Principles of Chemistry
Atomic Structure and Isotopes
Pearson Edexcel International GCSE Chemistry
Atoms and molecules
- Atom: the smallest particle of an element that can take part in a chemical reaction.
- Molecule: two or more atoms chemically bonded together.
Protons, neutrons and electrons
Protons and neutrons are in the nucleus; electrons are in shells around the nucleus.
| Particle | Relative charge | Relative mass |
|---|---|---|
| proton | +1 | 1 |
| neutron | 0 | 1 |
| electron | −1 | 0.0005 |
- Write −1 for the electron. Never write a minus sign without the 1 for the electron's charge.
- A relative mass has no sign. Never write a + or − sign on a relative mass.
- An atom has no overall charge: it has the same number of protons and electrons.
Atomic number and mass number
- Atomic number: the number of protons in the nucleus.
- Mass number: the number of protons and neutrons in the nucleus.
| Particle | Number in an atom | Number in an ion |
|---|---|---|
| protons | the atomic number | the atomic number |
| neutrons | mass number minus atomic number | mass number minus atomic number |
| electrons | the same as the number of protons | positive ion: fewer than the protons, by the charge; negative ion: more than the protons, by the charge |
A negative ion has more electrons than protons: electrons have a negative charge and protons have a positive charge.
Isotopes
- Isotopes are atoms of the same element with the same number of protons and different numbers of neutrons.
- Two isotopes of an element: the similarity is the number of protons, the difference is the number of neutrons. Never write isotopes have a different number of electrons.
- Isotopes of an element have the same chemical properties because they have the same electron configuration.
Relative atomic mass
Relative atomic mass, Ar: the average relative mass of the atoms of an element, taking into account the mass and abundance of each of its isotopes.
- For each isotope, multiply its mass number by its percentage abundance.
- Add these together for every isotope.
- Divide the total by 100.
- Give the answer to one decimal place.
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