The Nuclear Model of the Atom

2026 What You Need to Know (Syllabus Objectives)

Core:

Describe the structure of an atom (positively charged nucleus and negatively charged electrons in orbit)
Know how atoms form positive ions by losing electrons or negative ions by gaining electrons
Describe the composition of the nucleus (protons and neutrons)
State the relative charges of protons, neutrons and electrons as +1, 0 and –1
Define proton number (atomic number) Z and nucleon number (mass number) A
Calculate the number of neutrons in a nucleus
Use the nuclide notation A/Z X
Explain what an isotope is and know that elements can have more than one isotope

Supplement:

Describe how alpha particle scattering supports the nuclear model (small nucleus, empty space, nucleus has most mass, nucleus is positive)
Describe nuclear fission and fusion with nuclide equations and mass/energy changes
Know the relationship between proton number and relative charge on a nucleus
Know the relationship between nucleon number and relative mass of a nucleus

An atom is the smallest unit of matter that makes up everything around us. Think of it as the basic building block of all materials.

Every atom has two main regions:

The nucleus – This is the tiny, dense center of the atom. It is positively charged and contains almost all of the atom's mass.
Electrons in orbit – These are negatively charged particles that move around the nucleus in regions called shells or orbits. They are extremely light compared to the nucleus.

The nucleus itself is made up of two types of particles:

To understand how tiny atoms are:

Despite being so tiny, the nucleus contains almost all of the atom's mass. The atom is mostly empty space where the electrons orbit.