Rule of thumb · Materials and ElementsNº 102 / 162

Electron pairs get as far apart as they can

Count the things around the central atom — bonds and lone pairs alike — and

Why it works

Electron pairs are all negative, so they repel; the arrangement that survives is the one

When it fails

Wherever a lone pair is involved. A lone pair is held by one nucleus rather than shared

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

Open Molecules →
← PreviousCoffee ratios are about weight, not scoops Next →Multiplication binds tighter than addition

What shape will a molecule be?

Count the things around the central atom — bonds and lone pairs alike — and Electron pairs are all negative, so they repel; the arrangement that survives is the one .

Browse all 162 rules of thumb →