Types of Substance Flashcards
All 39 cards in this deck
What are the four types of substance that elements and compounds are classified into by structure and bonding?
Ionic, simple molecular (covalent), giant covalent and metallic.
Which types of substance conduct electricity when they are solid?
Metallic substances (graphite is the giant covalent exception).
How does solubility in water usually differ between ionic compounds and simple molecular substances?
Ionic compounds are usually soluble in water; simple molecular substances are usually insoluble.
True or false? All covalent substances have low melting points.
False. Simple molecular covalent substances do, but giant covalent substances such as diamond and silicon dioxide have very high melting points.
What are the steps to classify an unknown substance from its physical properties?
e.g. melting point 801 °C, soluble in water, conducts only when molten or in solution- Melting point high → giant structure (ionic, giant covalent or metallic): 801 °C is high.
- Test conduction as a solid: it does not conduct as a solid, so not metallic.
- Conducts when molten or dissolved → ionic: the substance is ionic.
Why do ionic compounds have high melting and boiling points?
Strong electrostatic forces of attraction between the oppositely charged ions; lots of energy is needed to break them.
Why can an ionic compound not conduct electricity when solid?
The ions are held in fixed positions in the lattice, so they cannot move.
When can an ionic compound conduct electricity, and why?
When molten or in aqueous solution, because the ions are then free to move.
Why do simple molecular substances have low melting and boiling points?
The forces between the molecules (intermolecular forces) are weak, so little energy is needed to separate them.
Why do simple molecular substances not conduct electricity?
They have no free electrons or ions, so there are no charged particles free to move.
True or false? When a simple molecular substance melts, its covalent bonds are broken.
False. Only the weak intermolecular forces between molecules are overcome; the covalent bonds inside each molecule stay intact.
What is a giant covalent (macromolecular) substance?
A huge lattice of many atoms joined throughout by strong covalent bonds.
e.g. diamond, graphite, silicon dioxideDescribe the structure of diamond.
Each carbon atom is covalently bonded to four others in a rigid three-dimensional giant lattice.
Describe the structure of graphite.
Each carbon atom is covalently bonded to three others forming layers of hexagonal rings, with weak forces between the layers and one delocalised electron per atom.
Why is graphite used to make electrodes?
Each carbon has one delocalised electron that is free to move through the structure, so graphite conducts electricity (and has a very high melting point).
Why is graphite used as a lubricant?
Its layers are held together by weak forces, so they can slide over each other.
Why is diamond used in cutting tools?
It is very hard, because every carbon is held by four strong covalent bonds in a rigid 3D lattice.
Describe the structure of a C60 fullerene molecule.
60 carbon atoms joined in a hollow ball of hexagons and pentagons, each carbon covalently bonded to three others.
Why does C60 fullerene have a low melting point compared with diamond?
It is simple molecular, so only weak intermolecular forces between the C60 molecules need to be overcome.
What is graphene?
A single layer of graphite, one atom thick, of carbon atoms arranged in hexagons.
Why is graphene a good conductor of electricity?
Each carbon atom uses three of its four outer electrons in bonds, leaving one delocalised electron free to move.
Why is graphene very strong for its mass?
Its atoms are held by many strong covalent bonds throughout the layer, yet the layer is only one atom thick.
True or false? Fullerenes such as C60 are giant covalent structures.
False. Fullerenes are simple molecular — separate molecules with weak forces between them.
Describe the structure of a simple polymer such as poly(ethene).
Very large molecules made of long chains of carbon atoms.
Why do polymers have higher melting points than small molecules like ethene?
Their molecules are much larger, so the intermolecular forces between the chains are stronger and more energy is needed to separate them.
Are polymers solid or gaseous at room temperature, and why?
Solid, because the intermolecular forces between the long chains are strong enough to hold the molecules together.
True or false? Poly(ethene) conducts electricity.
False. It has no delocalised electrons or free ions, so it is an insulator.
Describe the structure and bonding in a metal.
A giant lattice of positive metal ions surrounded by a sea of delocalised electrons.
Why do metals conduct electricity?
They contain delocalised (free) electrons that can move through the structure.
Why are metals malleable?
The layers of metal ions can slide over each other without breaking the metallic bonding.
Why do metals have high melting points?
There are strong forces of attraction between the metal ions and the delocalised electrons, needing lots of energy to break.
State the typical physical properties of most metals.
Shiny solids with high melting points, high density and good electrical conductivity.
True or false? Most non-metals have low boiling points and conduct electricity poorly.
True. Most non-metals are simple molecular with weak intermolecular forces and no free charged particles.
What does a dot and cross diagram show about a substance?
Which atom each outer-shell electron has come from, and how the electrons are arranged in the bonding.
State a limitation of dot and cross diagrams.
They do not show the shape of the substance, the relative sizes of atoms or ions, or the 3D arrangement of particles.
State a limitation of ball and stick models.
The atoms are shown too far apart with gaps between them, and bonds are shown as solid sticks, which they are not.
State a limitation of two-dimensional representations of structures.
They cannot show the true three-dimensional shape and arrangement of the particles.
State a limitation of any model used to show a giant structure such as diamond or sodium chloride.
It can only show a small part of the structure, so the lattice appears to have ends when it really continues.
True or false? In a ball and stick model the sizes of the atoms and the spaces between them are drawn to scale.
False. The atoms are drawn too small and too far apart, so the scale is wrong.