Electron Structure UNIT 2

Question 1

Selenium is in the same group of the periodic table as sulfur.
i. Complete the full electron configuration of a selenium atom.

1s2
[1]
ii. Sodium selenide reacts with hydrochloric acid to form a toxic gas, B, with a relative
molecular mass of 81.0.
Identify gas B and write an equation for this reaction.

Gas B

Equation [2]

Answer 1

i. (1s2) 2s22p63s23p63d104s24p4 ✓
Look carefully at (1s2) 2s22p63s23p6
– there may be a mistake

ii. Gas B
H2Se / Hydrogen selenide /
Selenium hydride ✓
Equation
Na2Se + 2HCl → 2NaCl + H2Se

Question 2

Solid barium nitride is reacted with water, forming an alkaline solution A and an alkaline
gas B.
Identify A and B.
Write an equation, including state symbols, for the reaction.[4]

Answer 2

Products of reaction
A = Barium hydroxide /
Ba(OH)2 ✔
B = Ammonia / NH3 ✔
Formula for barium nitride
Ba3N2 ✔
Balanced equation AND state
symbols
Ba3N2(s) + 6H2O(l) →
3Ba(OH)2(aq)
+ 2NH3(g) ✔
State symbols are required

Question 3

The electrons in the second shell of a nitrogen atom are found in an s-orbital and three p-orbitals.
i. State, in words, the 3D shape of an s-orbital and a p-orbital.
s-orbital …………………………………………………..
p-orbital …………………………………………………..
[1]
ii. Describe the relative energies of the 2s orbital and each of the three 2p orbitals in a
nitrogen atom.

[2]

Answer 3

i.s-orbital = spherical
AND
p-orbital = dumb-bell shape ✓

ii.p-orbitals have greater energy than
s-orbitals ✓
(three) p-orbitals have equal energy
✓

Question 4

A student adds a small volume of aqueous silver nitrate to an aqueous
solution of bromide ions in a test-tube. The student then adds a similar
volume of dilute aqueous ammonia to the same test-tube.
Describe what the student would see in the test-tube after the addition of
aqueous ammonia.

Answer 4

Cream AND precipitate ✔

Question 5

Sodium sulfide, Na2S, is an ionic compound of sodium, Na, and sulfur, S.
Draw a ‘dot-and-cross’ diagram to show the bonding in sodium sulfide.
Show outer electrons only[2]

Answer 5

Na shown with either 0 or 8
electrons
AND
S shown with 8 electrons with 6
dots and 2 crosses (or vice versa)
✓
Correct charges ✓

Question 6

At room temperature and pressure, the first four members of the alkanes are all gases but the
first four alcohols are all liquids.
Explain this difference in terms of intermolecular forces.

Answer 6

At room temperature and pressure, the first four members of the alkanes are all gases but the
first four alcohols are all liquids.
Explain this difference in terms of intermolecular forces.

Question 7

The table shows the boiling points of ammonia, fluorine and bromine.

Boiling point / °C
ammonia, NH3 – 33
fluorine, F2 – 188
bromine, Br2 59
Explain the different boiling points of NH3, F2 and Br2.
Include the names of any relevant forces and particles.
In your answer you should use appropriate technical terms, spelled correctly.

Answer 7

The table shows the boiling points of ammonia, fluorine and bromine.

Boiling point / °C
ammonia, NH3 – 33
fluorine, F2 – 188
bromine, Br2 59
Explain the different boiling points of NH3, F2 and Br2.
Include the names of any relevant forces and particles.
In your answer you should use appropriate technical terms, spelled correctly.
stronger than van der Waals’
forces in F2 ✔

Question 8

What is meant by electronegativity

Answer 8

The ability of an atom to attract
electrons ✔
(Electron pair) in a (covalent) bond
✔

Question 9

A chemist carries out reactions of barium and barium nitride, Ba3N2.

Reaction 1 Barium is reacted with water.
Reaction 2 Barium nitride is reacted with water, forming an alkaline solution and an alkaline gas.
Reaction 3 Barium is reacted with an excess of oxygen at 500°C, forming barium peroxide, BaO2.
i. Write equations for Reaction 1 and Reaction 2.
Ignore state symbols.

Answer 9

Reaction 1: Ba + 2H2O →
Ba(OH)2 + H2 ✔
Reaction 2: Ba3N2 + 6H2O →
3Ba(OH)2 + 2NH3
Correct products ✔
Balancing ✔

Question 10

The compounds SO2 and MgO both contain oxygen.
The table below shows the melting point of both compounds:

Compound Melting point / K
SO2 200
MgO 3125

Predict the type of structure and bonding of SO2 and MgO and explain the difference in their
melting points.[4]

Answer 10

MgO: giant ionic (1)
SO2: simple molecular (1)
ionic bonds (in MgO) are (much)
stronger than intermolecular bonds
(in SO2) (1)
ionic bonds (in MgO) need more
energy to overcome / break (than
intermolecular forces in SO2) (1)

Question 11

Orbital shapes

Answer 11

Note: You must be able to draw these!

Question 12

Displaying electron configuration
1. Electrons are filled up from lowest energy sub-shells first. ✓
2. Electrons fill orbitals with the same energy unpaired (singly) before they start pairing. ✓
3. For ions, we remove/add electrons from the highest shell. ✓
4. 4s electrons are added first and removed first. ✓

Answer 12

Displaying electron configuration
1. Electrons are filled up from lowest energy sub-shells first. ✓
2. Electrons fill orbitals with the same energy unpaired (singly) before they start pairing. ✓
3. For ions, we remove/add electrons from the highest shell. ✓
4. 4s electrons are added first and removed first. ✓

Question 13

2 bonding pairs and 0 lone pairs

Answer 13

Linear 180

Question 14

3 bonding pairs and 0 lone pairs

Answer 14

Trigonal planer 120

Question 15

4 bonding pairs and 0 lone pairs

Answer 15

Tetrahedral 109.5

Question 16

3 bonding pairs and 1 lone pairs

Answer 16

Pyramidal 107

Question 17

6 bonding pairs and 0 lone pairs

Answer 17

octahedral 90

Question 18

2 bonding pairs and 2 lone pairs

Answer 18

non-linear 104.5

Question 19

4 bonding pairs and 0 lone pairs

Answer 19

linear 180

Question 20

Density of ice and water

Answer 20

Ice is less dense than water. ✓
Hydrogen bonds hold H2O molecules apart ✓
Ice has a relatively high melting point ✓
Hydrogen bonds are relatively strong ✓