GCSE Chemistry · Chemistry
Atomic structure: a clear study guide
Atomic structure questions become easier when you separate the particles, their charges and their locations. Use the atomic number and mass number as anchors for every calculation.
The three subatomic particles
A proton has a relative charge of +1 and a relative mass of 1. A neutron has no charge and a relative mass of 1. An electron has a relative charge of −1 and a much smaller relative mass.
Protons and neutrons are in the nucleus. Electrons occupy shells around the nucleus. A neutral atom has equal numbers of protons and electrons.
- Atomic number = number of protons.
- Mass number = protons + neutrons.
- Neutrons = mass number − atomic number.
Isotopes and ions
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same chemical properties because they have the same electron arrangement.
An ion forms when an atom gains or loses electrons. Losing electrons creates a positive ion; gaining electrons creates a negative ion. The number of protons does not change when an ion forms.
Electronic structure and the periodic table
For the first twenty elements, electrons fill shells in the pattern 2, 8, 8, 2. The group number tells you the number of outer-shell electrons for the main groups, while the period number tells you the number of occupied shells.
Atoms often react to gain, lose or share electrons so their outer shell becomes more stable.
Quick check
An atom has atomic number 17 and mass number 35. How many neutrons does it have?
Answer: 18 neutrons: 35 − 17.
What changes when an atom becomes a positive ion?
Answer: It loses one or more electrons.
Common questions
Are isotopes different elements?
No. Isotopes have the same number of protons, so they are the same element; they differ in neutron number.
Why do atoms form ions?
They form ions by gaining or losing electrons, often to reach a more stable outer-shell arrangement.