Mass Number = Protons + Neutrons
Atomic Number = Protons or Electrons
Same element has the same no. of protons.
- Compounds are formed when atoms of two or more elements are chemically combined together.
- It's diificult to seperate the teo original elements out again.
- Different atomic forms of the same element, which have the same number of protons but a different number of neutrons
- Same atomic number, different mass number.
- E.g. Carbon-12 and Carbon-14
- Ionic Bonding is the transfering of electrons.
- Atom lose or gain electrons to form charged particle ions, which are then strongly attracted to one another becasue of the attraction of opposite charges.
Reaction of sodium and chlorine:
- Form giant regular ionic lattices
- They have very strong electrostatic forces of attraction between oppositely charged ions, in all directions.
- Example: A single crystal of sodium chloride (salt) is one giant ionic lattice, which is why salt crystals tend to be cuboid in shape. The Na+ and Cl- ions are held together in regular lattices
- High melting/boiling points due to the strong attraction between the ions. It takes a large amount of energy to overcome this attraction.
- When ionic compounds melt, the ions are free to move and they'll carry an electric current.
- Dissolve easily in water. The ions seperate and are all free to move in the solution, so they'll carry an electric current.
- Ions are atoms that have gained or lost electrons.
- Ions have the electronic structure of a noble gas.
- Elements in groups 1,2,6 & 7 are most ready to form ions.
- The charge of the positive ions is the same as the group number.
- Groups 1 and 2 elements lose electron to form positive ions.
- Groups 6 and 7 elements gain electrons to form negative ions.
- You can use the charge on the individual ions present to work out the formula for the ionic compound:
- charge must be balanced/equal zero
- Sodium Chloride Na+ and Cl- ions
- Magnesium Chloride Mg2+ and Cl- ions = MgCl2
Drawing Electronic Structures
Use a square bracket
+ or - to show the charge.
- Covalent Bonds share pairs of electrons.
- Atoms share electrons so they have a full outer shell.
- Full outer shell gives an electronic structure of a noble gas.
- Each covalent bond provide one extra shared electron from each atom
- Each atom involved has to make enough covalent bonds to fill up its outer shell.
- Substances with covalent bonds can either be simple molecular or giant structure.
- Covalent bonds are very strong
- The atoms form very tong covalent bond to form small molecules of several atoms
- The forces of attraction between these molecules are very weak.
- Weak intermolecular forces mean very low melting/boiling points, because the molecules are easily parted from each other.
- Most are gases or liquids at room temperature, but can be solids.
- They don't conduct electricity as there are no ions so there is no electric charge.
- Example: H2, Cl2, O2, HCl, H2O, NH4 and CH4
Giant Covalent Structure
- Giant Covalent Structures are similar to giant ionic lattices except they have no charged ions.
- All atoms form very strong covalent bonds.
- These very strong covalent bonds mean they have very high melting/boiling points.
- They don't conduct electricity, except graphite.
- The main macromolecules are Dimaond and graphite, made from carbon, and silicon dioxide (silica)
Diamond: Each carbon atom form four covalent bonds in a very rigid giant covalent structure. This structure makes it the hardest natural substance, so it is used for drill tips.
Silicon Dioxide (Silica): This is what sand is made of. Each grain of sand is one giant covalent structure of silicon and oxygen.
Graphite: Each carbon atom forms three covalent bonds. This creates layes which are free to slide over each other, so graphite is soft and slippery. The layers are only held together by weak-intermolecular forces. Each carbon atom has one delocolised electron, which can conduct heat and electricity, which is why grpahite is the only non-metal which is a good conductor of heat and electricity.
Metals consist of giant structures of atoms arranged in a regular pattern.
The electrons in the higherst occupied energy levels of the metal atoms are delocalised and so free to move through the whole structure.
These delocolised electrons in their structures:
- Hold the atoms throughout the metal together in a regular structure, as there are strong forces of electrostatic attraction between the positive metal ions and the negative electrons.
- Make metals good conductors of heat and electricity, as conduction depends on the ability of electrons to move freely.
Alloys are made from two or more different metals
The different sized atoms of the metal distort the layers in the structure of a metal, making it difficult for them to slide over each other and so make alloys harder than pure metals.