Solids have a definite shape and volume. They are always the same shape no matter what they are contained in; their volume is also the same because they don't change unless you add or take away from it.
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Liquids have an indefinite shape but definite volume. They expand to fill out the space they are contained in, but their volume doesn't change unless you take out or add more of the liquid.
Gases have an indefinite shape and volume. Gases expand to fill out the space they are in and also don't have a clear shape because they are not always in one form.
1 mole Cu ------------------ 6.02x10²³ atoms
0.90 moles Cu ------------ ??
0.90 x ( 6.02x10²³ ) / 1 =
= 5.41x10²³ atoms
