1.5 Types of Substance (Edexcel GCSE Chemistry)

In an ionic solution, the movement of ions allows electrical current to flow.

False.

Simple molecular structures have low melting and boiling points.

Covalent bonds are strong bonds that hold atoms together within a molecule, while intermolecular forces are weak attractive forces that exist between different molecules.

False.

Covalent compounds are poor conductors of electricity.

Covalent compounds are generally poor conductors of electricity because they do not contain free ions or electrons to carry the current.

False.

Intermolecular forces break when a covalent substance melts or boils..

As the relative molecular mass of a substance increases, the melting and boiling points also increase due to the increased number of electrons and stronger intermolecular forces.

Simple molecular structures have low melting and boiling points due to weak intermolecular forces which require small amounts of energy to overcome.

True.

Simple molecular structures are often gases or liquids at room temperature.

Giant covalent structures are macromolecules or giant lattices that consist of a huge number of non-metal atoms with strong covalent bonds in a fixed ratio.

Allotropes are different structural forms of the same element, such as diamond and graphite for carbon.

Diamond does not conduct electricity because all of the outer shell electrons are held in the covalent bonds, and there are no freely moving delocalised electrons to carry the current.

False.

Graphite is used in pencils, as an industrial lubricant, in inert electrodes, and in locks.

Diamond is used in cutting tools.

In graphite:

• Each carbon atom is bonded to three others.

• There are hexagonal layers.

• There is one free / delocalised electron per carbon atom.

False.

Diamond is harder than graphite.

Diamond does not conduct electricity, has a high melting point, and is extremely hard.

Graphite conducts electricity and heat, has a high melting point, and is soft and slippery.

Graphite conducts electricity because it has delocalised electrons that can move / flow (throughout the structure)

Diamond is hard because its rigid, tetrahedral structure contains a lot of covalent bonds that require a large amount of energy to break.

Graphite is soft because there are weak intermolecular forces betwen the layers, which allows the layers to slide over each other.

Fullerenes are a group of carbon allotropes that consist of molecules forming hollow tubes or spheres, such as C₆₀ (buckminsterfullerene or "buckyball").

True.

Fullerenes have a high surface area.

Buckminsterfullerene is a carbon allotrope with 60 carbon atoms forming a hollow sphere, also known as a "buckyball". Each carbon atom is bonded to three others and there is one free / delocalised electron per carbon atom.

C₆₀ fullerene is a poor electrical conductor because it does not have delocalised electrons that can move and carry the charge.

True.

Graphene has a high melting point because there are lots of covalent bonds that require a large amount of energy to break.

Graphene is a good conductor of electric current because:

• Each carbon has four outer shell electrons.

• 3 of these electrons are used in bonding.

• This leaves one delocalised electron that is free to move.

Fullerenes and graphene are giant covalent structures.

True.

Fullerenes are poor conductors of electricity because there is little / no movement of electrons between molecules.

Monomers are smaller molecules that link together to form polymers.

Repeat units in polymers are connected to adjacent units via covalent bonds.

Ethene monomers bonds together to form poly(ethene).

Many polymers are solid at room temperature because the intermolecular forces between the molecules in a polymer tend to be strong.

Poly(chloroethene) is commonly known as PVC.

The completed equation is:

Polymers are very large molecules which are built up by linking together 50 or more smaller molecules called monomers

Metallic bonding is the strong electrostatic force of attraction between delocalised electrons and positively charged metal ions.

Delocalised electrons are electrons in metals that do not belong to any particular metal atom and are free to move throughout the metal lattice.

Metals can conduct electricity because they have free electrons available to move and carry charge.

Metals are malleable and ductile because the layers of positive ions can slide over one another.

True.

Alloys are created by mixing a metal with another metal or non-metal.

Metals have high melting and boiling points because they contain many strong metallic bonds, which require a lot of heat energy to overcome.

Metal structures are positive metal ions arranged in a regular pattern, with a "sea" of delocalised electrons moving freely between them.

False.

Metallic bonds are strong due to the attraction between positive metal ions and delocalised electrons.

One advantage of using the a dot and cross diagram is:

• Useful for illustrating the transfer of electrons

• Indicates from which atom the bonding electrons come from

One disadvantage of using the a dot and cross diagram is:

• Fails to illustrate the 3D arrangements of the atoms and electron shells

• Doesn’t indicate the relative sizes of the atoms

One advantage of the ball and stick model to represent a molecule of ammonia is:

• Useful for illustrating the arrangement of atoms in space

• Especially useful for visualizing the shape of a molecule

One disadvantage of the ball and stick model to represent a molecule of ammonia is:

• Fails at indicating the movement of electrons

• The atoms are placed far apart from each other, which in reality is not the case as the gaps between atoms are much smaller

One advantage of the 2D representation of a molecule of methane is:

• Displayed formulae are 2D representations and are basically simpler versions of the ball and stick model

• Adequately indicate what atoms are in a molecule and how they are connected

One disadvantage of the 2D representation of a molecule of methane is:

• Fail to illustrate the relative sizes of the atoms and bonds

• Cannot give you an idea of the shape of a molecule and what it looks like in 3D space

One disadvantage of the 3D representations of an ionic solid is:

• 3D drawings and models depict the arrangement in space of the ions

• Also show the repeating pattern in giant lattice structures

One disadvantage of the 2D representation of a molecule of methane is:

• Only illustrate the outermost layer of the compound

• Are difficult and time-consuming to draw