electron, therefore this would be in group 6.
   3. 736 1450 3740 7740 10500 18000 21700 25600
      There is no big jump in energy, therefore this would be in group 8. (other answers have ben accepted depending on the reasoning)

 

9. Which would have a greater value for the second ionisation energy: magnesium or aluminium?

Mg⁺ has an electronic configuration of 1s²2s²2s⁶3s¹, Al⁺ has an electronic configuration of 1s²2s²2s⁶3s².

Shielding in both Mg⁺ and Al⁺ is the same.

Proton number increases.

Effective nuclear charge on the outermost electrons increases for Al⁺, making the atom smaller.

The Ionisation Energy increases for Al⁺

10. Which would have a greater value for the third ionisation energy: magnesium or aluminium?

Mg²⁺ has an electronic configuration of 1s²2s²2s⁶, Al²⁺ has an electronic configuration of 1s²2s²2s⁶3s¹.

Shielding in Mg²⁺ is lower than that of Al²⁺ since Al²⁺ will start filling a new shell.

Since the electrons are further away from the nucleus the pull on the outermost electrons will be less.

The Ionisation Energy for Mg²⁺ will be higher than that of Al²⁺.