- All atoms contain same number of protons and electrons.
- When an atom gains or loses an atom it is a charged ion.
- The atomic number is the number of electrons and protons.
- The mass number is the total number of protons and neutrons.
- Protons - posotive charge - mass of 1
- Neutrons - no charge - mass of 1
- Electrons - negative charge - mass of 0
Isotopes - isotopes of an element have the same number of protns but a different number of neutrons. So isotopes have the same atomic number but a different mass number.
Electrons occupy the lowest available shell, this is the one closest to the nucleus. The first shell may contain up to two electrons. The second and third shells may contain up to eight electrons. The electron structure shows how the atom will react.
IONIC AND COVALENT BONDING
All atoms wish to have a full shell.
Ionic bonding involved the transfer of electrons from one atom to another. metals in groups 1 and 2 such as sodium and calcium lose negative electrons to get a full outer shell. Overall they become positively charged.
Non-metals in Group 6 and 7 such as oxygen and chlorine gain negative electrons to fain a full outer shell. So overall the become negatively charged.
Opposite charged attract, this attraction holds the ions together and form an IONIC BOND.
Crystals are formed when millions of these ions bond together in a giant ionic structure.
Covalent bonding occurs between non-metals. The atoms share elctrons in the bond. Covalent bonding allows both atoms to feel that they have a stable, full outer shell.
Each bond consists of a shared pair of electrons with each atom contributing one electron. Covalent bonds creature two types of molecules:
- Simple Molecules
Two atoms of hydrogen will share their outer electron so that both atoms have a stable arrangement and form a covalent bond.
In the molecule of hydrogen each atom now has two electrons in the outer hell like and atom of Helium.
When two atoms bond covalently in this way they form a DIATOMIC molecule.
Diamond and Graphite are examples
Many substances consist of small molecules held together by covalent bonds. However some substances have GIANT structures where huge number of atoms are held together by a network of covalent bonds. Diamond is an example of a giant covalent structure where every carbon atom is covalently bonded to four other carbon atoms. This produces a rigid covalent lattice and this gives diamond its hardness.
Giant Ionic - high melting point - doesnt conduct electricity when solid but do when molton or dissolved in water - example. Sodium Chloride
Giant Covalent - high melting point - doesnt conduct electricity - example. Diamond
Simple Molecular - low melting point - doesnt conduct electricity - example. Water
Giant Metallic - high melting point - it conducts electricity (free electrons) - example. Zinc
An ionic compound has a giant regular structure of ions called a lattice. Ionic compounds are held together by electrostatic forces of attraction, between oppositely charged ions, which act in all directions.
There is a lot of energy needed to break the lattice apart because the bond is so strong. As a result ionic compound a high boiling and melting point.
- Do not conduct electricity when solid
- Conduct electricity when molten as a the high temperature to melt them provides energy to overcome the strong attractive forces between the ions. The ions are then free to move and will conduct electricity.
- Conduct electricity when dissolved in water, as the water splits the lattice, so the ions are free to move.
Giant covalent substances
- Have a large network of bonds
- Examples: Silicon Dioxide, Graphite, Diamond
- Giant latices & strong covalent bonds
- Very hard, high melting and boiling points.
- Chemically very non-reactive
In graphite : each carbon atom bonds to three others, forming layers.
- Weak forces between the layers
- Easily slide over each other
- Soft and slippery
- Free electrons within its structure
- de-localised electrons allow graphite to conduct heat and electricity
- very strong bonds
- Substances that consists of SM - H2, Cl2, HCl, H2O and CH4
- ^^^^^^^^ have only weak forces between molecules (intermolecular forces)
- It is these very weak intermolecular forces which are overcome when simple molecules melt or boil.
- Overcoming does not require much energy
- Low melting and boiling points
- Usually gases and liquid or solids with very low melting points
- No overall charge so can not carry any electrical charge
- Can not conduct electricity
GIANT METALLIC SUBSTANCES
- Atoms in pure metals are arranged in layers
- Layer can easily slide over each other, which means that the metal can be easily bent and shaped.
- Atoms in metals are held together in a giant structure by a sea of de-localised electrons. These de-localised electrons are able to go through the structure.
- Metals conduct heat and electricity because the de-localised electrons can flow through the giant metabolic lattice.