Elements, Compounds and Mixtures Notes
Cambridge IGCSE Chemistry: Revision notes
Key facts
- An element is made of only one type of atom.
- A compound is two or more elements chemically combined in fixed proportions; it has different properties from its elements.
- A mixture is not chemically combined; each part keeps its properties and the composition can vary.
- An atom has a nucleus of protons and neutrons, with electrons in shells.
- Shells fill from the inside: 2, 8, 8.
Three kinds of substance
Elements have one type of atom, compounds are chemically bonded elements, and mixtures are not chemically combined.
A compound has different properties from the elements it is made from. A mixture keeps the properties of its parts, and its composition can vary.
| Element | Compound | Mixture | |
|---|---|---|---|
| Made of | One type of atom | Two or more elements chemically combined in fixed proportions | Two or more substances not chemically combined |
| Separated by | Cannot be broken down into simpler substances | Only a chemical reaction that breaks the bonds | Physical methods such as filtration or distillation |
Element
- Made of:
- One type of atom
- Separated by:
- Cannot be broken down into simpler substances
Compound
- Made of:
- Two or more elements chemically combined in fixed proportions
- Separated by:
- Only a chemical reaction that breaks the bonds
Mixture
- Made of:
- Two or more substances not chemically combined
- Separated by:
- Physical methods such as filtration or distillation
Pure substance
Mixture
Which can be separated by a physical method?
Mixture or compound
Mixed iron and sulfur can be separated with a magnet, but once heated they form a new compound.
Iron and sulfur powders stay as separate substances when mixed, so a magnet pulls out the iron. Heating makes the two elements react to form iron sulfide, a new compound with different properties that the magnet no longer attracts.
- 1
Mix
iron and sulfur powders stay as separate substances
- 2
Separate
a magnet pulls out the iron, so it is a mixture
- 3
Heat
the two elements react
- 4
Iron sulfide
a new compound with different properties, not attracted by a magnet
After iron and sulfur are heated together, why is the magnet no longer able to remove the iron?
Inside the atom
Every atom has a tiny dense nucleus of protons and neutrons, surrounded by electrons in shells.
The nucleus contains protons and neutrons; electrons move around it in shells (energy levels). Almost all the mass is in the nucleus, but almost all the volume is empty space.
The neutral atom has equal numbers of protons and electrons.
Carbon
2,4
Proton
- Relative charge +1
- Relative mass 1
Neutron
- Relative charge 0
- Relative mass 1
Electron
- Relative charge −1
- Relative mass very small (often taken as 0)
Which particle has a relative charge of −1?
Proton and mass number
Proton number identifies the element; mass number counts protons plus neutrons.
Proton (atomic) number is the number of protons in the nucleus and defines the element. Mass (nucleon) number is the number of protons plus neutrons.
Sodium
2,8,1
- Neutronsmass number − proton number
Worked example
An atom of sodium has proton number 11 and mass number 23. How many protons, electrons and neutrons does it have?
- 1
Protons = proton number = 11.
- 2
A neutral atom has equal protons and electrons, so electrons = 11.
- 3
Neutrons = mass number proton number .
An atom has proton number 17 and mass number 35. How many neutrons?
Electrons in shells
Electrons fill shells from the nucleus outwards: the first three hold 2, 8 and 8.
The electronic configuration lists the electrons in each shell. An atom with 13 electrons is 2,8,3. The configuration of a neutral atom comes from its proton number, which equals its number of electrons.
Sodium
2,8,1
Chlorine
2,8,7
2,8,1
- Group 1
- 1 outer electron
- Period 3
- 3 shells
What is the electronic configuration of an atom with 13 electrons?
Try an exam question
Describe the differences between a compound and a mixture, using iron and sulfur as an example.
[4 marks]
- [1]In a mixture the substances are not chemically combined; in a compound the elements are chemically combined.
- [1]A mixture of iron and sulfur can be separated with a magnet.
- [1]Heating iron and sulfur forms iron sulfide, a compound.
- [1]The compound has different properties from the elements (it is not attracted to a magnet).
That's the notes covered.
Carry on to the next subtopic.