Atoms
1. What Is an Atom?
An atom is the smallest unit of matter that keeps the properties of an element.
Everything — solids, liquids, gases, and plasma — is made up of atoms.
Think of it like this:
If you take a piece of gold and keep cutting it smaller and smaller, you’d eventually
reach one gold atom — the tiniest piece that’s still gold.
2. The Structure of an Atom
B. The Electron Cloud (Outside)
An atom has two main regions:
Electrons move around the nucleus
A. The Nucleus (Center)
in energy levels or orbitals.
Contains protons and neutrons.
The electrons determine chemical
Is very small but extremely dense
behavior and bonding.
(holds most of the atom’s mass).
Particle
Proton
Symbol
p⁺
Charge
1
Location
Nucleus
3. Atomic Number, Mass,
Neutron
Electron
n⁰
e⁻
0
-1
Nucleus
Electron
and Isotopes
cloud
Term Definition
Number of protons. Defines the element
Atomic Number (Z)
(e.g., Hydrogen = 1, Oxygen = 8).
Mass Number (A) Number of protons + neutrons.
Atoms of the same element with different
Isotopes
neutrons (e.g., Carbon-12, Carbon-14).
Atom that gains or loses electrons →
Ion
becomes charged (e.g., Na⁺, Cl⁻).
1. What Is an Atom?
An atom is the smallest unit of matter that keeps the properties of an element.
Everything — solids, liquids, gases, and plasma — is made up of atoms.
Think of it like this:
If you take a piece of gold and keep cutting it smaller and smaller, you’d eventually
reach one gold atom — the tiniest piece that’s still gold.
2. The Structure of an Atom
B. The Electron Cloud (Outside)
An atom has two main regions:
Electrons move around the nucleus
A. The Nucleus (Center)
in energy levels or orbitals.
Contains protons and neutrons.
The electrons determine chemical
Is very small but extremely dense
behavior and bonding.
(holds most of the atom’s mass).
Particle
Proton
Symbol
p⁺
Charge
1
Location
Nucleus
3. Atomic Number, Mass,
Neutron
Electron
n⁰
e⁻
0
-1
Nucleus
Electron
and Isotopes
cloud
Term Definition
Number of protons. Defines the element
Atomic Number (Z)
(e.g., Hydrogen = 1, Oxygen = 8).
Mass Number (A) Number of protons + neutrons.
Atoms of the same element with different
Isotopes
neutrons (e.g., Carbon-12, Carbon-14).
Atom that gains or loses electrons →
Ion
becomes charged (e.g., Na⁺, Cl⁻).
