Grade 9 · Ions and Our Health · Ions
Cations and Anions
Naming, writing and predicting the charges of positive and negative ions.
Learning objectives
- Write correct symbols for common ions.
- Predict ion charge from group number.
- Explain why metals form cations and non-metals form anions.
NGSS alignment
HS-PS1-1
Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
HS-PS1-2
Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
HS-PS1-3
Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles.
MYP criteria
- Criterion A — Knowing and Understanding
ENGAGE
Two ions, two very different jobs
The difference between an atom and its ion is a single electron - and it changes everything about how the particle behaves.
Think about it
Sodium metal reacts violently with water, yet sodium ions are safe enough to eat every day. Why?
EXPLAIN
Naming and writing ions
Cations are positive and keep the element name (sodium ion, Na+). Anions are negative and usually end in -ide (chloride, Cl-).
| Group | Typical ion | Charge | Type |
|---|---|---|---|
| 1 | Na+, K+, Li+ | 1+ | Cation |
| 2 | Mg2+, Ca2+ | 2+ | Cation |
| 16 | O2-, S2- | 2- | Anion |
| 17 | F-, Cl-, Br- | 1- | Anion |
Shortcut
For main-group elements, group 1 and 2 metals lose electrons; groups 16 and 17 non-metals gain electrons to complete their outer level.
Interactive simulation · PhET
Build an Atom
Make one cation and one anion, then compare their charges.
While you explore
- Which particle changes to form an ion?
- Why are metals more likely to form cations?
Practice questions
0/2 correct
Level 1
Which of these is a cation?
Level 2
An element in group 16 usually forms an ion with which charge?
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