Difficulty: Beginner | Prerequisites: None
This is where all of chemistry starts. Atomic structure is the foundation for understanding elements, bonding, reactions, and essentially every topic that follows in a general chemistry course. If you can identify subatomic particles, know where they sit in an atom, and count them from a diagram or a periodic table entry, you have the toolkit for everything from isotopes to electron configuration. No prior chemistry knowledge is assumed here.
Atoms are the basic building blocks of all chemical elements. They are made of three subatomic particles: protons (positive, in the nucleus), neutrons (neutral, in the nucleus), and electrons (negative, orbiting outside). The number of protons defines the element; the number of electrons determines charge; the number of neutrons distinguishes isotopes.
Atom
The smallest unit of a chemical element that retains the properties of that element. In simple terms, atoms are the tiny particles that make up everything around you.
Proton
A positively charged subatomic particle found in the nucleus of an atom. Think of it as the particle that gives an element its identity: the number of protons equals the atomic number.
Neutron
A subatomic particle with no electrical charge, located in the nucleus alongside protons. In simple terms, neutrons add mass to the atom without changing what element it is.
Electron
A negatively charged subatomic particle that orbits the nucleus in energy levels (shells). Think of it as the particle responsible for chemical bonding and electrical behaviour.
Nucleus
The dense central core of an atom, containing protons and neutrons. It accounts for nearly all of the atom's mass despite being a tiny fraction of its total volume.
Atomic number
The number of protons in the nucleus of an atom, which defines the element. In simple terms, if you know the atomic number, you know what element you are looking at.
Mass number
The total count of protons plus neutrons in the nucleus. Think of it as the atom's weight tag, since electrons contribute negligible mass.
Subatomic particle
Any particle smaller than an atom. The three main ones in chemistry are protons, neutrons, and electrons.
Proton – positive charge (+1), located in the nucleus, relative mass of ~1 amu
Neutron – no charge (neutral), located in the nucleus, relative mass of ~1 amu
Electron – negative charge (–1), orbits the nucleus in energy levels/shells, relative mass of ~1/1836 amu (effectively negligible)
In a neutral atom, the number of protons equals the number of electrons, so the positive and negative charges cancel out. If an atom gains or loses electrons, it becomes an ion.
More electrons than protons = negative ion (anion)
Fewer electrons than protons = positive ion (cation)
The nucleus sits at the centre of the atom and contains both protons and neutrons. Electrons occupy the space around the nucleus, arranged in shells or energy levels. The nucleus is extremely small relative to the total volume of the atom, but it carries nearly all of the atom's mass.
Worksheet-style atom diagrams typically show the nucleus as a cluster of coloured spheres (protons and neutrons) with smaller particles orbiting around it (electrons).
Protons are usually marked with a "+" symbol inside the nucleus
Neutrons are the remaining particles in the nucleus, often a different colour, with no charge symbol
Electrons are the small particles shown orbiting outside the nucleus, marked with a "–" symbol
Atom 1 (Helium-4):
2 protons, 2 neutrons, 2 electrons
Atomic number = 2, mass number = 4
Neutral atom (protons = electrons)
Atom 2 (Carbon-10):
6 protons, 4 neutrons, 6 electrons
Atomic number = 6, mass number = 10
Neutral atom (protons = electrons)
Count the "+" particles in the nucleus = number of protons
Count the remaining (uncharged) particles in the nucleus = number of neutrons
Count the "–" particles outside the nucleus = number of electrons
Atomic number = protons
Mass number = protons + neutrons
Atomic number (Z) = number of protons
Mass number (A) = protons + neutrons
Number of neutrons = mass number – atomic number (A – Z)
Neutral atom: number of protons = number of electrons
Ion charge = protons – electrons
Understanding atomic structure is the basis for nuclear energy and nuclear medicine. Knowing how many protons and neutrons sit in a nucleus tells engineers whether an isotope is stable or radioactive, which determines whether it can fuel a reactor or serve as a tracer in a PET scan.
The electron arrangement around atoms is what drives all of chemistry: it determines which elements bond, how materials conduct electricity, and why metals behave differently from plastics.
Students often think the nucleus is large relative to the atom. It is not. If the atom were the size of a football stadium, the nucleus would be roughly the size of a marble. Almost all of the atom is empty space.
Students often confuse atomic number with mass number. Atomic number counts only protons. Mass number counts protons plus neutrons.
Students sometimes believe electrons are embedded in the nucleus. They are not. Electrons orbit outside the nucleus in energy levels.
Students occasionally think that neutrons carry a charge. Neutrons are electrically neutral, which is the origin of their name.
⚠️ Expect questions asking you to identify the number of protons, neutrons, and electrons from a diagram or from a periodic table entry.
⚠️ The relationship between atomic number and element identity is heavily tested. If the atomic number changes, the element changes.
⚠️ Calculating mass number (protons + neutrons) and working backwards to find neutrons (mass number – atomic number) appears on nearly every introductory chemistry exam.
⚠️ Know that the nucleus carries almost all the mass but occupies almost none of the volume. This is a common conceptual question.
True or False: The nucleus contains protons and electrons.
False. The nucleus contains protons and neutrons.
Fill in the blank: The number of ______ defines which element an atom is.
Protons.
True or False: In a neutral atom, the number of protons equals the number of electrons.
True.
Fill in the blank: Mass number = ______ + ______.
Protons + neutrons.
True or False: Electrons have a greater mass than protons.
False. Electrons have roughly 1/1836 the mass of a proton.
Q: What are the three subatomic particles and their respective charges?
A: Protons (positive, +1), neutrons (neutral, 0), and electrons (negative, –1).
Q: Where are protons and neutrons located in an atom?
A: In the nucleus, at the centre of the atom.
Q: An atom has 8 protons, 8 neutrons, and 8 electrons. What is its atomic number and mass number?
A: Atomic number = 8 (oxygen). Mass number = 8 + 8 = 16.
Q: An atom has an atomic number of 6 and a mass number of 12. How many neutrons does it have?
A: 12 – 6 = 6 neutrons.
Q: If an atom loses two electrons, does it become positively or negatively charged?
A: Positively charged. It now has more protons than electrons, making it a cation.
Q: Why does the nucleus account for nearly all of an atom's mass?
A: Because protons and neutrons each have a mass of approximately 1 amu, while electrons are roughly 1/1836 amu. The combined mass of the electrons is negligible compared to the nucleus.
This connects directly to isotopes, which are atoms of the same element (same protons) with different numbers of neutrons. Understanding atomic structure is a prerequisite for that topic.
Electron arrangement around the nucleus leads into electron configuration and orbital theory, which in turn underpins chemical bonding (ionic, covalent, metallic) and the periodic table's organisation.
The concept of ions (gaining or losing electrons) connects to electrochemistry and acid-base chemistry later in the course.
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