Subatomic Particles

New Evidence from a Tiny Space

Dalton's model made chemical reactions easier to count, but it still pictured atoms as solid spheres. That model left a simple problem: if atoms were truly solid and had no inner parts, why were atoms connected to electricity?

In the late 1800s, scientists began using low-pressure glass tubes and high voltage. A beam called a cathode ray appeared inside the tube. The beam could be bent by electric and magnetic fields. From this evidence, atoms started to look less like solid balls and more like structures with smaller particles inside.

OpenStax Chemistry Atoms First 2e, in the Evolution of Atomic Theory section, explains the evidence sequence from Thomson's cathode ray experiment, Rutherford's gold foil experiment, and Chadwick's discovery of the neutron. The source section can be opened through openstax.org.

Britannica also records that J. J. Thomson discovered the electron in $1897$ and showed that this particle was far lighter than an atom. The biography can be opened through britannica.com.

Particle Discovery Mini Lab

Switch the mode below. Do not start by memorizing particle names. First watch the direction of motion, which path bends, and which path stays straight.

Subatomic Particle Mini Lab

Three key experiments that made atoms no longer look like solid spheres with no inner structure.

The cathode ray bent toward the positive plate. That means the particles in the beam carried negative charge.

Particle: $e^-$
Evidence: Attracted to positive charge
Meaning: Atoms have inner parts

Electrons Opened the Door

Thomson used a cathode ray tube. Inside the tube, most air was removed, then a high voltage was applied. The beam that appeared was always attracted toward positive charge and pushed away from negative charge.

The electrical rule is simple: opposite charges attract, and like charges repel. Because the cathode ray was attracted to positive charge, its particles had negative charge. We now call those particles electrons.

\text{cathode ray attracted to } (+) \Rightarrow \text{particle has } (-) \text{ charge}

Thomson also found that cathode ray particles were far lighter than atoms and had the same properties even when the electrode metal changed. That matters because the electron was not a fragment from one specific metal. The electron is a common part of atoms.

To picture Thomson's model, imagine plum pudding. The pudding represents spread-out positive charge, while the plums represent embedded negative electrons. This picture only helps us read the model, not the real shape of an atom. Rutherford later improved Thomson's model.

A Tiny Dense Nucleus

Rutherford tested Thomson's model by firing alpha particles, which are positively charged particles, at a very thin sheet of gold foil. If positive charge were spread out evenly as Thomson's model suggested, almost all alpha particles should pass through with only small bends.

The result was more surprising:

most alpha particles passed through the gold foil with little change in direction
a small fraction bent
a very tiny fraction bounced backward

OpenStax explains that this pattern led Rutherford to two conclusions: atoms are mostly empty space, and positive charge is concentrated in a tiny, relatively heavy center. That small center is called the atomic nucleus.

To picture the size, imagine the nucleus as a small berry in the middle of a stadium. The atom would be about the size of the stadium. OpenStax Chemistry Atoms First 2e, in the Atomic Structure and Symbolism section, states that an atom's diameter is about $10^{-10}\ \text{m}$ , while the nucleus is about $10^{-15}\ \text{m}$ across, roughly $10^5$ times smaller. The reference can be opened through openstax.org.

Neutrons Closed the Mass Gap

Rutherford's model explained the positive nucleus and negative electrons, but there was still a gap: atomic masses were often larger than the mass of the protons alone. If protons explained positive charge, what added mass to the nucleus without adding charge?

In $1932$ , James Chadwick showed evidence for a neutral particle with almost the same mass as a proton. That particle is called the neutron. Neutral means its charge is $0$ , so neutrons can add mass to a nucleus without changing the atom's electric charge.

Nobel Prize facts for James Chadwick state that he proved radiation from beryllium hit by alpha particles consisted of neutral particles with about the same mass as protons. The summary can be opened through nobelprize.org.

Neutrons also help explain isotopes. Isotopes are atoms of the same element with different numbers of neutrons. The element stays the same because the proton number stays the same, while the mass can differ because the neutron number differs.

Comparing the Three Subatomic Particles

This table is a compact guide for reading atoms: which particle has which charge, where it is found, and how much mass it contributes.

Particle	Main location	Relative charge	Relative mass	How to picture it
Electron	Space around the nucleus	$-1$	About $\frac{1}{1800}$ of a proton's mass	A negative charge marker in atomic space
Proton	Nucleus	$+1$	About $1\ \text{u}$	The particle that determines element identity
Neutron	Nucleus	$0$	About $1\ \text{u}$	Mass inside the nucleus without extra charge

A neutral atom has the same number of protons and electrons. For example, if an atom has $6$ protons and is still neutral, it also has $6$ electrons.

\text{neutral atom} \Rightarrow \text{number of } p^+ = \text{number of } e^-

Hold these three relationships: protons determine element identity, neutrons help determine mass, and electrons determine many chemical behaviors. With this guide, charge, mass, and atomic identity become easier to read together.