Table of Contents
What Are Nucleons?
The tiny, dense core of an atom – the atomic nucleus – is built from just two kinds of particles:
- Protons
- Neutrons
Together, these particles are called nucleons. Electrons, which form the electron cloud around the nucleus, are not nucleons.
Nucleons are responsible for:
- Almost all of an atom’s mass
- Its nuclear charge (through the protons)
- Its nuclear stability, together with the forces acting between them
In this chapter we focus on what nucleons are like as particles and how they are counted and described.
Protons
Basic properties
A proton is a positively charged nucleon.
Key properties (approximate values):
- Mass:
$$m_p \approx 1.67 \times 10^{-27}\,\text{kg} \approx 1\,\text{u}$$
(1 u = 1 atomic mass unit, defined more precisely elsewhere in the course) - Electric charge:
$$q_p = +e \approx +1.602 \times 10^{-19}\,\text{C}$$ - Location in the atom:
In the nucleus.
Protons define which chemical element an atom is:
- The atomic number $Z$ is the number of protons in the nucleus.
- Hydrogen: $Z = 1$
- Carbon: $Z = 6$
- Oxygen: $Z = 8$, etc.
Atoms of different elements have different $Z$ and therefore different numbers of protons.
Proton as a “building block”
Because the proton is positively charged, it:
- Attracts electrons electrostatically
- Determines the number of electrons in a neutral atom (same number as protons)
- Fixes the chemical identity of the atom, since chemistry is governed mainly by electrons bound to the nucleus
Changing the number of protons in the nucleus changes the element itself – a nuclear process, not a chemical one.
Neutrons
Basic properties
A neutron is an electrically neutral nucleon.
Key properties (approximate values):
- Mass:
$$m_n \approx 1.67 \times 10^{-27}\,\text{kg} \approx 1\,\text{u}$$
Slightly heavier than the proton:
$$m_n > m_p$$ - Electric charge:
$$q_n = 0$$ - Location in the atom:
In the nucleus.
Because neutrons have nearly the same mass as protons but no charge, they add mass without adding electric charge.
Neutron as a “stabilizer”
Neutrons help to:
- Bind protons together: positively charged protons repel each other; neutrons contribute to the attractive nuclear forces without adding extra repulsion.
- Influence nuclear stability:
For many elements, there is a limited range of neutron numbers that give stable nuclei. Too few or too many neutrons can make a nucleus unstable (radioactive).
Atoms of the same element can have different numbers of neutrons. These variants are called isotopes and are handled in detail elsewhere; here the important point is that neutrons can vary while the element (number of protons) stays the same.
Counting Nucleons: $Z$, $N$, and $A$
To describe a nucleus, we often use three key numbers:
- $Z$ = number of protons (atomic number)
- $N$ = number of neutrons
- $A$ = mass number = total number of nucleons
By definition:
$$A = Z + N$$
Examples:
- $^1_1\text{H}$ (protium):
- $Z = 1$ (1 proton)
- $N = 0$ (no neutrons)
- $A = 1$
- $^{12}_6\text{C}$:
- $Z = 6$
- $N = 6$
- $A = 12$
- $^{16}_8\text{O}$:
- $Z = 8$
- $N = 8$
- $A = 16$
The symbol of a nuclide is often written as:
$$^{A}_{Z}\text{X}$$
where X is the element symbol, $Z$ is written as a subscript (bottom left), and $A$ as a superscript (top left).
Because each chemical element has a fixed $Z$, we can often omit $Z$ (it is implied by X) and write only $^AX$ in many contexts.
Mass of Nucleons and Nuclear Mass
Approximate nucleon mass and atomic mass unit
Protons and neutrons each have a mass very close to 1 atomic mass unit (1 u):
- $$m_p \approx 1.0073\,\text{u}$$
- $$m_n \approx 1.0087\,\text{u}$$
Since $A$ is the total number of nucleons, the mass of a nucleus with mass number $A$ is roughly $A\,\text{u}$ – but the actual nuclear mass is slightly less due to binding effects (addressed elsewhere).
This simple counting makes $A$ a convenient way to estimate nuclear and atomic masses in basic calculations.
Mass number vs. atomic mass
For a single nuclide:
- The mass number $A$ is an integer (just counts nucleons).
- The actual mass (in u or kg) is not exactly equal to $A$ because:
- Proton and neutron masses are not exactly 1 u.
- Binding energy in the nucleus causes a mass defect.
When you see a number like “carbon-12”:
- “12” is the mass number $A$ (12 nucleons)
- The actual isotopic mass is almost exactly 12 u by definition for $^{12}\text{C}$.
Charge and Composition of the Nucleus
Net charge of the nucleus
Because:
- Protons have charge $+e$
- Neutrons have charge 0
The total charge $Q_{\text{nucleus}}$ of a nucleus is:
$$Q_{\text{nucleus}} = Z \cdot (+e) = +Ze$$
For a neutral atom, the number of electrons $n_e$ equals the number of protons $Z$, so the negative charge of the electrons balances the positive nuclear charge.
Summary of composition
A nucleus is fully specified (for our purposes) by:
- $Z$ protons: determine the element and total positive charge
- $N$ neutrons: determine the isotope and strongly influence stability
- $A = Z + N$: the total count of nucleons, closely related to mass
Internal Structure of Nucleons (Conceptual Overview)
At the nuclear-chemistry level used in this course, protons and neutrons are treated as elementary building blocks of nuclei. On a deeper level (beyond what we use for basic nuclear chemistry), both protons and neutrons:
- Are composite particles
- Belong to a family called baryons
- Are made of more fundamental constituents called quarks, held together by the strong interaction
However, in most nuclear-chemistry descriptions, you work with nucleons as if they were indivisible particles characterized by their mass and charge, without needing to use their quark structure.
Nucleons in Nuclear Reactions (Outlook)
While detailed nuclear reactions are covered elsewhere, it is useful here to note how nucleons appear in nuclear equations:
- In nuclear reactions, protons and neutrons can be rearranged between nuclei.
- The total number of nucleons $A$ is usually conserved in simple nuclear reactions:
$$\sum A_{\text{reactants}} = \sum A_{\text{products}}$$ - The total charge (and thus total number of protons) is also conserved:
$$\sum Z_{\text{reactants}} = \sum Z_{\text{products}}$$
Understanding protons, neutrons, and how they are counted with $Z$, $N$, and $A$ is therefore essential for reading and writing nuclear reaction equations, discussing isotopes, and analyzing nuclear stability in later parts of this course.