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?
Nothing new was burned or heated - yet the liquid suddenly carries charge. Something in the salt must be able to move and carry electricity.
Think about it
Pure water barely conducts electricity. Add a spoonful of table salt and a small bulb lights up. What has changed?
Hint: Think about what happens to sodium chloride when it dissolves.
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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