Structure of the Atom (Junior Cert Science): Revision Notes

Structure of the Atom

What is an atom?

An atom is the smallest particle of an element that still has all the properties of that element. Everything around us is made up of millions of tiny atoms. Even though atoms are incredibly small, scientists have discovered that they have their own internal structure made up of even smaller particles.

Subatomic particles

Atoms contain three different types of tiny particles called subatomic particles. Each type of particle has its own special properties and location within the atom.

The three types of subatomic particles

Protons are particles that carry a positive electrical charge. They are found at the centre of the atom in a region called the nucleus. Protons are relatively heavy particles compared to electrons.

Neutrons are particles that have no electrical charge – they are neutral. Like protons, neutrons are also located in the nucleus at the centre of the atom. Neutrons have approximately the same mass as protons.

Electrons are particles that carry a negative electrical charge. Unlike protons and neutrons, electrons are found outside the nucleus in a region called the electron cloud. Electrons are extremely light particles – much lighter than protons or neutrons.

Properties of subatomic particles

The table above summarises the key properties of each subatomic particle. Let's look at these properties in more detail.

Location in the atom

The atom has two main regions:

• The nucleus is the central core of the atom where protons and neutrons are found. The nucleus is very small compared to the overall size of the atom. If an atom were the size of a sports pitch, the nucleus would be like a marble at the centre.
• The electron cloud is the region surrounding the nucleus where electrons move around in paths called orbits or shells. This region takes up most of the space in an atom.

Relative mass

The relative mass tells us how heavy each particle is compared to the others.

• Protons have a relative mass of $1$ unit
• Neutrons have a relative mass of $1$ unit
• Electrons have a relative mass of $\frac{1}{1840}$ unit

This means that protons and neutrons have approximately the same mass, while electrons are much, much lighter. In fact, an electron has only $\frac{1}{1840}$ of the mass of a proton!

Relative charge

The relative charge tells us about the electrical charge on each particle.

• Protons have a relative charge of $+1$ (positive)
• Electrons have a relative charge of $-1$ (negative)
• Neutrons have a relative charge of $0$ (no charge)

Atoms are normally electrically neutral overall. This is because they contain equal numbers of protons and electrons, so the charges balance out and cancel each other.

Atomic number

Every element has its own unique atomic number. The atomic number tells us the number of protons in the nucleus of an atom of that element.

The atomic number is very important because it identifies which element an atom belongs to. If you know the atomic number, you know the element.

Since atoms are electrically neutral, the number of electrons must equal the number of protons. This means:

$\text{Number of electrons} = \text{Atomic number} = \text{Number of protons}$

Mass number

The mass number of an atom tells us about the total mass of particles in the nucleus. Since nearly all of an atom's mass comes from protons and neutrons, the mass number is calculated by adding these together:

$\text{Mass number} = \text{number of protons} + \text{number of neutrons}$

We use a special unit called the atomic mass unit (amu) to measure atomic masses. The mass number tells us the mass of an atom in atomic mass units.

Calculating the number of neutrons

If you know the mass number and the atomic number of an atom, you can calculate how many neutrons it has. Simply rearrange the mass number formula:

$\text{Number of neutrons} = \text{mass number} - \text{atomic number}$

Nuclear formula

Scientists use a special notation called the nuclear formula to write down information about an atom. The nuclear formula shows both the mass number and the atomic number:

• The mass number is written at the top left of the element symbol
• The atomic number is written at the bottom left of the element symbol

Structure of the first 10 elements

Let's look at how the atomic structure varies for the first $10$ elements in the periodic table:

Element	Atomic number	Number of protons	Number of electrons	Mass number	Number of neutrons
H	$1$	$1$	$1$	$1$	$1-1=0$
He	$2$	$2$	$2$	$4$	$4-2=2$
Li	$3$	$3$	$3$	$7$	$7-3=4$
Be	$4$	$4$	$4$	$9$	$9-4=5$
B	$5$	$5$	$5$	$11$	$11-5=6$
C	$6$	$6$	$6$	$12$	$12-6=6$
N	$7$	$7$	$7$	$14$	$14-7=7$
O	$8$	$8$	$8$	$16$	$16-8=8$
F	$9$	$9$	$9$	$19$	$19-9=10$
Ne	$10$	$10$	$10$	$20$	$20-10=10$

Remember!