Inside the atom.

To explain chemical behavior, start with the particles that make up every atom—and the small set of rules connecting them.

By the end, you can:

Relate protons, neutrons, and electrons to atomic identity and charge.
Use atomic number and mass number to determine particle counts.
Recognize isotopes as atoms of the same element with different neutron counts.

Core idea

Three particles, three different roles

An atom contains a compact nucleus made of protons and neutrons. Electrons occupy the much larger region surrounding that nucleus. Most chemical changes rearrange electrons; they do not change the nucleus.

p⁺ n⁰

Particle relationships

Protons carry +1 charge, electrons carry −1 charge, and neutrons have no charge. A neutral atom has equal numbers of protons and electrons.

Identity comes from protons.The number of protons is the atomic number. Change the proton count and you have changed the element itself.

Atomic notation

Mass number counts the nucleus

The mass number is the total number of protons and neutrons in one particular atom. Electrons contribute so little mass that they are not included in this count.

mass number = protons + neutrons

Atoms of the same element always have the same number of protons, but they can have different numbers of neutrons. These variants are called isotopes.

Do not confuse the two numbers.Atomic number identifies the element. Mass number identifies a particular isotope of that element.

Worked example

Decode an isotope

Problem

A neutral atom of chlorine-37 has atomic number 17. How many protons, neutrons, and electrons does it contain?

Protons: atomic number = 17, so the atom has 17 protons.
Neutrons: mass number − protons = 37 − 17 = 20 neutrons.
Electrons: the atom is neutral, so electrons equal protons: 17 electrons.

Notice that “chlorine” already fixes the proton count. The isotope label tells us which neutron count this atom has.

Practice checkpoint

Original question

A neutral atom has 12 protons and a mass number of 26. Which statement is correct?

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