What Gives an Atom its Mass and Size?

Following up on the previous post on “What is an atom and why are atoms important?”, we are now going to find out what gives an atom its mass and size.


The mass of an atom.

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The mass of an atom. (Credit: JJ. Smith)

It is the nucleus (i.e. the protons + neutrons) that make up the mass of an atom.

- Most (> 99%) of the mass of an atom is in the dense nucleus 
(i.e. protons + neutrons).
- A proton has a mass of about 1800 times greater than the mass of an electron.
- If you thought an atom was tiny, an electron is much, much smaller than an atom … it is minuscule.
- An electron is so so tiny, it hardly has any mass.
- Electrons make up less than 0.06% of the total mass of an atom.


The size of an atom.

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The size of an atom. (Credit JJ. Smith)

When it comes to the size of an atom, it is the electron that is largely responsible for the size of an atom.

- The nucleus of the atom is much, much smaller than the total size of the atom.
- The size of the atom is at least 10,000 times larger than than the nucleus of the atom.
- It is the cloud of electrons around the nucleus that gives the atom its size. It is where the electrons whiz about in space.
- While the electrons are extremely small (compared to the protons and neutrons in the atom), the space that the electrons and the electron cloud occupy is huge – relative to the nucleus of the atom.
- So, most of the atom is taken up by the electron cloud, which consist of tiny, tiny electrons whizzing about in space – therefore an atom is mostly empty space – and so, most of matter is empty space

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Mass versus size of an atom. (Credit: JJ Smith)

Summary: So now you know that:

- It is the nucleus which makes up the mass of the atom.
(Remember – the nucleus is made up of protons and neutrons.)
- It is the electron cloud that makes up the size of the atom.


Don’t miss our next post on: “How to find an atom’s mass and size.”

If you can’t wait, or want more detailed information see:
“Atoms: An Introduction to Atoms”
(contains 31 PowerPoint slides with teaching notes on each slide)


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