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Analyse Simple and Giant Covalent Structures

Worksheet Overview

QUESTION 1 of 10

Ionic and metallic bonding each only make one type of structure. Covalent bonding is a bit harder, because there are two types of structure that form. Don't panic, because you will soon understand both of them.

Most covalent compounds form simple structures, also called simple molecules, or simple molecular structures. Simple molecules are tiny; many of them only contain two or three atoms. The forces between the atoms in each molecule are made by covalent bonds, so they are very strong. Scientists call these "intramolecular forces" ("intra" at the beginning of a word means "inside").

 

The forces between separate molecules are very very weak. Scientists call these forces "intermolecular forces" ("inter" at the beginning of
a word means "between"- think of international, or internet). That means that the molecules in oxygen gas look like this:

The properties of simple covalent structures depend on the weak intermolecular forces; it doesn't really matter that the forces within the molecule are strong. Simple covalent molecules:
have very low melting and boiling points (oxygen turns from a solid to a liquid at -219 °C, and from a liquid to a gas at -183 °C). Larger molecules have higher boiling points, but still lower than for ionic or metallic bonding.
are usually gases or liquids at room temperature.
dissolve easily  in water
do not conduct electricity, because there are no mobile electrons or ions.

Although most covalent substances make simple molecules, a few make giant structures- more like the structures formed by metallic or ionic bonding. The important point about covalent giant structures is that all the bonds linking atoms are covalent, so all the bonds are very strong indeed. Giant covalent structures are usually based on carbon or silicon. The reason for this is that they can make four covalent bonds per atom, which is more than most other elements. You need four covalent bonds to make a giant structure with bonds in three dimensions. Diamond is a giant covalent structure made of carbon atoms:

This means that covalent giant structures:
are very hard, because the covalent bonds are very strong.
have very high melting temperatures, because huge amounts of energy are needed to break covalent bonds.
do not dissolve in water, because water cannot weaken covalent bonds.
do not conduct electricity, because there are no mobile electrons or ions.

The key difference between molecules and giant structures is that giant structures only have covalent bonds. To make a liquid or solid from molecules, we need other (very weak) forces to hold the molecules together.

Complete the gaps in this paragraph. Use these words:

covalent

intermolecular

low

small

strong

weak

Match the properties with their reasons

Column A

Column B

Electrical insulator
because there are weak forces between molecules
Low melting temperature
because there are no mobile charged particles.
High melting temperature
is not true.
Which of these chemicals would you expect to be simple molecules?
C3H8

O2

NaCl

Mg

CO2

C

Match these descriptions with the likely types of bonding:

Column A

Column B

A gas
covalent giant structure
A solid which dissolves in water
ionic
A solid which doesn't dissolve in water
simple molecular

These compounds all have simple molecular structures:

Compound Formula Melting temperature (ºC)
propane C3H8 -190
butane C4H10 -138
pentane C5H12  
hexane C6H14 -95
heptane C7H16 -91
 
Which of these statements is true?
Larger molecules have higher melting temperatures

All of these substances are solid at room temperature

A graph of melting temperature against molecule size would be a straight line

The melting temperature of pentane is -117 ºC

Look at this chain of ideas. Highlight the sentence where it goes wrong.

As the size of the molecule increases, because the intermolecular forces become stronger. This means that it is harder to pull molecules apart. As a result, the boiling temperature decreases.

Complete the gaps in this paragraph. Use these words; you might need to use some more than once, and some not at all.

all

covalent

giant

higher

intramolecular

intermolecular

melt

no

weak

 

 

As the size of the molecule increases, because the intermolecular forces become stronger. This means that it is harder to pull molecules apart. As a result, the boiling temperature decreases.

Which Group of the Periodic Table contains the elements which form giant covalent structures?

Which of these experiments would let you tell if a substance is made of simple molecules or a giant structure?

Measuring its melting temperature

Measuring its electrical conductivity

Testing its solubility in water

Observing its colour

What do you expect the chemical formula and properties of germanium oxide to be? (Hint: check where germanium is on the periodic table before you start.)

Measuring its melting temperature

Measuring its electrical conductivity

Testing its solubility in water

Observing its colour

  • Question 1

Complete the gaps in this paragraph. Use these words:

covalent

intermolecular

low

small

strong

weak

CORRECT ANSWER
EDDIE SAYS
The confusing thing about molecular solid is the way that the intramolecular bonds are strong, but the intermolecular bonds are weak. The weak parts of the structure mean that the strength and melting point are both low.
  • Question 2

Match the properties with their reasons

CORRECT ANSWER

Column A

Column B

Electrical insulator
because there are no mobile charg...
Low melting temperature
because there are weak forces bet...
High melting temperature
is not true.
EDDIE SAYS
The melting temperature of materials is linked to how strong the bonds are. Electrical conductivity depends on how easily it is for charged particles(electrons or ions) to move.
  • Question 3
Which of these chemicals would you expect to be simple molecules?
CORRECT ANSWER
C3H8
O2
CO2
EDDIE SAYS
You can't tell a molecular compound just by looking at the numbers in the formula. For giant structures (like ionic NaCl, or covalent C) we cancel the numbers of atoms down to their ratio, so the numbers are still small. Most covalent stubstances are molecular; the main exceptions are C and SiO2.
  • Question 4
Match these descriptions with the likely types of bonding:
CORRECT ANSWER

Column A

Column B

A gas
simple molecular
A solid which dissolves in water
ionic
A solid which doesn't dissolve in...
covalent giant structure
EDDIE SAYS
This classification isn't foolproof (at the top grades, you will get messy examples to think about!). Some ionic compounds don't dissolve in water, but no giant covalent structures do, because covalent bonds are too strong.
  • Question 5

These compounds all have simple molecular structures:

Compound Formula Melting temperature (ºC)
propane C3H8 -190
butane C4H10 -138
pentane C5H12  
hexane C6H14 -95
heptane C7H16 -91
 
Which of these statements is true?
CORRECT ANSWER
Larger molecules have higher melting temperatures
The melting temperature of pentane is -117 ºC
EDDIE SAYS
Although larger molecules have higher melting temperatures, the pattern isn't a straight or smooth line. Taking the midpoint of -138 ºC and -95 ºC is a reasonable first guess, but it isn't right (the actual melting temperature for pentane is -130 ºC)
  • Question 6

Look at this chain of ideas. Highlight the sentence where it goes wrong.

CORRECT ANSWER
As the size of the molecule increases, because the intermolecular forces become stronger. This means that it is harder to pull molecules apart. As a result, the boiling temperature decreases.
EDDIE SAYS
This is a nicely structured paragraph, except that if it's harder to pull molecules apart, the boiling temperature increases. Each sentence has one idea, and a link to what happened in the previous sentence. This is good writing, because you can more easily see any mistakes and correct them.
  • Question 7

Complete the gaps in this paragraph. Use these words; you might need to use some more than once, and some not at all.

all

covalent

giant

higher

intramolecular

intermolecular

melt

no

weak

 

 

CORRECT ANSWER
EDDIE SAYS
Melting temperatures depend on the weakest links in the structure. That is why molecular solids (with very weak bonds between molecules) melt much more easily than giant structures.
  • Question 8

Which Group of the Periodic Table contains the elements which form giant covalent structures?

CORRECT ANSWER
4
four
EDDIE SAYS
The main elements to look for are carbon and silicon. Germanium is also in Group 4, and also makes a giant structure, but is less common. The key thing about these elements is that they all make four covalent bonds.
  • Question 9

Which of these experiments would let you tell if a substance is made of simple molecules or a giant structure?

CORRECT ANSWER
Measuring its melting temperature
Testing its solubility in water
EDDIE SAYS
Melting temperature would work, because giant structures have a much higher melting temperature than simple molecules. Solubility is also valid- giant structures don't dissolve in water. Neither type of covalent substance conducts electricity, and colour wouldn't tell us much.
  • Question 10

What do you expect the chemical formula and properties of germanium oxide to be? (Hint: check where germanium is on the periodic table before you start.)

CORRECT ANSWER
EDDIE SAYS
Ge is in group 4, so it makes 4 covalent bonds- like silicon. The formula will be GeO2 in he same way that silicon makes SiO2. Because it makes a giant covalent structure, it has similar properties to other giant covalent structures.
---- OR ----

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