Learning focus
Develop precise chemical language, connect particle and bonding models to observations, interpret diagrams and data, and apply ideas to unfamiliar examples.
The atom
An atom has a very small central nucleus containing protons and neutrons. Electrons occupy shells around the nucleus. Almost all atomic mass is concentrated in the nucleus, while most atomic volume is associated with the electron region.
Protons
A proton has relative charge +1 and relative mass 1. Proton number defines the element. If proton number changes, the species is a different element.
Neutrons
A neutron has charge 0 and relative mass 1. Different neutron numbers produce isotopes. Neutrons contribute to mass number but not to electrical charge.
Electrons
An electron has relative charge -1 and relative mass about 1/1840. A neutral atom has equal numbers of protons and electrons. Chemical reactions mainly involve outer-shell electrons.
The shell model
The shell model is simplified and is used in this syllabus for the first 20 elements. Electrons are not literally small planets following fixed circular paths, but the model predicts ion formation and bonding successfully.
Practical or data skill
Complete a table of proton, neutron and electron numbers. Identify neutral atoms, ions and isotopes from the data.
Examination tip
Most mass is in the nucleus; electrons contribute negligible relative mass.
Review questions and suggested answers
Question 1
Which particle defines an element?
Suggested answer
The proton.
Question 2
Where is most atomic mass?
Suggested answer
In the nucleus.
Question 3
How many electrons are in a neutral atom with 13 protons?
Suggested answer
13.
