Everything around us is made of atoms – but what is an atom, really?

From the screen you’re reading this on to the water you drink, atoms are everywhere. They’re the basic building blocks of all matter — but many students find the idea of atoms a bit confusing at first.

This blog is written just for you — a simple, clear guide to help you understand atoms, step by step.

In this post, you’ll learn:

• What an atom actually is

• What parts make up an atom

• How scientists discovered the structure of atoms over time

Let’s break it down together — nice and easy.

What Is an Atom?

Think about this: you’re crushing a piece of gold.

First, you break it into two parts. Then into smaller and smaller bits. But no matter how many times you break it, it’s still gold — it doesn’t lose its identity. It looks smaller, but it’s still the same substance.

Now here’s the question: how small can you really go?
Even if you break it until you can’t see it anymore, is there a limit?

Scientists asked the same thing. They went far beyond what our eyes can see — and discovered something amazing.

They found that all substances around us — gold, air, water, wood — are made from tiny building blocks called atoms.

These atoms are so small that you can’t see them without special equipment. But they are the basic units of matter — meaning everything you see, touch, or feel is made of atoms.

What Are the Parts of an Atom?

At first, scientists had no idea what atoms looked like. Technology wasn’t advanced back then, so they couldn’t see atoms directly. Instead, they used different experiments and guessed what atoms might look like.

Because of that, many different atomic models were suggested over time. Some were just ideas. Others were based on results from real experiments.

Today, scientists understand atoms much better  but in GCSE and school-level chemistry, we often use the Dalton Model or the basic Bohr Model to explain atomic structure.

According to Bohr’s atomic model, an atom has a tiny central nucleus, and electrons move around it in fixed shells or paths  just like planets around the sun (but much, much smaller).

At first, scientists believed that the atom was the smallest unit of matter,  that it couldn’t be divided any further. But later, with better experiments and technology, they discovered that atoms are actually made of even smaller particles.

These smaller parts are called subatomic particles, and there are three main ones you need to know:

• Proton

• Neutron

• Electron

Each one has a different charge, mass, and place inside the atom  and they all work together to form the structure of an atom.

Back in the day, scientists couldn’t see atoms  and definitely not what was inside them. They had no microscopes strong enough. So how did they find out that atoms are made of smaller particles?

They didn’t just guess. Instead, they did clever experiments and looked at how atoms behaved. By studying electricity, radiation, and how particles moved, they slowly discovered the proton, neutron, and electron.

So, What Makes These Sub Atomic Particles Different?

Let’s break them down simply:

• Some have positive charge, some have none, and some are negative

• Some have mass, some are lightweight

• And they don’t all live in the same part of the atom

Here’s a clear table to help you understand:

Subatomic Particles Summary Table

Protons and neutrons stay packed together in the center of the atom, that’s the nucleus.
Electrons are much smaller and move around the nucleus in shells or energy levels.

So when we take a closer look at an atom, we can notice that it has some important characteristics  things like:

• Atomic number

• Mass number

• And whether the atom has a charge or not

These help us identify the atom and understand how it behaves in chemical reactions.

Let’s break each one down clearly so it’s easy to understand.

Atomic Number and Mass Number

When we look at an atom, two of the most important things we can find are its atomic number and mass number. These numbers help us understand what the atom is, how heavy it is, and how it might react with other atoms.

Atomic Number – The Atom’s Identity Card (A)

The atomic number is a constant for each element. That means it never changes. The atomic number acts like an identification number for every atom. It’s unique  no two different elements can have the same atomic number.

We can identify any atom just by knowing its atomic number, because it tells us how many protons are in the nucleus.

For example:

• If an atom has 1 proton, it’s always hydrogen. No other element has 1 proton.

• If an atom has 8 protons, it’s oxygen — always. Oxygen will never have 7 or 9 protons.

• Fluorine has 9 protons → atomic number = 9

So:

Atomic number = number of protons = the identity of the element

Even if the atom gains or loses electrons, or becomes part of a compound — its atomic number stays the same, because the protons inside the nucleus don’t change.

That’s why the atomic number is so important in chemistry — it helps us identify atoms just like a name or fingerprint.

Tip for Remembering Atomic Number

The atomic number of an element is also its position in the Periodic Table.

That means:

• The 1st element in the Periodic Table is Hydrogen, and its atomic number is 1

• The 8th element is Oxygen, and its atomic number is 8

• The 11th element is Sodium, and its atomic number is 11

Atomic number = the element’s order in the Periodic Table

What Is Mass Number? (Z)

The mass number tells us how heavy an atom is.

In real life, we measure weight using units like grams, kilograms, or milligrams. But as we already know, atoms are extremely small — so small that our normal weighing scales can’t measure them at all.

That’s why scientists use a different method to talk about the “mass” of atoms.

Instead of grams, we just count the number of particles inside the atom. And the ones that matter are:

• Protons

• Neutrons

These two particles are found in the nucleus, and they have mass.

So the formula is:
Mass number = number of protons + number of neutrons

But wait — what about electrons? Aren’t they part of the atom too?

Yes, they are! But electrons are so small and so light that their mass is almost nothing compared to protons and neutrons.

To help you understand this, imagine you place a sensitive digital scale in open air. Even though air is touching it, the scale doesn’t show any weight — because the air is too light to register.
That’s how small an electron’s mass is — we don’t include it when calculating the atom’s mass.

Examples: How to Calculate Mass Number

Example: Isotopes of Hydrogen

Let’s take hydrogen as an example.

Hydrogen is the first element in the Periodic Table and always has 1 proton. But it actually has three different isotopes — and they all are still hydrogen!

So why are there different types?
Because each one has a different number of neutrons in the nucleus.

• Protium is the most common form — it’s what you find in normal water.

• Deuterium is heavier and is used in scientific research and nuclear reactors.

• Tritium is radioactive and used in glow-in-the-dark devices and nuclear fusion experiments.

Even though they have different neutrons and mass numbers, all three are still hydrogen — because they all have 1 proton, and that’s what makes them hydrogen.

That’s the key idea of isotopes:
Same element, different versions

Do Isotopes Behave Differently?

Now you might be wondering:

If isotopes have different numbers of neutrons, does that change how they react in chemistry?

Here’s the simple answer:

Chemical properties stay the same

Because chemical reactions are all about electrons — and all isotopes of an element have the same number of protons and electrons they behave the same way in chemical reactions.

So:

• Protium, Deuterium, and Tritium all react like hydrogen

• Chlorine-35 and Chlorine-37 both form compounds like NaCl (salt)

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Physical properties may change

But since isotopes have different mass numbers, their physical properties can be slightly different.
Examples:

• Heavier isotopes may move slower in reactions

• Their melting or boiling points might be slightly higher or lower

• Tritium is radioactive — that’s a physical property that’s very different from Protium

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Summary: