SC2 - Structure and Bonding Flashcards
How are ions formed?
Atoms are more stable (less reactive) if they have a full outer shell. In order to get a full outer shell, atoms either gain or lose electrons by transferring them to other atoms. They have become ions (atoms with a charge due to uneven numbers of protons and electrons).
What is an ionic bond?
There are forces between all positively and negatively charged objects (electrostatic forces of attraction). They hold oppositely charged ions together, forming an ionic bond.
What structure do ionic bonds form?
Giant ionic lattice - ionic bonds are very strong so they allow billions of ions to be packed together in a regular repeating pattern called a lattice. Ionic compounds often form crystals because of this regular lattice structure.
How do you work out the formula of an ionic compound?
Work out the charges of the ions involved by looking at their group on the periodic table (which tells you the number of electrons on the outer shell), then use the crossover rule.
Calcium chloride CaCl2
Ca Cl
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2+ 1-
What are polyatomic ions?
Polyatomic ions are ions which contain more than one atom (e.g. sulphate ions SO4 2-). If an ionic formula contains two or more of the same polyatomic ions then the formula of them must be written inside brackets Al2(SO4)3
Properties of ionic compounds.
- Form crystals due to regular pattern of lattice
- high melting and boiling points due to strong electrostatic forces of attraction
- do not conduct electricity when solid because negative ions are fixed in place so can’t move to conduct electricity, but they can conduct electricity when molten or in aqueous solution because the negative ions can move, so can carry the charge.
- Most are soluble in water
How are covalent bonds formed?
Covalent bonds are formed between two non-metals and are produced by sharing pairs of electrons, so it is as if both atoms have a full outer shell. You can have single, double or even triple covalent bonds.
Describe the bonds in a simple molecular substance.
The bonds between atoms are strong covalent bonds (strong because of electrostatic forces between positive nuclei and negative electrons), but the bonds between molecules are weak (weak intermolecular forces).
What is the valency of an element?
The number of electrons needed to compete the outer shell (e.g. oxygen’s valency is 2). The valency also equals the number of bonds an atom forms (e.g. carbon’s valency is 4, so it always forms 4 bonds).
Properties of simple molecular compounds.
- low melting and boiling points due to weak intermolecular forces not requiring much energy to break.
- do not conduct electricity as the electrons are held in place so cannot flow.
- Slightly soluble in water
- often exist as liquids or gases at room temperature
How can bonds be represented in diagrams?
Dot - cross diagram - S- shows how electrons are shared, W- doesn’t show the structure formed, suggest the electrons on different atoms are different, when they are the same.
Metallic model - S- shows how the metal ions are held together, does not show that the metal ions would be vibrating all the time
Ball and stick model - S- shows which atoms are joined together and the structure, W- shows the atoms as too far apart
3D space filling model, S- shows which atoms are bonded together, W- does not shows how the electrons are bonded.
Describe the structure and properties of polymers.
Monomers are small, simple molecules which can be joined in a chain to form a polymer. Polymer molecules can have different lengths, longer polymers have more intermolecular forces and tend to get tangled up more, so generally have higher melting and boiling points than shorter polymers.
What is an allotrope?
A different structural form of the same element.
What are fullerenes and describe their uses?
Fullerenes are an allotrope of carbon. They often form nanotubes or spheres. The molecules are very strong due to covalent bonds, but they have low melting points due to weak intermolecular forces. Uses include drug delivery, catalysts, technology/electrics and strengthening materials.
What is graphene and describe its uses?
Graphene is a single layer of graphite with each carbon atom bonded to 3 others. It can be found as a sheet or rolled into a tube, it is just one atom thick, so is the lightest known material, but is extremely strong due to covalent bonds. It has a high melting point and can conduct electricity (due to one delocalised electron per carbon atom being free to move). Uses include touch screen layers.
