Atomic theory 9

Question 1

Define electronegativity (L.C)

Answer 1

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

Question 2

Predict the type of bond formed between carbon and chlorine atoms in a CCl₄ molecule (L.C)

Answer 2

Polar covalent

Question 3

State and account for the shape of a tetrachloromethane molecule. (L.C)

Answer 3

• Tetrahedral

- Four pairs electrons and no lone pairs

Question 4

Define electronegativity (L.C)

Answer 4

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

Question 5

Why is there an increase in electronegativity value moving from gallium to germanium in the periodic table? (L.C)

Answer 5

• Nuclear charge increasing

- Atomic radius decreasing

Question 6

Use electron pair repulsion theory to predict the shape of a molecule of boron trifluoride. (L.C)

Answer 6

• Trigonal planar

Question 7

Define electronegativity (L.C)

Answer 7

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

Question 8

Define electronegativity (L.C

Answer 8

The relative power of attraction an atom of an element has for the shared pair of electrons in a covalent bond.

Question 9

How many (i) sigma bonds, (ii) pi bonds, result from sharing of the valence electrons between the atoms in a molecule of nitrogen. (L.C)

Answer 9

(i) 1 sigma

(ii) 2 pi

Question 10

Distinguish between intramolecular bonding and intermolecular forces. (L.C)

Answer 10

Intramolecular : forces between atoms in molecules

Intermolecular : forces between molecules

Question 11

Explain in terms of intramolecular bonding and intermolecular forces, why the boiling point of hydrogen (20 K) is significantly lower than that of oxygen (90.2 K). (L.C)

Answer 11

• hydrogen smaller

- weaker intermolecular forces

Question 12

Explain in terms of intramolecular bonding and intermolecular forces, why iodine has a very low solubility in water. (L.C)

Answer 12

• Iodine is pure covalent

- Water is a polar solvent

Question 13

Explain in terms of intramolecular bonding and intermolecular forces, why when a charged rod is held close to a thin stream of water flowing from a burette, the stream of water is deflected. (L.C)

Answer 13

Charge on rod attracts opposite charge on polar water molecule.

Question 14

Distinguish between sigma (σ) and pi (π) covalent bonding. (L.C)

Answer 14

Sigma - When orbitals overlap ‘head-on’. This can occur between two s orbitals, an ‘s’ and a ‘p’ orbital or two ‘p’ orbitals.
Pi - When orbitals overlap ‘side-ways’. This can occur between two ‘p’ orbitals only.

Question 15

Give one reason why electronegativity values exhibit general increase across the second period of the periodic table. (L.C)

Answer 15

Increase in nuclear charge / decrease in atomic radius

Question 16

Show that the ammonia molecule has polar covalent bonding. (L.C)

Answer 16

• There is an electronegativity difference between N and H.
• N with greater attraction
• H with smaller attraction

Question 17

Describe the processes involved when ammonia dissolves in water. (L.C)

Answer 17

• Hydrogen bonds between slightly negative O of water and H of ammonia and between slightly positive H of water and N of ammonia.
• Breaking of hydrogen bonds in water
• Forming of hydrogen bonds between ammonia and water.

Question 18

Distinguish between sigma (σ) and pi (π) covalent bonding. (L.C)

Answer 18

Sigma - When orbitals overlap ‘head-on’. This can occur between two s orbitals, an ‘s’ and a ‘p’ orbital or two ‘p’ orbitals.
Pi - When orbitals overlap ‘side-ways’. This can occur between two ‘p’ orbitals only.

Question 19

State two factors that cause electronegativity values to increase across a period in the periodic table of the elements. (L.C)

Answer 19

• Increasing effective nuclear charge

- Decreasing atomic radius

Question 20

State which of the following compounds contain intermolecular hydrogen bonds, HCl, H₂O, NH₃. Justify your answer. (L.C)

Answer 20

H₂O and NH₃

- Hydrogen bonded to a small highly electronegative element (bonded to F, O or N)

Question 21

Suggest a reason why the boiling point of ammonia (-33°C) is significantly lower than that of water (100 °C) (L.C)

Answer 21

• Weaker hydrogen bonding in ammonia

- Stronger hydrogen bonding in water

Question 22

Explain the term intermolecular forces (L.C)

Answer 22

Attractive forces (repulsive) between molecules

Question 23

A thin stream of liquid is flowing from the burette. A stream of water is deflected towards a positively charged rod whereas a stream of cyclohexane is undeflected. Account for these observations. (L.C)

Answer 23

• Polarity of water causes attraction to charged rod

- Non-polarity of cyclohexane means it is not affected by charged rod

Question 24

Use electronegativity values to predict the type of bond expected between hydrogen and sulphur. (L.C)

Answer 24

• Weakly polar
• Almost non-polar
• Covalent bond

Question 25

Write the chemical formula for hydrogen sulphide. (L.C)

Answer 25

H₂S

Question 26

Distinguish between sigma and pi covalent bonding.(L.C)

Answer 26

Sigma - When orbitals overlap ‘head-on’. This can occur between two s orbitals, an ‘s’ and a ‘p’ orbital or two ‘p’ orbitals.
Pi - When orbitals overlap ‘side-ways’. This can occur between two ‘p’ orbitals only.

Question 27

Define atomic covalent radius. (L.C) (same as atomic radius)

Answer 27

The atomic radius is obtained by getting half the distance between the centres of singly bonded atoms of the same element

Question 28

Under what circumstances can ionic compounds conduct electricity? (L.C)

Answer 28

In water / in the molten state

Question 29

A thin stream of liquid is flowing from the burette. A stream of water is deflected towards a positively charged rod whereas a stream of cyclohexane is undeflected. Explain what would happen in the case of the stream of water if the positively charged rod were replaced by a negatively charged rod. (L.C)

Answer 29

Stream of water still attracted to rod as molecules arrange themselves with positive pole towards rod.

Question 30

What is the valency of carbon in tetrachloromethane?

Answer 30

4

Question 31

Suggest a reason why ammonia has the highest boiling point of the three hydrides. (L.C)

Answer 31

Hydrogen bonding in ammonia is stronger than the other intermolecular forces.

Question 32

Suggest a reason why phosphine’s boiling point is lower than that of arsine. (L.C)

Answer 32

Phosphine molecules are smaller.