Back to PHYSICAL SCIENCE
Unit 2•Lesson 1 3 min read

The Atom: Protons, Neutrons, and Electrons

7/18

Learning Objectives

Describe the basic structure of an atom.
Identify the three main subatomic particles: protons, neutrons, and electrons.
State the charge and location of each subatomic particle.
Define atomic number and mass number, and use them to determine the composition of an atom.

The Building Blocks of Matter

All matter, from the air you breathe to the chair you're sitting on, is made of tiny particles called atoms. An atom is the smallest unit of a chemical element that retains the properties of that element. For a long time, people thought atoms were the smallest things possible, but we now know they are made of even smaller, or subatomic, particles.

The Parts of an Atom

A simple model of an atom looks a bit like a miniature solar system. It has a central core, called the nucleus, with other particles orbiting around it. The three main subatomic particles are protons, neutrons, and electrons.

1.Proton (p⁺)
Charge: Has a positive (+) electrical charge.
Location: Found inside the nucleus.
Mass: Has a relative mass of approximately 1 atomic mass unit (amu).
2.Neutron (n⁰)
Charge: Is neutral, meaning it has no charge.
Location: Found inside the nucleus.
Mass: Has a relative mass of approximately 1 amu (slightly heavier than a proton).
3.Electron (e⁻)
Charge: Has a negative (-) electrical charge.
Location: Orbits the nucleus in a region called the electron cloud.
Mass: Has a very, very tiny mass, about 1/1836th the mass of a proton. Its mass is so small it's considered negligible when calculating the total mass of an atom.

Identifying Atoms: Atomic Number and Mass Number

The nucleus is the dense center of the atom, containing the protons and neutrons. Since protons are positive and electrons are negative, they are attracted to each other, which is what keeps the electrons in orbit around the nucleus.

In a neutral atom, the number of protons is equal to the number of electrons, so the positive and negative charges balance out.

We can identify an atom by the number of particles in its nucleus.

Atomic Number (Z): This is the number of protons in the nucleus of an atom. The atomic number is what defines an element. Every atom of Carbon has 6 protons. Every atom of Oxygen has 8 protons.
Mass Number (A): This is the total number of protons and neutrons in the nucleus. It's a whole number that tells you the approximate mass of the atom.

You can find the number of neutrons in an atom by subtracting the atomic number from the mass number.

Number of Neutrons = Mass Number - Atomic Number

Example: An atom of lithium has an atomic number of 3 and a mass number of 7.

Number of Protons = 3 (from the atomic number)
Number of Electrons = 3 (to balance the protons in a neutral atom)
Number of Neutrons = 7 - 3 = 4

Key Terms

**Atom
The smallest particle of a chemical element that can exist.
**Subatomic Particle
A particle smaller than an atom. The three main ones are the proton, neutron, and electron.
**Proton (p⁺)
A subatomic particle with a positive charge, found in the nucleus of an atom.
**Neutron (n⁰)
A subatomic particle with no charge (neutral), found in the nucleus of an atom.
**Electron (e⁻)
A subatomic particle with a negative charge, found in the electron cloud orbiting the nucleus.
**Nucleus
The dense, positively charged central region of an atom, composed of protons and neutrons.
**Atomic Number (Z)
The number of protons in the nucleus of an atom, which determines the chemical element.
**Mass Number (A)
The total number of protons and neutrons in an atomic nucleus.

Check Your Understanding

1

What is the electrical charge of a neutron?

2

An atom has 8 protons, 8 electrons, and 8 neutrons. What is its atomic number?

3

A neutral atom has an atomic number of 12 and a mass number of 25. How many neutrons does it have?

Previous
Electricity and Simple Circuits
Next
The Periodic Table