iGCSE Chemistry - Principles of Chemistry

Atoms

 

• Solids have fixed volume and shape.
• Liquids have fixed volume but no definite shape.
• Gases have no fixed volume or shape.
• All matter is made from elements.
• Elements cannot be broken down into simpler substances.
• The number of atoms in a mole is called Avogadro’s constant.
• Moles of atoms = mass/RAM
• Avogadro’s number is 6 x 10²³.

 

Atomic Structure

 

• The smallest amount of an element that behaves like that element is an atom.
• Atoms are made up of 3 subatomic particles: electrons, protons, and neutrons.

Subatomic particle	Relative mass	Relative Charge
Proton	1	+1
Neutron	1	0
Electron	1/2000	-1

• Protons and neutrons are found in the centre of the atom in the nucleus.
• Hydrogen is the only element that has no neutrons.
• Atoms of the same element with the same number of protons and electrons but different numbers of neutrons are called isotopes.
• Isotopes have the same chemical properties but different physical properties.
• The relative atomic mass is the average mass of all isotopes.
• Electrons are arranged in shells around the nucleus.

Electron Shell	Maximum number of electrons
1	2
2	8
3	18

• The arrangement of electrons in an atom is called electronic configuration.
• Number of electrons in outer shell is called periodicity.
• Noble gases have filled outer shells and thus they are stable and unreactive.

 

Relative formula masses and molar volumes

 

• The relative formula mass of a molecule can be calculated by adding up the relative atomic masses of the atoms in the molecule.
• The relative atomic mass of an element tells you the mass of a mole of atoms of that element.
• The relative formula mass tells you the mass of a mole of that substance.
• The volume of one mole of gas contains Avogadro’s constant number of molecules of that gas.
• One mole of any gas occupies the same volume under the same conditions of pressure.
• One mole of any gas occupies 24,000cm³ (24dm³) at room temperature and pressure (rtp).
• Moles = volume in cm³/24,000.

 

Chemical Formulae and Chemical Equations

 

• To find the formula of magnesium oxide:
1. Measure mass of magnesium.
2. Heat magnesium so that it oxidises.
3. Measure mass of magnesium oxide formed.
4. Find difference in mass to ascertain how much oxygen has bonded with the magnesium.
5. Convert mass change into moles.
6. Convert original magnesium into moles.
7. The moles come out to be equal, so formula is MgO.
• To find the formula of water:
1. Electrolyse water.
2. Hydrogen is produced at the cathode and oxygen is produced at the anode.
3. Observe that twice as much gas is produced at the cathode than at the anode.
4. Thus, formula is H₂O.

• The formula of a compound can be calculated if the number of moles of the combining elements is known.
• Empirical formulae show the simplest ratio of atoms present.
• Molecular formulae show the actual number of atoms of each element in one molecule.
• Percentage yield = actual yield/predicted yield x 100
• Percentage purity = amount of pure substance/total amount of substance x 100
• In chemical equations starting chemicals are reactants and finishing chemicals are products.
• Diatomic elements, i.e. exist as molecules containing two atoms:

 

Element	Form in which it exists
Hydrogen	H2
Oxygen	O2
Nitrogen	N2
Chlorine	Cl2
Bromine	Br2
Iodine	I2

 

• Balancing numbers can only be put in front of formulae.
• States and their symbols:

 

State	Symbol
Solid	s
Liquid	l
Gas	g
Solution	aq

 

• Ions and their charges:

 

Positive Ions (cations)		Negative ions (anions)
Ammonium	NH4+	Bromide	Br-
Hydrogen	H+	Chloride	Cl-
Copper	Cu+	Iodide	I-
Potassium	K+	Hydroxide	OH-
Sodium	Na+	Nitrate	NO3-
Silver	Ag+	Carbonate	CO32-
Calcium	Ca2+	Oxide	O2-
Magnesium	Mg2+	Sulphate	SO42-
Copper (II)	Cu2+	Sulphite	SO32-
Iron (II)	Fe2+	Sulphide	S2-
Zinc	Zn2+	Phosphate PO43-
Aluminium	Al3+
Iron (III)	Fe3+

 

• Ionic equations show reactions involving ions.
• In many ionic reactions some of the ions play no part in the reaction. These ions are called spectator ions.
• A solution is made when a solute dissolves in a solvent.
• Moles present in solution = volume (cm³) x mol cm³.

 

Ionic Compounds

 

• Metals lose electrons from their outer shell to form positive ions.
• Non-metals gain electrons in their outer shell to form negative ions.
• Ionic bonding can be represented in dot and cross diagrams.

 

Properties of Ionic compounds

High melting + boiling points

Form giant lattices

Strong forces between ions

Electrolytes – conduct electricity when molten or dissolved in water

 

Covalent Substances

 

• Covalent bonds involve electron sharing.
• Covalent bonds only occur between non-metals.
• The displayed formula of H₂ is written as H—H.
• Covalent bonds can be single, double, or triple. This means they each contribute one, two, or three (respectively) electrons to the molecule formed.
• Shapes of molecules:

          Carbon dioxide is a linear molecule.

          Water molecules are v-shaped.

          Methane molecules are tetrahedral.

          Molecules of ammonia are pyramidal.

• Covalent compounds can form simple molecular crystals.

 

Properties of covalent compounds

Low melting + boiling points

Very strong intramolecular forces

Weak forces between ions

Do not conduct electricity

 

• Not all covalently bonded compounds exist as simple molecular structures: diamond exists as a giant structure.
• Different forms of the same element are called allotropes.
• Diamond ore is called kimberlite.

 

Electrolysis

 

• Substances that can conduct electricity are called electrolytes.
• For electrolysis to occur, the substance must be molten or dissolved in water, and it must contain ions.
• Electrode connected to positive terminal is the anode.
• Electrode connected to negative terminal is called cathode.
• The negative ions are attracted to the anode and lose electrons (oxidised).
• The positive ions are attracted to the cathode and gain electrons (reduced).
• A Faraday is one mole of electrons.
• Reactions at electrodes can be shown as ionic half equations.
• e^- stands for electron.
• Electroplating is the process by which electrolysis is used to coat an object with a thin film of metal. The object is used as the cathode.
• To purify metals, make the anode a large impure block and the cathode a thin pure block

 

Metallic Crystals

 

• Metals are giant structures with high melting and boiling points.
• Metal atoms give up electrons to form cations.
• These electrons given off form a swarm around the metal ion, and thus are free to move (delocalised) meaning metals conduct electricity.

 

Properties of Metals

High melting + boiling points

Malleable (can be hammered into a sheet)

Ductile (can be drawn into a wire)

Conduct heat and electricity