C1: Atomic structure

1.1: Atoms

There are about 100 different elements from which all substances are made - the periodic table is a list of the elements

Each element is made of one type of atom

Atoms are represented by chemical symbols, e.g. Na for an atom of sodium

The elements in the periodic table are arranged in columns, called groups - the elements in a group usually have similar properties

Atoms have a tiny nucleus surrounded by electrons

When elements react, their atoms join with atoms of other elements - compounds are formed when 2 or more elements combine together

1.2: Chemical equations

Chemical equations show the reactants and the products in a reaction

Word equations give the names of the reactants and products only

In chemical reactions, the atoms get rearranged - symbol equations show the numbers and types of atoms in the reactants and products

Atoms are neither created nor destroyed in a chemical reaction - the number of atoms remains the same before and after the reaction

When writing a symbol equation you should always balance the equation - a balanced symbol equation has the same number of each type of atom on both sides of the equation

You can include state symbols in balanced symbol equations

solid

liquid

gas

aqueous solutions (substances dissolved in water)

Law of conservation of mass

No atoms are made or lost during a chemical reaction - so the mass of products is equal to the mass of reactants

In some reactions, the law seems to be broken because the mass appears to change - this is usually when gases are reactants or products e.g. if a product is a gas it may escape into the air

Sometimes substances increase when heated in air - the extra mass is because oxygen gas from the air is a reactant

1.3: Separating mixtures

A mixture is made up of 2 or more substances that are not chemically combined together

The chemical properties of each substance in the mixture are unchanged

Mixtures are separated by a physical process - physical processes do not involve chemical reactions, so no new substances are made

Physical processes include

Filtration

Separates substances insoluble in a solvent from those that are soluble in a solvent

Crytillisation

Separates a soluble solid from a solvent

Distillation

Separates a solvent from soluble solids dissolved in the solvent

1.4: Fractional distillation and paper chromatography

1.5: History of the atom

1.6: Structure of the atom

1.8: Electronic structures

C1.7: Ions, atoms and isotopes

Fractional distillation

Fractional distillation is a way to separate mixtures of miscible liquids

Miscible liquids dissolve in each other, mixing completely - they don't form separate layers

The liquids have different boiling points - the liquid with the lowest boiling point is collected first

To aid separation you can add a fractionating column to the distillation apparatus

Fractional distillation is a way to separate ethanol from a fermented mixture in the alcoholic drinks industry

Paper chromatography

Paper chromatography is a way to separate substances from mixtures in a solution - it work because some compounds are more soluble that others in the solvent

Paper chromatography is a way to separate ink, dye, or food colourings

The ancient Greeks were the first to have ideas about atoms

In the early 1800's, Dalton linked his ideas to strong experimental evidence - Dalton suggested atoms were tiny, hard spheres - these atoms could not be divided or split

At the end of the 1800's, Thomson discovered a tiny negatively charged particle called the electron

Thomson proposed the 'plum pudding' model for the atom - the model suggested that negative electrons were embedded in a ball of positive charge - like the electrons as the bits of plum in a plum pudding

Ten years later, Geiger and Marsden were experimenting with alpha particles (dense positively charged particles) - they fired alpha particles at a very thin piece of gold foil

A few alpha particles were repelled showing that there must be a tiny spot of positive charge in the centre of the atom - Rutherford then proposed the nuclear model

In the nuclear model, electrons orbit around a nucleus (centre of the atom) - the nucleus contains positively charged protons

Bohr then revised the nuclear model - he suggested that the electrons were orbiting the nucleus in energy levels (or shells) - the electrons were a set distance from the nucleus - Bohr's theoretical calculation matched the experimental observations

In 1932, Chadwick provided the experimental evidence that showed the existence of uncharged particles called neutrons in the nucleus

The nucleus at the centre of the atom contains 2 types of sub-atomic particle, called protons and neutrons - protons have a positive charge and neutrons have no charge

Electrons are tiny negatively charged particles that move around the nucleus - an atom has no overall charge - that is because the number of protons is equal to the number of electrons and their relative charges are equal and opposite

All atoms of an element contain the same number of protons - this number is called the atomic number (or proton number) of the element - elements in the periodic table are arranged in order of their atomic number - the atomic number is also the number of electrons in an atom of the element

The mass number is the total number of particles in the nucleus of an atom, so it is the number of protons + the number of neutrons

An ion is a charged atom (or group of atoms)

If an atom gains electrons, it becomes a negative ion - it has an overall negative charge because it has more negative electrons than positive protons

If an atom loses electrons, it becomes a positive ion - it has an overall positive charge because it has less negative electrons than positive protons

You can't see atoms because they are so incredibly small - an atom is about a tenth of a billionth of a metre across (0.000,000,000,1m)

Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons

Atoms of the same element with different numbers of neutrons are called isotopes

Isotopes of an element have different physical properties, but always have the same chemical properties

Electrons are arranged around the nucleus in shells - each shell represents a different energy level

Electrons occupy the lowest available energy levels (those closest to the nucleus)

The first and lowest energy level (shell) can hold up to 2 electrons - the second can hold up to 8

You can show the arrangement of electrons in an atom by drawing diagrams or writing down the numbers of electrons in each energy level - after 8 electrons occupy the third shell, the fourth starts to fill

The electronic structure shows the number of electrons in each energy level

Elements in the same group have the same number of electrons in their outer shell - called the outer electrons

Because all the elements in the same group have the same amount of electrons in their outer shell, they all react chemically in the same way

The elements in group 0 are the noble gases, and they're very unreactive because they have a very stable arrangement of electrons - noble gases have 8 electrons in their outer shell, except helium which only has 2