What the lab asks
The periodic table is a map. Elements in one period (row) have the same number of shells. Elements in one group (column) have the same number of outer electrons. This lab checks two things by experiment:
- How does the activity of elements change across period 3?
- How does it change down group 1 and group 17?
Activity of a metal means how easily it gives away electrons. Activity of a non-metal means how strongly it pulls electrons in.
Safety: the teacher handles sodium and potassium in very tiny pieces behind a shield. Wear goggles. Chlorine and bromine are used only in dilute water form, in a well-aired room.
Across period 3: metals Na, Mg, Al
Clean strips of Mg and Al with sandpaper (they have a thin oxide skin). Put a tiny piece of each metal in water and watch the bubbles of hydrogen gas.
| Metal | In cold water | Activity |
|---|---|---|
| Na | Fizzes fast, a hot ball skates on water | High |
| Mg | A few slow bubbles (more in hot water) | Medium |
| Al | No change (oxide layer; reacts only with acid or steam) | Low |
Why? From Na to Al the nuclear charge grows but the shells stay the same. The nucleus pulls the outer electrons harder, so the metal gives them up less easily.
Across period 3: non-metals Si, P, S, Cl
For non-metals the story turns round. From Si to Cl the pull on outer electrons grows, so the elements grab electrons more strongly. Chlorine is the strongest in this row.
The oxides show the change from base to acid:
| Element | Its oxide in water |
|---|---|
| Na | Strong base (NaOH) |
| Mg | Weak base (Mg(OH)₂) |
| Al | Both: reacts with an acid and with a base |
| Si, P | Weak acids (silicic, phosphoric) |
| S, Cl | Strong acids (sulfuric, perchloric) |
Rule: left to right, metallic character falls and non-metallic character rises.
Down group 1: Li, Na, K
The alkali metals all have one outer electron. Add a tiny piece to water: Li reacts slowly, Na reacts quickly and melts, K reacts very fast with a lilac flame. Test the water with indicator: it turns basic (alkaline), because each metal makes a hydroxide.
Metal + water → metal hydroxide + hydrogen. For example: 2Na + 2H₂O → 2NaOH + H₂.
Why? Down the group there are more shells, the outer electron is farther from the nucleus and held less tightly. So metal activity rises down a group.
Down group 17: the halogen displacement test
Shake a halogen water (Cl₂, Br₂) with a salt solution (KCl, KBr, KI). A stronger halogen takes the place of a weaker one. Add a little hexane to see the colour of the free halogen.
| Mixture | What you see | Meaning |
|---|---|---|
| Cl₂ + KBr | Orange | Br₂ is made: Cl is stronger than Br |
| Cl₂ + KI | Brown | I₂ is made: Cl is stronger than I |
| Br₂ + KI | Brown | I₂ is made: Br is stronger than I |
| Br₂ + KCl | No change | Br cannot push out Cl |
Example: Cl₂ + 2KBr → 2KCl + Br₂. Non-metal activity falls down group 17 (Cl > Br > I): bigger atoms hold on to an extra electron less strongly.
Try it: predict, then check
Before step 6, predict: which element has the highest metal bar, and which has the highest non-metal bar? Then tap them. At home: drop a clean iron nail, a piece of aluminium foil and a copper coin each in a little lemon juice and compare the bubbles after an hour. Keep to a fair test: same amount of juice, same time, same size.
Key formulas and definitions
- Across a period (left to right): metallic character falls, non-metallic character rises
- Down a group: metal activity rises (Li < Na < K)
- Down group 17: non-metal activity falls (Cl > Br > I)
- 2Na + 2H₂O → 2NaOH + H₂
- Cl₂ + 2KBr → 2KCl + Br₂ and Cl₂ + 2KI → 2KCl + I₂ and Br₂ + 2KI → 2KBr + I₂
Worked examples
1. Magnesium bubbles slowly in water but sodium bubbles fast. Which is more active, and why?
Sodium is more active. It has only 1 outer electron and a weaker pull from the nucleus than magnesium, so it gives away that electron more easily.
2. Chlorine water is added to potassium iodide solution. The mixture turns brown. Write the equation and explain.
Cl₂ + 2KI → 2KCl + I₂. Chlorine is a stronger non-metal than iodine, so it pushes iodine out of the salt. The brown colour is free iodine.
3. Rank Li, Na and K by how quickly they react with water, and give a reason.
K > Na > Li. Down the group the outer electron is farther from the nucleus and is lost more easily.
4. Will bromine water react with potassium chloride? Explain.
No. Bromine is less active than chlorine, so it cannot push chlorine out of KCl. There is no change.
Common mistakes
- Thinking activity always rises left to right. For metals it falls; for non-metals it rises.
- Mixing up group trends: group 1 metals get MORE active going down, group 17 non-metals get LESS active going down.
- Forgetting to clean Mg and Al with sandpaper, so the oxide skin hides the real reaction.
- Using big pieces of sodium or potassium. Always use a piece the size of a rice grain and a shield.