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I am really sorry if this is a beginner question but please still help me. In this website, which explains balancing chemical equations mentioned the word formula unit here:

$\ce{3Ca(NO3)2}$ (just the oxygens) ---> There are 18. The 3 on the nitrate times 2 outside the parenthesis equals 6 oxygen in one formula unit. The coefficient of three times the 6 gives the final answer of 18.

My question: What does formula unit mean?

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  • $\begingroup$ The formula $3$ $\ce{Ca(NO3)2}$ means that you have taken three times the same atomic assembly, or the same formula unit. Each assembly, or each formula unit, is made of one Calcium atom surrounded by two groups of atoms. And each group of atoms is made of one Nitrogen atom surrounded by three oxygen atoms. The formula unit is simply $\ce{Ca(NO3)2}$ $\endgroup$ – Maurice Aug 30 '20 at 16:39
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    $\begingroup$ Formula unit in calculations is one complete molecular formula. For instance, here it is $\ce{Ca(NO3)2}$. For example, in organic chemistry, empirical formula of benzene is $\ce{CH}$. However its molecular formula is $\ce{C6H6}$, which is used as the formula unit in calculations. $\endgroup$ – Mathew Mahindaratne Aug 30 '20 at 16:42
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    $\begingroup$ "Formula unit in calculations is one complete molecular formula. For instance, here it is Ca(NO3)2.", that really helped explain it simply. Thanks a lot @MathewMahindaratne $\endgroup$ – Saifeldin Farid Aug 30 '20 at 16:50