1.1-Atoms

Atoms contain protons, neutrons and electrons

Atoms have a radius of about 10 nanometres(1x10(-9)). There are a few different modern models of the atom.

The nucleus

• It’s in the middle of the atom

It contains protons and neutrons

• The nucleus has a radius of around 1x10(-14) which is around 1/10,000 the radius of an atom

• It has a positive charge because of the protons

• Almost the whole mass of the atom is concentrated in the nucleus

The electrons

• Move around the nucleus in electron shells

• They’re negatively charges and tiny, but they cover a lot of space

• Electrons have virtually no mass

Particles, relative mass and charge

• Proton:1 and +1

• Neutron:1 and 0

• Electron:Very small and -1

Number of protons equals number of electrons

• Atoms are neutral-they have no charge overall(unlike ions).

• This is because they have the same number of protons as electrons

• The charge on the electrons is the same size as the charge on the protons, but opposite-so the charges cancel out

• In an ion, the number of protons doesn’t equal the number of electrons.

  • This means it has an overall charge. For example, an ion with a 2- charge, has two more electrons than protons

Atomic Numbers and Mass Number describe an Atom

• The nuclear symbol of an atom tells you its atomic(proton) number and mass number

• The atomic number tells you how many protons there are

• The mass number tells you the total number of protons and neutrons in the atom

• To get the number of neutrons, just subtract the atomic number from the mass number

  

1.2-Elements

An element is a substance made up of atoms that all have the same number of protons in their nucleus

Elements consist of atoms with the same atomic number

• Atoms can have different numbers of protons, neutrons and electrons.

• It’s the number of protons in the nucleus that decides what type of atom it is

  • For example, an atom with one proton in its nucleus is hydrogen and an atom with two protons is helium.

    • There are about 100 different elements

• So all the atoms of a particular element have the same number of protons and different elements have atoms with different numbers of protons

Atoms can be represented by symbols

• Atoms of each element can be represented by a one or two letter symbol-it’s a type of shorthand that saves you the bother of having to write the full name of the element.

  • e.g. C=carbon, O-oxygen, Mg=magnesium

  • or e.g. Na=sodium, Fe=iron, Pb-lead

Isotopes are the same except for extra neutrons

• Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons

• So isotopes have the same atomic number but different mass numbers

• A very popular example of a pair of isotopes are carbon 12 and carbon 13

• Carbon 12:

  • 6 protons

  • 6 electrons

  • 6 neutrons

• Carbon 13

  • 6 protons

  • 6 electrons

  • 7 neutrons

• Because many elements can exist as a number of different isotopes, relative atomic mass is used instead of mass number when referring to the element as a whole.

  • This is an average mass taking into account the different masses and abundances of all the isotopes that make up the element

• You can use a formula to work out the relative atomic mass:

  • Relative atomic mass = sum of(isotopes abundance x isotope mass number) / sum of abundance of all the isotopes

1.3-Compounds

Atoms can join together to make compounds

• When elements react, atoms combine with other atoms to form compounds

  • Compounds: substances formed form two or more elements

  • The atoms of each are in fixed properties throughout the compound and they’re held together by chemical bounds

• Making bonds involves atoms giving away, taking or sharing electrons.

• Only the electrons are involved-the nuclei of the atoms aren’t affected at all when a bond is made

• It’s usually difficult to separate the original element of a compound out again

  • A chemical reaction is needed to do this

• A compound which is formed from a metal and a non-metal consists of ions.

  • The metal atoms lose electrons to form positive ions and the non-metal atoms gain electrons to form negative ions.

  • The opposite charges(positive and negative) of the ions mean that they’re strongly attracted to each other.

  • This is called ionic bonding.

    • Examples of compounds which are bonded ionically include sodium chloride, magnesium oxide and calcium oxide

• A compound formed from non-metals consists of molecules.

• Each other shares an electron with another atom-this is called covalent bonding.

  • Examples of compounds that are bonded covalently include hydrogen chloride gas. carbon monoxide and water

• The properties of a compound are usually totally different from the properties of the original elements.

  • For example, if iron and sulfur react, the compound formed is a dull grey solid lump, and doesn’t behave anything like either iron or sulfur

A formula shows what atoms are in a compound

• Just as elements can be represented by symbols, compounds can be represented by formulas.

• The formulas are made up of elemental symbols in the same proportions that the elements can be found in the compound

1.4-Chemical Equations

Chemical changes are shown using chemical equations

• One way to show a chemical reaction is to write a word equation.

• It’s not as quick as using chemical symbols and you can’t tell straight away what’s happened to each of the atoms, but it’s dead easy.

  • Here’s an example-you’re told that methane burns in oxygen giving carbon dioxide and water

    • Methane + Oxygen - Carbon Dioxide + Water

    • The molecules on the left-hand side of the equation are called reactants(because they react with each other)

    • The molecules on the right-hand side are called the products(because they’ve been produced from the reactants

Symbol equations show the atoms on both sides

• Chemical changes can be shown in a kind of shorthand using symbol equations.

• Symbol equations just show the symbols or formulas of the reactants and products

  • Magnesium + Oxygen - Magnesium Oxide

  • 2Mg + 02 - 2Mg0

Symbol equations need to be balanced

• There must always be the same number of atoms on both sides-they can’t just disappear

• You balance the equation by putting numbers in front of the formulas where needed.

  • Take this equation for reacting sulfuric acid with sodium hydroxide:

    • H2S04 + Na0H - Na2S04 + H20

• The formulas are all correct but the numbers of some atoms don’t match up on both sides

• You can’t change formulas like H2SO4 to H2SO5. You can only put numbers in front of them.

• The more you practise, the quicker you get, but all you do is this:

  • Find an element that doesn’t balance and pencil in a number to try and sort it out

  • See where it gets you. It may create another imbalance, but if so, pencil in another number and see where that gets you

  • Carry on chasing unbalanced elements and it’ll sort itself out pretty quickly