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What is an atom?

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Used in a sentence

University Course Reader · STEM

A single carbon atom can bond with four others, which is why carbon builds such long chains.

The reader highlighted one word in a textbook passage. Clicked broke down the concept “atom” into plain English:

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Overview

An atom is the smallest piece of an element that is still that element. Break one apart and you no longer have gold or oxygen. You have the pieces every atom is made from, and those pieces are the same whichever element you started with. What makes gold gold is how many protons sit in the middle.
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Overview

An atom is the smallest scrap of an element that still counts as that element. Split a single gold atom and the pieces are not gold any more, just protons, neutrons and electrons. What decides which element you have is a headcount of the protons in the middle. Very small detail to hang an entire periodic table on. 😎

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Detail

An atom is the smallest piece of an element that is still that element. It has a dense middle, called the nucleus, packed with protons and usually neutrons. Electrons occupy the space around it. That space is nearly all of the atom's size, while nearly all its weight sits in the nucleus. The proton count names the element: one makes hydrogen, six makes carbon, seventy-nine makes gold. Change that count and you have changed the element itself. Chemistry only ever rearranges the electrons on the outside, so burning, rusting and digesting all leave the nuclei untouched. That is why turning lead into gold was never a chemistry problem. It is also the whole difference between a chemical reaction and a nuclear one.
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Detail

An atom is the smallest scrap of an element that still counts as that element. It has a heavy clump at the core and a fuzzy cloud around it. Nearly all the mass sits in that clump and nearly all the room belongs to the cloud, so the thing is mostly nothing. What decides which element you are holding is a headcount of the protons in that core. One is hydrogen, seventy-nine is gold, eighty-two is lead. Alchemists burned centuries trying to turn lead into gold with furnaces. That is a three-proton problem, and the protons sit in a core no furnace can reach. Ordinary reactions only ever shuffle the outer cloud. Burn something, rust it, eat it, and the core comes out the far side unbothered. 😎

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Analogy

An atom is built like a stadium with a marble at the centre. The marble is the nucleus. It holds nearly all the weight. The electrons are somewhere in the stands, and everything between them and the marble is empty. Solid objects feel solid because the electrons on each surface push against each other, and that push is what you feel when you touch something.
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Analogy

An atom is a house on a street, and the number on the door is the proton count. That number is the only thing that says which house it is. Repaint it, refurnish it, knock through the back wall, and the address never moves. Change the number and you are somewhere else entirely, further down the street. Almost all of the house is rooms and air, and none of that decides the address. 😎
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Formal definition — The same term, explained the usual way

An atom is the smallest unit of a chemical element that retains that element's identity. It comprises a nucleus of protons and neutrons, bound by the strong nuclear force, surrounded by electrons occupying probabilistic orbitals. The number of protons, the atomic number, determines the element; variation in neutron number produces isotopes, and variation in electron number produces ions. Chemical reactions involve only the electron configuration, whereas altering the nucleus constitutes a nuclear process.

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