Grade 9 · Science · Unit 4 of 6
Electrons & Compounds
Why atoms bond and what they make
What your child practises
How valence electrons decide whether atoms gain, lose or share electrons, ionic and covalent compounds, and reading simple chemical formulas.
BC Curriculum learning standard: The arrangement of electrons determines the compounds formed by elements: ionic and covalent bonding
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
Table salt (NaCl) is made of…
- carbon and hydrogen
- sodium and chlorine joined by ionic bonds
- two gases sharing electrons
- metals only
Show answer
sodium and chlorine joined by ionic bonds
Hint: Sodium gives an electron to chlorine, and the ions attract.
Question 2
Why is oxygen's formula in water H₂O, with two hydrogens?
- Hydrogen is a noble gas.
- Oxygen has two protons.
- Water has no bonds.
- Oxygen needs two more electrons, and each hydrogen shares one.
Show answer
Oxygen needs two more electrons, and each hydrogen shares one.
Hint: Oxygen has 6 valence electrons and needs 2 more, so it shares with two hydrogen atoms.
Question 3
Which pair would most likely form a covalent compound?
- calcium and oxygen
- carbon and oxygen
- sodium and chlorine
- potassium and bromine
Show answer
carbon and oxygen
Hint: Two non-metals usually share electrons.
Question 4
What is the chemical formula of water?
- HO₂
- H₂O
- H₂O₂
- H₂
Show answer
H₂O
Hint: Water has 2 hydrogen atoms and 1 oxygen atom.
Question 5
Why does chlorine (7 valence electrons) form a negative ion?
- It loses a proton.
- It never bonds.
- It gains one electron to fill its outer shell.
- It loses seven electrons.
Show answer
It gains one electron to fill its outer shell.
Hint: Gaining one electron gives chlorine 8 outer electrons and a charge of −1.
Question 6
What charge does an ion of a Group 2 element (like calcium) usually have?
- 0
- +1
- −2
- +2
Show answer
+2
Hint: Group 2 elements lose their 2 valence electrons.