Chemistry · Lesson
Atoms, nuclei, electrons and isotopes
Use three particle counts to describe an atom or ion. Separate a useful bookkeeping model from a picture of what an atom actually looks like.
Back to the Chemistry mapBy the end, you can
- Count protons, neutrons and electrons from an isotope and charge.
- Distinguish atomic number from mass number.
- State what a particle diagram leaves out.
Start with three counts
An atom contains a small nucleus and electrons associated with it. The nucleus contains protons and, for most isotopes, neutrons. A proton carries one positive elementary charge; an electron carries one negative elementary charge. A neutron has no net electric charge. These names describe different particles, not three colours of the same object. Hydrogen-1 is an important exception to the familiar proton-and-neutron picture: its nucleus has one proton and no neutron.
For introductory bookkeeping, count the protons, neutrons and electrons separately. A neutral atom has equal numbers of protons and electrons, so their charges cancel. If an atom loses an electron, the proton count has not changed, but the net charge becomes more positive. The particle counts tell us composition and charge; they do not specify the full energy state or spatial distribution of the electrons.
Two numbers answer different questions
The atomic number, written Z, is the number of protons. It determines which chemical element the atom belongs to. Every carbon atom has six protons. A nucleus with seven protons belongs to nitrogen, even if its total mass is close to that of a particular carbon isotope. Changing the proton count is a nuclear change, not the electron rearrangement of an ordinary chemical reaction.
The mass number, written A, counts protons plus neutrons in one nucleus. It is a whole-number count: A = proton number + neutron number. Therefore the neutron number is A − Z. The name “mass number” can mislead: it is not an exact measurement of the atom’s mass in grams or daltons. Nuclear binding and electron masses matter when actual masses are measured.
Read notation one piece at a time
The name carbon-13 says that the atom is carbon and its mass number is 13. Since carbon has Z = 6, carbon-13 contains six protons and seven neutrons. A neutral carbon-13 atom also has six electrons. In a full isotope symbol, the mass number can be written at the upper left of the element symbol and the atomic number at the lower left. A charge belongs at the upper right.
Use that placement as a language rule. The “13” in carbon-13 is not the number of carbon atoms in a molecule. A lower-right subscript in a chemical formula has a different job, introduced later. When a plain-text display cannot show isotope notation clearly, a name such as “carbon-13, neutral atom” can prevent ambiguity.
| Atom | Protons, Z | Neutrons, A − Z | Electrons | Mass number, A |
|---|---|---|---|---|
| Hydrogen-1 | 1 | 0 | 1 | 1 |
| Carbon-12 | 6 | 6 | 6 | 12 |
| Carbon-13 | 6 | 7 | 6 | 13 |
Each row describes one neutral atom. The totals are counts, not particle sizes or measured atomic masses.
A drawing is a model with limits
A classroom drawing may put coloured circles in a nucleus and dots around it. This helps count charges and distinguish the nucleus from electrons. It does not show actual particle colours, sizes or measured electron paths. The nucleus is much smaller than the whole atom; a convenient diagram rarely preserves that scale.
Electrons are described with quantum models, developed in the electronic-structure chapter. A drawn dot should not be read as a tiny planet following a definite circular orbit. For now, retain the reliable counting relations while leaving the detailed electron distribution open. A useful simplified model states both what it explains and what it leaves out.
Worked examples
Decode carbon-13
How many protons, neutrons and electrons are present in a neutral carbon-13 atom?
- Carbon fixes the proton count at 6.
- The mass number is 13, so the neutron count is 13 − 6 = 7.
- Neutrality requires six electrons to balance the six positive proton charges.
Six protons, seven neutrons and six electrons. The count 13 describes the nucleus, not thirteen carbon atoms.
Try it yourself
Must every atom contain a neutron?
Show the answer and reasoning
No. Hydrogen-1 has no neutron.
Its nucleus consists of one proton; its mass number and atomic number are both 1.
A neutral oxygen-18 atom has Z = 8. Give its neutron and electron counts.
Show the answer and reasoning
10 neutrons and 8 electrons.
A − Z = 18 − 8 = 10; neutrality gives an electron count equal to Z.
Is A = 13 an exact atomic mass of 13 Da?
Show the answer and reasoning
No. A is a particle count.
Actual atomic mass is measured and is not generally equal to the integer mass number.
A drawing shows an electron on a circle. Does the drawing establish a measured circular orbit?
Show the answer and reasoning
No.
The drawing is a simplified representation. Electron distributions require a quantum description.
What to carry forward
- Z counts protons and fixes element identity.
- A counts protons plus neutrons in one nucleus.
- Neutral atoms have equal proton and electron counts.
- Use particle pictures for their stated purpose, with their scale and orbital limits.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.