Chemistry · Grades 9–12

General chemistry · Quick reference

Reference sheet for Grades 9 to 12: periodic trends, predicting ion charges and writing ionic formulas. Across a period, more protons pull the same electron shells closer, so atoms get smaller and hold electrons more strongly.

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Rule and key idea

Periodic trends

Across a period, more protons pull the same electron shells closer, so atoms get smaller and hold electrons more strongly. Down a group, extra shells make atoms larger.

radius: ↓ across a period, ↑ down a group · electronegativity: ↑ across, ↓ down

Which atom has the larger atomic radius: barium (Ba) or calcium (Ca)? → barium (Ba)

Predicting ion charges

Main-group atoms tend to lose or gain electrons until they have the same arrangement as the nearest noble gas. Metals form positive ions; non-metals form negative ions.

group 1 → 1+ · group 2 → 2+ · Al → 3+ · group 15 → 3− · group 16 → 2− · group 17 → 1−

What charge does an ion of sulfur (S) usually have in ionic compounds? → 2−

Writing ionic formulas

Combine the ions in the smallest whole-number ratio that makes the total charge zero. The metal is written first.

total positive charge = total negative charge

Which is the correct formula for the ionic compound formed from lithium (Li) and oxygen (O)? → Li₂O

Check yourself

Four practice questions on this topic. Try each one before you open the answer.

A model sample has mass 219 g and stated molar mass 113 g/mol. Find its amount. Round to 3 decimal places if needed. Answer in mol.
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1.938 mol n = m/M 219/113 = 1.938
Which atom has more valence electrons: iodine (I) or silicon (Si)?
- iodine (I)
- silicon (Si)
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iodine (I) Valence electrons: I has 7 (group 17); Si has 4 (group 14).
Which atom has more valence electrons: oxygen (O) or beryllium (Be)?
- beryllium (Be)
- oxygen (O)
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oxygen (O) Valence electrons: O has 6 (group 16); Be has 2 (group 2).
A model sample has mass 124 g and stated molar mass 59 g/mol. Find its amount. Round to 3 decimal places if needed. Answer in mol.
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2.102 mol n = m/M 124/59 = 2.102