Grade 9 · Ions and Our Health · Ions
Understanding Ions
What ions are, how atoms become charged, and why your body depends on them.
Learning objectives
- Explain what an ion is.
- Describe how atoms become ions.
- Distinguish between cations and anions.
- Predict the charge of common ions.
- Explain why ions are important in biological systems.
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.
MS-PS1-1
Develop models to describe the atomic composition of simple molecules and extended structures.
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
- Criterion D — Reflecting on the Impacts of Science
ENGAGE
Why does salt water conduct electricity?
EXPLORE
Predict first
EXPLAIN
What is an ion?
An atom is neutral because it has equal numbers of protons (+) and electrons (-). An ion forms when an atom gains or loses electrons, so those numbers are no longer balanced.
Losing electrons: cations
Metals tend to lose their outer electrons. A sodium atom (11 protons, 11 electrons) loses one electron and becomes Na+, a positive ion called a cation.
Gaining electrons: anions
Non-metals tend to gain electrons. A chlorine atom (17 protons, 17 electrons) gains one electron and becomes Cl-, a negative ion called an anion.
| Particle | Protons | Electrons | Charge |
|---|---|---|---|
| Na atom | 11 | 11 | 0 |
| Na+ ion | 11 | 10 | 1+ |
| Cl atom | 17 | 17 | 0 |
| Cl- ion | 17 | 18 | 1- |
Remember
The number of protons never changes when an ion forms. Only electrons move - which is why an ion is still the same element.
Common misconception
Ions are not damaged or broken atoms. A Na+ ion is still sodium; it simply has one electron fewer than protons.
Predicting charges from the periodic table
- Group 1 metals lose 1 electron: 1+ (Na+, K+)
- Group 2 metals lose 2 electrons: 2+ (Mg2+, Ca2+)
- Group 16 non-metals gain 2 electrons: 2- (O2-, S2-)
- Group 17 non-metals gain 1 electron: 1- (Cl-, F-)
Back to the phenomenon
Dissolved sodium chloride separates into free-moving Na+ and Cl- ions. Those moving charges are what carry the current and light the bulb.
INVESTIGATION
Does concentration affect conductivity?
research question
How does the concentration of a salt solution affect its electrical conductivity?
hypothesis
As the concentration of sodium chloride increases, the conductivity will increase, because more ions are available to carry charge.
variables
{ "dependent": "Conductivity measured with a conductivity probe (microsiemens/cm)", "controlled": [ "Temperature of solution", "Volume of solution", "Depth and separation of electrodes", "Same probe and same salt sample" ], "independent": "Concentration of NaCl solution (0.1, 0.2, 0.4, 0.8, 1.0 mol/dm3)" }
method
[ "Prepare five solutions of known concentration using distilled water.", "Rinse the probe with distilled water between readings.", "Place the probe to a fixed depth and record conductivity once the reading is stable.", "Repeat each concentration three times and calculate a mean.", "Record the temperature of each solution." ]
safety
[ "Wear eye protection.", "Keep electrical equipment away from spills.", "Wipe up spills immediately to avoid slips." ]
analysis
[ "Plot mean conductivity (y) against concentration (x).", "Describe the trend and state whether it is linear.", "Use the particle model to explain the trend." ]
SOURCES
Sources & references
Watch
What's an Ion?
Tyler DeWitt · 6:51
This video shows electron transfer as an animation, which makes the change in charge much easier to picture.
Before you watch: Before watching: what do you think happens to the electrons when sodium meets chlorine?
- Which particle moves when an ion forms?
- Why does a metal usually form a positive ion?
- Why is an ionic compound neutral overall?
Interactive simulation · PhET
Build an Atom
Open the Ion tab. Add or remove electrons and watch the net charge change.
While you explore
- What happens to the charge when you remove one electron?
- How many electrons must a sodium atom lose to become Na+?
- Does changing electrons change which element it is? Why not?
Key vocabulary
- Ion
- An atom that has gained or lost electrons, so it now has an electrical charge.
- Cation
- A positive ion - it has lost electrons.
- Anion
- A negative ion - it has gained electrons.
- Electrolyte
- A substance that lets a solution carry electricity because it releases ions.
- Electrostatic attraction
- The pull between opposite charges.
Practice questions
0/3 correct
Level 1
An atom becomes an ion when it...
Level 2
A magnesium atom has 12 protons and 12 electrons. What is the charge on a magnesium ion that has lost two electrons?
Level 3
Solid sodium chloride does not conduct electricity, but the same salt dissolved in water does. The best explanation is that...
MYP criterion tasks
Level 4 · Criterion A
Explain why an ionic compound such as magnesium chloride has no overall electrical charge.
Level 4 · Criterion C
A student measures the conductivity of salt solutions at 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm3 and records 410, 790, 1150, 1520 and 1880 microsiemens/cm. Describe the trend, state a conclusion, and evaluate one weakness in the method.
Level 4 · Criterion D
Lithium ions are essential for rechargeable batteries. Evaluate the environmental and social impacts of extracting lithium.
Reflect & track
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