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Evaluate Giant Ionic Structures

In this worksheet, students will learn about the structure and properties of giant ionic compounds, and compare different ways of representing their structures.

Worksheet Overview

QUESTION 1 of 10

Once we know how atoms bond together in compounds, we can understand why the compounds have the properties they do. In this activity, you will learn what ionic compounds are like, and why.

In ionic compounds, metal atoms lose electrons to become positively charged ions, and non-metal atoms gain electrons to become negatively charged ions. Positive and negative charges attract each other, so we get ionic bonds holding the ions together. Positive attracts negative, negative attracts positive, and so on. The only thing that stops the pattern is when you run out of atoms. Since the structure can have many many millions of atoms in it, we call it a giant structure. Giant ionic structures are crystalline, because they have a repeating pattern.

In this picture, there are equal numbers of purple Na and green Cl ions, so we can tell that the formula of the structure is NaCl. In ZnS, there equal numbers of Zn (blue) ions and S (yellow) ions. The ions aren't really coloured, but it helps us imagine the structure.

 

Different people draw the same structures in slightly different ways. In a space-filling diagram (like the NaCl picture), we have a realistic picture of the size of the different ions and how they bond. Unfortunately, it's hard to see all the atoms, because some are hidden. In a ball-and-stick model, we shrink the atoms (the balls) and link them by bonds (the sticks). We can then see more of the atoms more clearly through the gaps. We can also clearly see where pairs of atoms are linked (bonded). To really visualise the structure, it's even better to look at a model or a computer animation.

We can work out the composition of a compound by counting atoms. In this picture of CaTiO3, there are four green Ca ions, four grey Ti ions and 12 red O ions. Instead of writing Ca4Ti4O12, we cancel through by four to get CaTiO3.

All ionic compounds have these properties:
The have high melting and boiling temperatures. This is because there are strong ionic bonds in all directions. To break these bonds, large amounts of energy is needed, which comes from high temperatures.
Most ionic compounds dissolve in water. This happens because water weakens the electrostatic bonds holding positive and negative ions together.
All ionic compounds are insulators (they don't conduct electricity). Although the ions carry charge, their positions are fixed in the crystal structure, so they can't move.
Molten ionic compounds can conduct electricity, because the ions in a liquid can move. When they move, they carry electrical charge with them.
If ionic compounds dissolve in water, the resulting solution also conducts electricity. 

Not all ionic compounds have exactly the same properties. The bond between Na+ and Cl- only involves one electron. It is a lot weaker than the bond between Mg2+ and O2-. As a result, the melting point for MgO is much higher than for NaCl, even though the patterns of ions and bonds are the same.

When you think about giant ionic structures, remember that they are made of positive and negative ions. The properties of giant ionic structures work because there are strong forces between the ions, which stop ions moving. However, we can use water to weaken those bonds, which makes a solution where the ions move around easily in the water.

Pick words to complete this.

Metal ions make...

 

What is meant by a giant structure?

it has many atoms

atoms are in a repeating pattern

it is made of ions

Match these half sentences

Column A

Column B

Giant ionic structures have high melting temperatu...
because water weakens the ionic bonds.
Giant ionic structures are electrical insulators
because ions carry charge but cannot move
Many giant ionic structures dissolve in water
because large amounts of energy are needed to brea...

Which is the best explanation of why giant ionic structures have high melting temperatures?

There are strong bonds

There are stronger bonds in some directions than others

There are strong bonds in all directions

Underline the incorrect sentence.

Sodium chloride and magnesium oxide are both ionic compounds. To form magnesium oxide, two electrons need to move to the oxygen. Because two electrons move, magnesium oxide is a good conductor of electricity.

A grain of salt is about 2 mm x 2 mm x 2 mm. Estimate how many ions are in that grain of salt. (Two of the answers in the list are correct).

1018

1000000

1000000000000

106

1000000000000000000

1012

Suppose you have a substance which has a high melting point, does not conduct electricity as a solid, and does not dissolve in water. Could it be an ionic compound?

yes

no

Which of these will conduct electricity?

pure water

sea water

Mark these sentences about ionic compounds by ticking the correct ones.

All ionic compounds dissolve in water.

When this happens, the fixed ions become able to move.

This means that solutions of ionic compounds can conduct electricity.

Solid ionic compounds can conduct electricity as well.

Imagine you have samples of two ionic compounds, labeled A and B. You know that one of them is KF, and the other is CaO, but you don't know which one is which. Sample A melts at about 2500 C, and sample B melts at about 850 C. Which sample is CaO?

A

B

  • Question 1

Pick words to complete this.

Metal ions make...

 

CORRECT ANSWER
EDDIE SAYS
The electrons are negatively charged, so when a metal loses electrons, the remaining ion is positively charged.
  • Question 2

What is meant by a giant structure?

CORRECT ANSWER
it has many atoms
atoms are in a repeating pattern
EDDIE SAYS
Giant structures have many atoms in a repeating pattern. They can be ionic, but don\'t have to be.
  • Question 3

Match these half sentences

CORRECT ANSWER

Column A

Column B

Giant ionic structures have high ...
because large amounts of energy a...
Giant ionic structures are electr...
because ions carry charge but can...
Many giant ionic structures disso...
because water weakens the ionic b...
EDDIE SAYS
The key thing for all these ideas is that the bonds between the ions are very strong. That explains the melting temperature and the inability of giant ionic structures to conduct electricity.
  • Question 4

Which is the best explanation of why giant ionic structures have high melting temperatures?

CORRECT ANSWER
There are strong bonds in all directions
EDDIE SAYS
If there are any weak bonds in a structure, they are the ones which will break most easily, and that will reduce the melting point.
  • Question 5

Underline the incorrect sentence.

CORRECT ANSWER
Sodium chloride and magnesium oxide are both ionic compounds. To form magnesium oxide, two electrons need to move to the oxygen. Because two electrons move, magnesium oxide is a good conductor of electricity.
EDDIE SAYS
Electrons do move between ions in ionic substances, but once they get to their new ion, they are fixed, so there isn\'t anything that can conduct electricity.
  • Question 6

A grain of salt is about 2 mm x 2 mm x 2 mm. Estimate how many ions are in that grain of salt. (Two of the answers in the list are correct).

CORRECT ANSWER
1018
1000000000000000000
EDDIE SAYS
The radius of an atom is about 1 nm, so its diameter is about 2 nm. 2 mm ÷ 2 nm = 1000000, so there are about a million atoms along one edge of the grain of salt. So in the cube, there are 1000000 x 1000000 x 1000000 = 1018 atoms.
  • Question 7

Suppose you have a substance which has a high melting point, does not conduct electricity as a solid, and does not dissolve in water. Could it be an ionic compound?

CORRECT ANSWER
yes
EDDIE SAYS
Many ionic substances do dissolve in water, but not all of them
  • Question 8

Which of these will conduct electricity?

CORRECT ANSWER
sea water
EDDIE SAYS
Sea water has sodium chloride dissolved in it; the mobile ions are able to conduct electricity, but the water itself can\'t.
  • Question 9

Mark these sentences about ionic compounds by ticking the correct ones.

CORRECT ANSWER
When this happens, the fixed ions become able to move.
This means that solutions of ionic compounds can conduct electricity.
EDDIE SAYS
Many ionic substances dissolve in water, but not all of them. Solid ionic substances can't conduct electricity, because the ions can't move.
  • Question 10

Imagine you have samples of two ionic compounds, labeled A and B. You know that one of them is KF, and the other is CaO, but you don't know which one is which. Sample A melts at about 2500 C, and sample B melts at about 850 C. Which sample is CaO?

CORRECT ANSWER
A
EDDIE SAYS
Calcium makes +2 ions, and oxygen makes -2 ions. Lithium makes +1 ions, and fluorine makes -1 ions. That means that the Ca-O bond is much stronger than Li-F, so the melting point of CaO will be higher than that of LiF.
---- OR ----

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