SC4-SC7 Flashcards
What is an ion?
An ion is an atom or group of atoms with a positive or negative charge.
How do ions form?
- Metals atoms lose electrons to form positively charged ions.
- Non-metal atoms gain electrons to form negatively charged ions.
Give an example of how you would show ion charges.
Na+ - It has lost 1 electron in order to obtain a full outer shell and become positively charged.
How can the number of protons, neutrons and electrons in an ion be calculated?
From its atomic number, mass number and ionic charge.
What do you need to remember?
- Atomic number = protons
- Mass number = protons + neutrons
- Electrons = protons (if atom is neutral)
- Positive ions have lost electrons
- Negative ions have gained electrons
What are positively charged ions called?
Cations.
What are negatively charged ions called?
Anions.
How do cations and anions form?
When a metal reacts with a non-metal, by transferring electrons. The oppositely charged ions are strongly attracted to each other, forming ionic bonds.
What can a dot and cross diagram model?
The transfer of electrons from metal atoms to non-metal atoms. The electrons from one atoms are shown as dots, and the electrons from the other atoms are shown as crosses.
What are the ions in a solid ionic compound not?
Randomly arranged.
What do they have instead?
A regular, repeating arrangement called an ionic lattice.
Why is the lattice formed?
Because the ions attract each other and form a regular pattern with oppositely charged ions next to each other.
How is the ionic lattice held together?
By ionic b0onds.
What are ionic bonds?
Strong electrostatic forces between oppositely charged ions.
How can the physical properties of ionic compounds be explained?
By thinking about their structure and bonding.
What are ionic compounds at room temperature?
Solids.
What are melting and boiling?
State changes.
What has to be transferred to a substance in order to melt or boil it?
Energy.
What is this energy needed for?
To break the bonds between particles in the substance:
- Some bonds are overcome during melting.
- All remaining bonds are overcome during boiling.
What happens if more energy is needed?
The higher the melting or boiling point of the substance will be.
How can a substance conduct electricity?
If:
- It contains charged particles, and
- these particles are free to move from place to place.
When do ionic compounds conduct electricity?
When molten to form a liquid or dissolved in water to form an aqueous solution. This is because both processes make their ions free to move from place to place. Ionic compounds cannot conduct electricity when solid, as their ions are held in fixed positions and cannot move.
What can you use to deduce the formulae of ionic compounds?
The formulae of their ions can be used.
How do you work out the formula?
The formula for an ionic compound must contain the same number of positive and negative charges so that the charges are balanced and it is neutral overall.
What is a polyatomic ion?
Charged particle consisting of 2 or more atoms joined together.
What do the names of ionic compounds end in?
- -ide if it contains just 2 elements.
- -ate if it contains 3 or more elements, one of which is oxygen.
What is a covalent bond?
When a pair of electrons is shared between 2 atoms, usually non-metals.
What is a molecule?
A molecule consists of a group of two or more atoms joined together by covalent bonds. Molecules of the same element or compound will have a set size.
What can be used to model a simple molecule?
A dot and cross diagram can model the bonding in a simple molecule:
- The outer shell of each atom is drawn as a circle
- Circles overlap where there is a covalent bond
- Electrons from 1 atom are drawn as dots, and electrons from another atom are drawn as crosses.
How do atoms form covalent bonds?
By sharing electrons to get a full outer shell.
How do you work out how many circles to draw for a simple molecular substance?
Look at the formula. For example, the formula for ammonia is NH3. For this, draw 4 circles, 1 labelled N and 3 labelled H. Each of the 3 H circles overlaps the N circle.
Nitrogen is in group 5 so it forms 3 covalent bonds. There are 3 shared spaces between the circles, so add a dot and cross to each one.
What do you finally do?
Add in the non-bonding outer electrons. Nitrogen atoms have 5 outer electrons. Three of these are shared, leaving 2 non-bonding electrons. Add these to the diagram to complete the diagram (hydrogen atoms only have 1 electron, so they have no non-bonding electrons).
What do some molecules contain?
A double bond, which consists of 2 shared pairs of electrons. For example, oxygen molecules contain of 2 oxygen atoms joined together. Oxygen atoms can form 2 covalent bonds, so to link the 2 oxygen atoms together, a double bond forms.
What do simple molecular substances generally have?
Low melting and boiling points and are often liquids or gases at room temperature.
What forces are there between simple molecules?
Intermolecular forces.
What are these intermolecular forces much weaker than?
The strong covalent bonds in molecules. When simple, molecular substances melt or boil, it is these weak intermolecular forces that are overcome. The covalent bonds are not broken. Very little energy is needed to overcome the intermolecular forces, so simple molecular substances usually have low melting and boiling points.
What do simple molecules not have?
An overall charge, or charged particles that can separate, so simple molecular substances cannot conduct electricity, even when liquid or dissolved in water.
What does covalent bonding lead to?
The formation of molecules. These can be:
- Simple molecules, which contain a set number of atoms joined by covalent bonds.
- Giant covalent substances, which contain many atoms joined by covalent bonds.
What is the main compound found in sand?
Silica. It is an example of a giant covalent substance. It contains many silicon and oxygen atoms. These are joined together by covalent bonds in a regular arrangement, forming a giant covalent network or lattice structure. There is no set number of atoms joined together in this type of structure, so these covalent lattices are not classed as molecules. However, the atoms in the compound will be present in the ratio indicated by the chemical formula.
What are giant covalent substances at room temperature and what do they have?
Solids and they have very high melting points and boiling points. Covalent bonds are strong, so a lot of energy is needed to break up these large structures during melting and boiling.
What do giant covalent substances not have?
An overall charge, so they cannot conduct electricity. Graphite, a form of carbon, which can conduct electricity is an exception.
What can a substance do in water if it forms strong enough attractions with water molecules?
Dissolve.
Why are giant covalent substances insoluble in water?
Because they cannot form these strong attractions with water.
What are diamond and graphite different forms of?
The element carbon.
What do both diamond and graphite consist of?
Giant covalent network structures of carbon atoms, joined together by covalent bonds. However the shape of their structures and their properties are different.
What is diamond?
Diamond is a giant covalent substance in which:
- Each carbo atom is joined to 4 other carbon atoms by covalent bonds.
- The carbon atoms form a regular tetrahedral network.
- There are no free electrons.
What are the properties and uses of diamond?
The rigid network of carbon atoms, held together by strong covalent bonds, makes diamond very hard. This makes it useful for cutting tools, such as diamond-tipped glass cutters and oil rig drills.
What is graphite?
Graphite is a giant covalent substance in which:
- Each carbon atoms is joined to 3 other carbon atoms by covalent bonds.
- The carbon atoms form a hexagonal layered network structure.
- The layers have weak forces between them and can slide over each other.
- Each carbon atoms has one un-bonded outer electron.
- These un-bonded electrons are delocalised (electrons that are not associated with a particular atom), and are free to move.
What are the properties and uses of graphite?
Delocalised electrons are free to move through the structure of graphite, so graphite can conduct electricity. This makes it useful for electrodes (a conductor used to establish electrical contact with a circuit) in batteries and for electrolysis.
What can the layers in graphite do?
Slide over each other because the forces between them are weak. This makes graphite slippery, so it is useful as a lubricant.
What is graphene?
Another form of carbon. Its structure resembles a single layer of graphite. Graphene has a very high melting point and is very strong because of its large regular arrangement of carbon atoms joined by covalent bonds. Like graphite it conducts electricity well because it has delocalised electrons that are free to move across its surface.
What is a fullerene?
A molecular form of the element carbon. 2 examples of fullerenes are nanotubes and buckyballs.
What is a nanotube?
A nanotube resembles a layer of graphene, rolled into a tube shape. Nanotubes have high tensile strength, so they are strong in tension and resist being stretched. Like graphene, nanotubes are strong and conduct electricity because they have delocalised electrons.
What are buckyballs?
Buckyballs are spheres or squashed spheres of carbon atoms. They are made of large molecules so are not classed as giant covalent networks. Weak intermolecular forces exist between buckyballs. These need little energy to overcome, so substances consisting of buckyballs are slippery and have lower melting points than graphite or diamond.
What do simple polymers consist of?
Large molecules that contain chains of carbon atoms of no net size.
