- The nucleus of an atom is made up of protons are neutrons.
- Protons have a positive charge, electrons have negative charge and neutrons have no charge.
- The atomic number (proton number) of an element = the number of protons in the nucleus of its atoms.
- Elements are arranged in order of their atomic numbers in the periodic table.
Arrangements of electrons in an atom
- Electrons in atoms can be represented by shells as energy levels.
- Electrons in the lowest energy level are in the shell closest to the nucleus.
- All elements in a group of the periodic table have the same number of electrons in their outer shell.
- Electrons occupy energy levels from the lowest first eg: neon with 10 electrons is 2.8.
- Noble gases are unreactive becasue they have stable arrangements of their electrons.
- Atoms of other electrons can achieve stable electronic structures by gaining or losing electrons to form ions, or by sharing electrons to form colvalent bonds.
- Group 1 lose their single outer shell electron eg: Sodium Na (2.8.1) forms sodium ions, Na+ (2.8) ---> group 7 gains one electron to form ions eg: Chlorine Cl (2.8.7) forms chloride ions Cl- (2.8.8)
- Positive and Negative Ions attract and form ionic bonds.
- Ionic Bonding holds oppositely charged ions together in giant structures.
- Strong electrostatic forces of attraction act in all directions.
- Each ion in the lattice is held together firmly by surrounding ions with the opposite charge.
- The Sodium Chloride structure contains equal numbers of sodium ions and chloride ions as shown by its formula NaCl.
- Both ions alternate to form a cubic lattice.
- The ratio of ions in the formula and structure nof an ionic compund depensd on the charge on the ions eg: Magnesium ions are Mg²+ and Chloride ions are Cl¯ so chloride needs 2 to make the formula balanced.---> It would make MgCl².
- The ratio would then be 1:2
- A covalent bond is a strong bond between two non metal atoms by sharing a pair of electrons from the outer shell.
Hydrogen and chlorine can each form one covalent bond, oxygen two bonds, nitrogen three, while carbon can form four bonds.
Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.
Atoms can form more than one covalent bond.
There is a quick way to figure out how many covalent bonds an atom has.
Group 4Group 5Group 6Group 7 example carbon nitrogen oxygen chlorine number of bonds 8 - 4 = 4 8 - 5 = 3 8 - 6 = 2 8 - 7 = 1
Hydrogen forms one covalent bond. The noble gases in Group 0 do not form any!
Covalent Bonding Continued......
Double and triple Bonds:
- A molecule of oxygen (O2) consists of two oxygen atoms held together by a double bond, like this: O=O. A molecule of nitrogen (N2) has two nitrogen atoms held together by a triple bond, like this:
Dot and Cross Diagrams:
Ionic bonds form when a metal reacts with a non-metal. Metals form positive ions; non-metals form negative ions. Ionic bonds are the electrostatic forces of attraction between oppositely charged ions.
- The oppositely charged ions are arranged in a regular pattern to form giant ionic lattices. Ionic Compounds often form crystals as a result.
Properties of Ionic Compounds:
- High Melting and Boiling Points- Ionic bonds are very strong - a lot of energy is needed to break them. So ionic compounds have high melting and boiling points.
- Conductive when liquid - Ions are charged particles, but ionic compounds can only conduct electricity if their ions are free to move. Ionic compounds do not conduct electricity when they are solid - only when dissolved in water or melted.
Covalent bonding occurs between non metal atoms. Each bonds consists of a shared pair of electrons, and is very strong. There are two main types of covalently bonded substances: 1.) Simple Molecules and 2.) Giant Covalent Structures.
- Contain only a few atoms held together by strong covalent bonds.
- An example of this could be Carbon Dioxide, where one atom of Carbon is bonded with two atoms of Oxygen.
Properties of simple molecular substances:
- Low Melting and Boiling Points - This is becasue the weak intermolecular forces break down easily.
- Non - Conductive - Substances with a simple molecular structure do not conduct electricity. This is because they do not have any free electrons or an overall electric charge.
Continued...(Higher Tier Only)
- Hydrogen, ammonia, methane and water are also simple molecules with covalent bonds.
- They all have very strong bonds between the atoms, but much weaker forces holding the molecules together.
- When one of these subtances melts or boils, it is these 'weak intermolecular forces' that break, not the strong covalent bonds.
- Simple molecular substances are gases, liquids or solids with low melting and boiling points.