chapter 7: covalent and metallic bonding

Question 1

what is a covalent bond?

Answer 1

• is formed by the sharing of electrons between two atoms

Question 2

what can covalent bonds be formed between? and what do they form?

Answer 2

can be formed between:

• atoms of the same element
• atoms of different elements

> when atoms combine by sharing electrons, molecules are formed

Question 3

what are diatomic molecules and what are some examples of it?

Answer 3

• oxygen, chlorine, hydrogen

> each of these elements is made up of two atoms that are joined together by sharing electrons

Question 4

what are molecules of compounds?

Answer 4

-when atoms of different elements are joined together by covalent bonding

Question 5

draw the dot-and-cross diagram of methane and carbon dioxide.

Answer 5

answer is on page 111 :)

Question 6

what are the physical properties of substances with simple molecular structures?

Answer 6

• have low melting and boiling points
• soluble in organic solvents and insoluble in water
• do not conduct electricity in solid, liquid or gaseous state

Question 7

why do simple molecular substances have low melting and boiling points?

Answer 7

• within each molecule, atoms are held together by strong covalent bonds
• BUT between the molecules, are only weak intermolecular forces holding them together
  > can be easily overcomed
  > low bp and mp

Question 8

what is an example simple molecular substances that are insoluble in water and soluble in organic solvents?

Answer 8

• oil dissolves in dichloromethane but not water

exceptions: alcohol and sugar are covalent compounds that can be dissolved in water

Question 9

why do simple molecular substances not conduct electricity in any state?

Answer 9

• simple molecular substances do not have any free moving electrons or mobile ions to act as charge carriers

Question 10

what is a giant molecular structure and what are some examples of substances with it?

Answer 10

• a giant network of atoms that are covalently bonded
• eg. diamond, graphite and silicone dioxide
• *diamond & graphite - allotropes of carbon
  > different forms of carbon

Question 11

what are the physical properties of substances with giant molecular structures?

Answer 11

• very high melting points
• do not conduct electricity (except for graphite)
• are insoluble in both water and organic solvents

Question 12

why do giant molecular substances have high melting and boiling points?

Answer 12

• consists of a large number of atoms that are held together by strong covalent bonds
• to melt or boil, strong bonds must be broken
  > requires a large amount of energy
• solids at room temperature

Question 13

why do giant molecular substances not conduct electricity?

Answer 13

• all the outer electrons of the atoms are used
  > to form covalent bonds
  > no free electrons that move through the structure to conduct electricity

Question 14

why are giant molecular substances insoluble in both organic solvents and water?

Answer 14

• all atoms are held together by strong covalent bonds
• any forces of attraction between the giant molecule and solvent molecules
  > not strong enough to break the strong covalent bonds

Question 15

what is diamond and what is it used for?

Answer 15

• one of the allotropes of carbon
• when well cut and polished, > valuable gemstone
• tips of drills & the cutting edges of tools
  > exceptional hardness and high bp and mp

Question 16

what is graphite what is it used for?

Answer 16

• another allotrope of carbon
• soft, slippery, fairly unreactive & good conductor of electricity
  > used to make dry lubricants, inert electrodes
• can be mixed with clay to make pencil lead

Question 17

what are the physical properties of graphite and why?

Answer 17

1. high m.p and b.p
   - each carbon atom covalently bonded to 3 other C atoms
   > which is bonded to 3 more carbon atoms
   > forms a continuous layer of hexagons
   - difficult to break strong covalent bonds
2. soft and slippery
   - layers of carbon atoms held loosely by weak intermolecular forces of attraction
   > layers of carbon atoms can slide over each other when force is applied
3. conducts electricity
   - C atom has 1 outer electron not used to form covalent bonds
   - electrons can move freely along the layers from one C atom to another
   > delocalised
   -free moving electrons act as charge carriers and allows graohite to conduct electricity

Question 18

what is a metallic bond?

Answer 18

• force of attraction between positive ions and the ‘ sea of mobile electrons’

Question 19

what are the physical properties of metals?

Answer 19

1. good conductors of electricity
2. good conductors of heat
3. high densities, melting points and boiling points
4. malleable and ductile

Question 20

why are metals good conductors of electricity?

Answer 20

• while atoms of metal - tightly packed
  > outer electrons can break away easily from the atoms
• outer electrons able to move freely within the metal lattice
  > mobile electrons allow metals to conduct electricity

Question 21

why are metals good conductors of heat?

Answer 21

• due to the movement of mobile electrons within the metal lattice
  > heat energy transferred easily by mobile electrons in structure

Question 22

why do metals have high densities, melting points and boiling points?

Answer 22

• atoms packed tightly in layers & held together by strong metallic bonds
  > large amount of energy needed to break these strong metallic bonds
• this accounts for high densities, melting pints and boiling points of metals

Question 23

why are metals malleable and ductile?

Answer 23

malleable: can be hammered into different shapes
ductile: drawn into wires without breaking
- when force applied on metal, layers of metal atoms
> can slide over each other through ‘ sea of electrons’

a) before force applied,
> metal atoms form an orderly and rigid arrangement

b) when force applies, one layer of metal ions slides over the though the ‘ sea of electrons’