When all the atoms in a substance are the same, it is called a pure element.

Here's why:

* Element: An element is a fundamental substance that cannot be broken down into simpler substances by ordinary chemical means. Each element is made up of only one type of atom.

* Pure: This means the substance is not mixed with any other elements or compounds.

For example:

* Gold (Au) is a pure element, consisting only of gold atoms.

* Oxygen (O2) is a pure element, consisting of only oxygen atoms bonded together.

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