Types of Substance Flashcards
All 32 cards in this deck
What are the four types of structure that elements and compounds are classified into?
Ionic, simple molecular (covalent), giant covalent and metallic.
What are the steps to classify a substance from its physical properties?
e.g. melting point 801 °C, does not conduct as a solid, conducts when molten- Check the melting point: very high, so it is a giant structure.
- Check conduction as a solid: does not conduct, so not metallic (or graphite).
- Check conduction when molten or dissolved: conducts, so it is ionic.
Why do simple molecular substances not conduct electricity?
They have no delocalised electrons and no ions, so there are no charged particles free to move and carry the current.
True or false? Silicon dioxide is a simple molecular substance.
False. Silicon dioxide is giant covalent, with strong covalent bonds and very high melting and boiling points.
Why do ionic compounds have high melting and boiling points?
There are strong electrostatic forces of attraction between the oppositely charged ions, and a lot of energy is needed to overcome them.
When can an ionic compound conduct electricity, and why?
When molten or dissolved in water, because the ions are then free to move; in the solid they are fixed in the lattice.
Why do simple molecular substances have low melting and boiling points?
The intermolecular forces between the molecules are weak, so little energy is needed to overcome them.
True or false? When a simple molecular substance melts or boils, the covalent bonds between its atoms are broken.
False. Only the weak forces between the molecules are overcome; the covalent bonds inside the molecules stay intact.
Describe the structure of diamond.
A giant covalent lattice in which each carbon atom is joined to four other carbon atoms by strong covalent bonds in a 3D (tetrahedral) arrangement.
Describe the structure of graphite.
A giant covalent lattice of layers; each carbon is joined to three others by strong covalent bonds, with weak forces between the layers and one delocalised electron per carbon.
Why is graphite used as a lubricant?
The forces between its layers are weak, so the layers can slide over each other.
Why is graphite used to make electrodes?
It has delocalised electrons between its layers that can move to carry the current.
Why is diamond used in cutting tools?
Strong covalent bonds join all the carbon atoms in a 3D giant lattice, needing a very large amount of energy to overcome, so diamond is very hard.
Why does diamond not conduct electricity?
All four outer-shell electrons of each carbon atom are used in covalent bonds, so there are no free (delocalised) electrons.
Describe the structure of graphene.
A giant covalent lattice one atom thick, in which each carbon atom is joined to three other carbon atoms by strong covalent bonds.
Why is graphene a good conductor of electricity?
Carbon has four outer-shell electrons but each carbon forms only three bonds, so one electron per carbon is delocalised and free to move.
Why does C₆₀ fullerene have a much lower melting/sublimation point than graphene or diamond?
Fullerene has a molecular structure with weak forces between the molecules, so little energy is needed to separate them.
True or false? C₆₀ fullerene conducts electricity as well as graphene does.
False. C₆₀ has delocalised electrons, but they cannot easily move from one molecule to the next, so solid C₆₀ is a poor conductor.
Describe the structure of poly(ethene).
Large molecules consisting of long chains of carbon atoms.
True or false? The carbon atoms in a poly(ethene) chain are joined by covalent bonds.
True. Each carbon atom in the chain is covalently bonded to the next.
Describe the structure of a metal.
A giant lattice of positive metal ions surrounded by a sea of delocalised electrons.
Why do metals conduct electricity?
Their delocalised electrons are free to move through the metallic lattice and carry the current.
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 is strong electrostatic attraction between the positive metal ions and the delocalised electrons, which takes a lot of energy to overcome.
How do the general physical properties of most metals and most non-metals differ?
Most metals are shiny solids with high melting points, high density and good electrical conductivity; most non-metals have low boiling points and are poor conductors.
What is a limitation of a dot and cross diagram?
It shows only the outer electrons in bonding — not the shape of the substance, the relative sizes of the atoms or how the particles are arranged in space.
What is a limitation of a ball and stick model?
Atoms are not solid coloured balls and bonds are not rigid sticks; the spaces between the atoms are shown far too large.
What is the main limitation of a two-dimensional representation of a structure?
It cannot show the true three-dimensional shape or arrangement of the particles.
True or false? In a real substance the electrons drawn as dots can be told apart from those drawn as crosses.
False. All electrons are identical; dots and crosses are only used to show which atom each electron came from.
True or false? Giant covalent substances such as diamond, graphite and silicon dioxide dissolve in water.
False. Giant covalent substances are insoluble in water.
Describe the structure of buckminsterfullerene, C₆₀.
A simple molecule of 60 carbon atoms joined by covalent bonds into a hollow ball (sphere).
What state are most simple molecular substances in at room temperature?
Gases or liquids, because the weak intermolecular forces give them boiling or melting points below room temperature.
e.g. methane and carbon dioxide are gases; water is a liquid.