lesson
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If you enlarged an atom to the size of a football stadium, the nucleus at the center would be the size of a single marble — yet it holds over 99.9% of the atom's entire mass.
Everything in our universe is assembled from these tiny building blocks, each constructed from just three subatomic particles: protons, neutrons, and electrons.
What gives each particle its unique identity, and how are they arranged inside an atom?
The Three Subatomic Particles
An atom consists of a central nucleus containing protons and neutrons, orbited by electrons arranged in energy levels called shells.
In 1897, J.J. Thomson discovered the electron, followed by Ernest Rutherford discovering the dense positive nucleus in 1911 and James Chadwick confirming the neutron in 1932.
📊A clean visual comparison card showing an atom model alongside a summary of the 3 subatomic particles. Left side: an atom diagram with a dense central nucleus containing red circles (+ for protons) and gray circles (0 for neutrons), surrounded by dotted concentric rings holding tiny blue circles (- for electrons). Right side: 3 comparative cards. Card 1 (Proton, red): Location: Nucleus, Relative Mass: 1, Relative Charge: +1. Card 2 (Neutron, gray): Location: Nucleus, Relative Mass: 1, Relative Charge: 0 (neutral). Card 3 (Electron, blue): Location: Shells, Relative Mass: 1/1840 (negligible), Relative Charge: -1. Clean modern UI, #1e2945 text, pastel backgrounds (#fee2e2, #f3f4f6, #dbeafe), responsive 350px width.
Notice that an electron's mass is only 18401 of a proton, which is so small we treat it as virtually negligible.
Because each proton has a +1 charge and each electron has a −1 charge, every neutral atom contains exactly equal numbers of protons and electrons.
How do we read a periodic table entry to find out how many of each particle an atom actually has?
Reading Nuclear Notation
The atomic number (Z) identifies the element and tells you the exact number of protons in the nucleus.
The mass number (A) tells you the total count of protons plus neutrons combined.