Science · Chemistry foundations
Atoms, Elements, Ions, and the Periodic Table
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Atoms contain protons, neutrons, and electrons.
- Atomic number equals the number of protons.
- Mass number equals protons plus neutrons.
- Isotopes have different numbers of neutrons.
- Ions form when atoms gain or lose electrons.
- Periodic-table groups often share valence-electron patterns.
What you’ll be able to do
- Identify protons, neutrons, and electrons.
- Find protons, neutrons, and electrons from atomic information.
- Tell isotopes from ions.
- Use group patterns to predict valence electrons.
Learn the parts of an atom, read atomic notation, and use simple periodic-table patterns.
Inside the atom
An atom has a tiny nucleus surrounded by electrons. The nucleus contains positive protons and neutral neutrons. Negative electrons occupy the space outside the nucleus.
Protons and neutrons have most of the atom’s mass. Electrons have much less mass. A neutral atom has equal numbers of protons and electrons.
Charge comes from the balance of protons and electrons. Each proton is +1 and each electron is −1. For example, 11 protons and 10 electrons give a 1+ charge. Neutrons do not affect charge.
The proton count is also called the atomic number. Find it before working out neutrons or charge. Changing an electron count changes charge, not the element.
Two models of atomic structure
Protons and neutrons are in the center, called the nucleus. Electrons are outside it. The rings help show energy levels; electrons do not travel on fixed circular tracks. The cloud shows where electrons are likely to be. Sizes are not to scale.
Two models of atomic structure — OpenStax College, Anatomy & Physiology (2014 CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
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