Edexcel IGCSE Chemistry Section A

• The steam from 5 cubic cm will fill a bucket
• Liquids are less dense than the solids they came from
• You can move through water but you can't move through a wall

Particles in a solid are often regularly packed - they can only vibrate

In a solid - the particles have strong forces attracting them

In a liquid - the particles are mainly touching but there are some gaps

In a gas - almost no forces of attraction between the particles

Melting - when a solid is heated, the particles vibrate faster + faster until the forces are no longer strong enough to keep them together

Freezing - if a solid is cooled, the particles move more slowly until the forces of attraction are strong enough to hold them together in a solid

Boiling - when a liquid is heated to strongly that the particles have enough energy + are moving fast enough to break all forces of attraction

Evaporation - at each temp. some particles will be moving faster than others. Some v. fast particles on the surface will have enough energy to break away to form a gas

Condensing - same concept as freezing but with liquids and gases

Sublimation - does not involve a liquid. A solid will turn directly to a gas

In Gases:

• Each substance particle is bouncing off endless air particles as it diffuses
• Diffusion is the spreading out of particles
• Even gases with different densities will diffuse evenly
• Some particles travel faster than others

In Liquids:

• There are only small gaps in liquids for other particles to diffuse into - this is why it is so slow

• Proton - charge: +1, mass: 1
• Neutron - charge: 0, mass: 1
• Electron - charge: -1, mass: 1/1800
• Mass is concentrated in the nucleus
• Masses and charges are relative because they are so small
• Atomic/proton number - number of protons in the nucleus
• Mass number(nucleon number) - the total number of protons and neutrons in the nucleus
• Isotopes - atoms with the same atomic number but different mass numbers i.e. same number of protons, different number of neutrons
• Radioactive Isotope - an isotope that has an unstable nucleus + gives off radiation to try to stabilise itself
• The radioactivity of Carbon-14 is used in Carbon dating
• The number of electrons = number of protons

• Arranged in order of increasing atomic number in periodic table
• The smaller number is the atomic number
• The larger number will be the mass number, the most common isotope or the relative mass

Electron Arrangement:

• Electrons are found at large distances in shells or energy levels
• Each energy level can hold a certain number of electrons
• Low energy levels are always filled before the higher ones
• 1st shell: 2 electrons
• 2nd shell: 8 electrons
• 3rd shell: 8 electrons

Columns of the periodic table are called groups. Groups contain elements with similar properties. This is because they have the same number of electrons in their outer shell.

The number of electrons in the outer shell is the same as the group number for groups 1-7

The elements in group 0 have 8 electrons in their outer shell except helium which has 2

• Group 0 are known as noble gases - they are almost completely unreactive
• Electrons in their shells can be shown by circles with dots or crosses showing the electrons

In any bond, particles are held together by electrical attractions between a positive charge and a negative charge

A covalent bond is a bond between two non-metals

In a covalent bond, a pair of electrons is shared between two atoms because both nuclei are attracted to the same pair of electrons

Covalent bonds are often shown with dots and crosses

e.g. Hydrogen molecules are said to be diatomic because the bond is so strong that hydrogen atoms are found joined together in pairs

Alkenes, Alkanes and Alkynes all have covalent bonds because they are made from non-metals

e.g. Nitrogen gas consists of triple bonds which are very hard to break. This is why nitrogen gas is very unreactive

Ionic bonding occurs between a metal and a non-metal

One of the atoms is is attracted to the electron pair much more strongly than the other one. The electron pair is then pulled very close to that atom, and away from the other one. In effect, one of the atoms has given it's electron to the other.

The charged particles are called ions

Ion - an atom that carries an electrical charge

Cation - carries positive charge

Anion - carries negative charge

Ionic bonding is where there has been a transfer from one atom to another to produce ions. The substance is held together by strong attractions between the positive and negative ions.

Metallic bonding occurs between two metals. Metals have high melting points and this means that the forces holding the particles in the metal together are very strong