KAHIBARO
Table of Contents
Matter Inside the Nucleus
The atomic nucleus is made of two kinds of particles, protons and neutrons. These particles are called nucleons. They are packed into a very small region at the center of the atom. Although the nucleus is tiny compared with the whole atom, it contains almost all of the atom's mass.
Protons and neutrons are similar in many ways. They have nearly the same mass, and both behave as particles of the nucleus. But they differ in electric charge, and this difference is extremely important for the structure of atoms.
The Proton
A proton carries a positive electric charge. Its charge is equal in magnitude to the charge of the electron, but opposite in sign. If the proton charge is written as $+e$, then
$$
e = 1.602 \times 10^{-19}\,\text{C}
$$
This means each proton contributes one unit of positive charge to the nucleus.
The mass of a proton is approximately
$$
m_p \approx 1.673 \times 10^{-27}\,\text{kg}
$$
In atomic mass units, this is about
$$
m_p \approx 1.0073\,\text{u}
$$
Because the nucleus contains protons, the nucleus is positively charged. This positive charge attracts the negatively charged electrons that surround the nucleus.
Important fact: A proton has charge $+e$ and mass about $1.67 \times 10^{-27}\,\text{kg}$.
The Neutron
A neutron has no electric charge. Its charge is zero, so it does not contribute to the net electric charge of the nucleus. Even though it is electrically neutral, it is still a major part of nuclear matter.
The mass of a neutron is slightly larger than the mass of a proton:
$$
m_n \approx 1.675 \times 10^{-27}\,\text{kg}
$$
In atomic mass units,
$$
m_n \approx 1.0087\,\text{u}
$$
The neutron is very important because it helps build nuclei without increasing the electric repulsion that exists between positively charged protons.
Important fact: A neutron has zero charge and a mass slightly greater than the proton mass.
Comparing Protons and Neutrons
Protons and neutrons are close relatives in nuclear physics. Their masses are nearly equal, which is why both are grouped together as nucleons.
| Property | Proton | Neutron |
|---|---|---|
| Symbol | $p$ | $n$ |
| Electric charge | $+e$ | $0$ |
| Charge in coulombs | $+1.602 \times 10^{-19}\,\text{C}$ | $0$ |
| Mass in kg | $1.673 \times 10^{-27}$ | $1.675 \times 10^{-27}$ |
| Mass in u | $1.0073$ | $1.0087$ |
| Location in atom | Nucleus | Nucleus |
The small mass difference matters in nuclear processes, but for many basic ideas it is enough to remember that both have masses of about $1\,\text{u}$.
Why Both Are Needed
If nuclei contained only protons, the positive charges would strongly repel each other because of the electric force. Neutrons help make larger nuclei possible because they add mass and participate in the nuclear interaction without adding electric repulsion.
A simple way to think about it is that protons determine the positive charge of the nucleus, while neutrons help provide extra nuclear matter and stability.
Protons, Neutrons, and Atomic Identity
The number of protons in a nucleus determines which chemical element the atom is. For example, every hydrogen nucleus has one proton, and every helium nucleus has two protons. Changing the number of protons changes the element itself.
Neutrons do not change the element's identity, but changing the number of neutrons changes the form of that element. These different forms are called isotopes, which are discussed in their own chapter.
Key rule: The number of protons determines the element. Neutrons affect the mass and stability, but not the element's identity.
A Note on Their Internal Structure
Protons and neutrons are not truly fundamental particles. Each is made of smaller particles called quarks. At this stage, the most important point is simply that protons and neutrons are the building blocks of the nucleus. Their deeper internal structure belongs to particle physics.
Summary
Protons and neutrons are the two particles that make up atomic nuclei. A proton has positive charge and a mass close to $1\,\text{u}$. A neutron has no charge and a slightly larger mass. Both are nucleons, and together they account for nearly all the mass of ordinary matter.
Essential summary:
$$
\text{Nucleus} = \text{protons} + \text{neutrons}
$$
Protons give the nucleus its positive charge, and neutrons add mass and help nuclei exist and remain stable.