Atoms, elements and compounds - AQA

Okay difficulty.

What is the smallest part of an element that can exist?

An atom.

How are elements arranged into groups?

They are arranged into groups based on similar properties.

What are groups numbered to?

1-7, then 0

What non-metal elements exist as molecules?

• iodine, I₂

• bromine, Br₂

• chlorine, Cl₂

• fluorine, F₂

• oxygen, O₂

• nitrogen, N₂

• hydrogen, H₂

What is a compound?

A substance that contains two or more elements

Common ion charges

1. Sodium - Na⁺

2. Magnesium - Mg²⁺

3. Chloride - Cl^-

4. Oxide - O^2-

In an ionic compound, how can we predict the formula?

(according to its charges)

The number of ions in a formula must give an equal number of positive and negative charges (so that it is neutral).

e.g. Al³⁺ O^2- —> Al₂O₃

Word equation form

reactants → products

What are the state symbols?

1. (s) - Solid

2. (l) - Liquid

3. (g) - Gas

4. (aq) Aqueous solution

What is an aqueous solution?

An aqueous solution forms when a substance dissolves in water.

What is a pure substance?

A substance that consists of one element or one compound.

What is a mixture

A combination of two or more substances that are not chemically bonded and retain their individual properties.

How can mixtures be separated?

Mixtures can be separated by physical processes. They do not involve chemical reactions, and no new substances are made.

Filtration

• A physical process used to separate solid particles from liquids or gases using a barrier that only allows the fluid to pass through.

• This works as the filter paper has tiny holes or pores in it.

Crystallisation

A process used to separate a solid that has dissolved in a liquid by evaporating the liquid to form crystals of the solid.

Simple distillation

• Simple distillation is used to separate a solvent from a solution.

Simple distillation works because the dissolved solute has a much higher boiling point than the solvent. When the solution is heated, solvent vapour leaves the solution. It moves away and is cooled and condensed. The remaining solution becomes more concentrated as the amount of solvent in it decreases.

Fractional distillation

• A process used to separate different liquids from a mixture of liquids.

Fractional distillation works because the different liquids have different boiling points. When the mixture is heated:

• vapours rise through a column which is hot at the bottom, and cooler at the top

• vapours condense when they reach a part of the column that is below the temperature of their boiling point

• each liquid is led away from the column

There are two ways of obtaining different liquids from the column:

• by collecting different liquids from different parts of the column - the substance with the lowest boiling point is collected at the top of the column

• by continuing to heat the mixture to increase the temperatures in the column - the substance with the lowest boiling point is collected first

Chromatography

• Used to separate mixtures of soluble substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Chromatography relies on two different ‘phases’:

• the stationary phase, which in paper chromatography is very uniform, absorbent paper

• the mobile phase is the solvent that moves through the paper, carrying different substances with it

The different dissolved substances in a mixture are attracted to the two phases in different proportions. This causes them to move at different rates through the paper.

Who was John Dalton?

John Dalton published his ideas about atoms in 1803. He thought that all matter was made of tiny particles called atoms, which he imagined as tiny spheres that could not be divided.

Who was J J Thomson?

J J Thomson carried out experiments and discovered the electron. This led him to suggest the plum pudding model of the atom. In this model, the atom is a ball of positive charge with negative electrons embedded in it.

What did Ernest Rutherford do?

In 1909 Ernest Rutherford designed an experiment to test the plum pudding model. In the experiment, positively charged alpha particles were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions.

This evidence led Rutherford to suggest a new model for the atom, called the nuclear model. In the nuclear model:

• the mass of an atom is concentrated at its centre, the nucleus

• the nucleus is positively charged

What did Niels Bohr do?

Niels Bohr adapted Ernest Rutherford's nuclear model. Bohr did calculations that led him to suggest that electrons orbit the nucleus in shells. The shells are at certain distances from the nucleus. The calculations agreed with observations from experiments.

Further experiments led to the idea that the nucleus contained small particles, called protons. Each proton has a small amount of positive charge.

What did James Chadwick do?

In 1932 James Chadwick found evidence for the existence of particles in the nucleus with mass but no charge. These particles are called neutrons. This led to another development of the atomic model, which is still used today.

What is the atom structure?

An atom has a central nucleus. This is surrounded by electrons arranged in shells.

How big is an atom?

• the radius of an atom is about 0.1 nm (1 × 10^-10 m)

• the radius of a nucleus (1 × 10^-14 m) is less than 110,000 of the radius of an atom

Relative mass of a proton

1

Relative mass of a neutron

1

Relative mass of an electron

very small

Relative charge of a proton

+1

Relative charge of a neutron

0

Relative charge of an electron

-1

What is the atomic number?

The number of protons in an atom

What does an atom have an equal amount of?

An atom contains equal numbers of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms have no overall electrical charge.

What is atomic mass?

The mass number of an atom is its total number of protons and neutrons.

What is an isotope?

Atoms of the same element that have the same number of protons, but they have different numbers of neutrons.

What is relative atomic mass?

The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes. It takes account of the abundance of each of the isotopes of the element.

How do you calculate relative atomic mass?