We already know the three particles that make up atoms: protons, neutrons, and electrons. Now the focus is not “who discovered them,” but which properties make them different.
Three properties matter most for reading simple atoms:
charge, the electric sign of the particle
mass, how much the particle contributes to atomic mass
location, the part of the atom where the particle is usually discussed
The Atomic Structure and Symbolism section from OpenStax gives a table of subatomic particle charge, mass, and location. The source table can be opened through OpenStax's Properties of Subatomic Particles.
For more precise constants, NIST publishes CODATA recommended values. The summary can be opened through NIST's CODATA 2022 wall chart.
Particle Property Mini Lab
Switch the mode below. First watch the big pattern: direction of bending, mass comparison height, and particle location inside the atom.
Subatomic Particle Property Map
One visual for reading the charge, mass, and location of protons, neutrons, and electrons.
Protons are positive, electrons are negative, and neutrons are neutral. In an electric field, charge sign controls the bending direction.
Electric charge can be read as a particle's “interaction sign.” A proton is positive, an electron is negative, and a neutron has no total charge.
The proton and electron have the same charge magnitude, but opposite signs:
e=1.602176634×10−19C
That means:
a proton has charge +e
an electron has charge −e
a neutron has charge 0
In an electric field, a charged particle feels a force. The short way to read it is:
F=qE
The symbol q is the particle charge, and E is the electric field. If q is positive, the force points with the field. If q is negative, the force points the opposite way. If q=0, the particle is not bent by a simple electric field.
An electric field can be pictured like wind that only charged objects feel. Protons and electrons both feel the push, but in opposite directions. A neutron is neutral, so in this simple model it goes straight.
Subatomic particle masses are tiny, so we often use the unified atomic mass unit, written u. It is defined from the mass of a 612C atom.
Particle
Relative charge
Relative mass
Absolute mass
Main location
Proton
+1
1.0073u
1.67262×10−24g
Nucleus
Neutron
0
1.0087u
1.67493×10−24g
Electron
−1
0.00055u
9.10939×10−28g
The table shows two important ideas. First, protons and neutrons are almost equally heavy. Second, electrons are extremely light compared with protons and neutrons.
A simple comparison is:
memp≈1836
So one proton has about the same mass as 1836 electrons. That is why almost all atomic mass comes from the nucleus, not from the electrons.
If two objects receive the same push, a tennis ball changes motion more easily than a bowling ball. This is only a mass comparison, not the shape of a particle. In an atom, an electron is much lighter, so a force can change its motion much more than it changes a proton's motion.
Particle location gives each particle a different role.
Particle
Location
Common role in chemistry
Proton
Nucleus
Determines element identity.
Neutron
Nucleus
Adds mass and distinguishes isotopes.
Electron
Space around the nucleus
Strongly affects reactions and ion formation.
The number of protons determines the element. For example, an atom with 6 protons is carbon. If the neutron number changes, the element is still carbon, but the isotope is different. If the electron number changes, the atom becomes an ion.
For a neutral atom, the number of protons and electrons is the same:
neutral atom⇒p+=e−
That sentence does not mean protons and electrons have the same mass. It means the total positive and negative charges balance.
Atomic symbols use three important pieces of information. On Atomic Symbols, that information is written around the element symbol. Subatomic particle properties tell us how to read it:
the number of protons is read as the atomic number
the number of protons plus neutrons is read as the mass number
the difference between protons and electrons is read as the ion charge
So do not memorize the table as disconnected numbers. Read the main relationship: protons give identity, neutrons add mass, and electrons control the atom's outer charge.
