the discovery of the electron disproved the idea that atoms were tiny spheres that couldn't be… what?

The discovery of the electron disproved the idea that atoms were tiny spheres that could not be divided (i.e., that they were indivisible).

What idea was disproved?

Before J. J. Thomson’s work, John Dalton’s atomic theory treated atoms as the smallest possible, solid, indivisible spheres of matter.

When Thomson identified the electron as a subatomic particle contained within atoms, it showed that atoms themselves have smaller parts, so they are not indivisible spheres.

Put simply

• Atoms used to be thought of as solid, structureless balls.

• The electron’s discovery revealed internal structure inside atoms, so they can be broken into smaller pieces.

So the completed sentence is:

“The discovery of the electron disproved the idea that atoms were tiny spheres that couldn’t be divided (were indivisible).”

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