From Döbereiner's triads to Group VII halogens — master patterns, trends, and reactions in the most useful chart in chemistry! ⚗️
| Scientist | Year | Contribution | Limitation/Outcome |
|---|---|---|---|
| Döbereiner | 1817–29 | Triads: groups of 3 similar elements where the middle element's mass ≈ average of the outer two. E.g. Li(7) + K(39) ÷ 2 ≈ Na(23) | Only worked for a few groups; ignored most elements |
| Newlands | 1865 | Law of Octaves: arranged 56 elements by atomic mass; every 8th element had similar properties | Broke down for heavier elements; ridiculed by peers |
| Mendeleev | 1869 | Arranged elements by atomic mass; left gaps for undiscovered elements; correctly predicted gallium's properties | A few elements had to be swapped — order not always correct |
| Moseley | 1914 | Arranged by atomic number (proton number) using X-ray experiments — this is the modern periodic table | Fixed all anomalies in Mendeleev's table |
Elements are arranged in order of increasing atomic number.
| Group | Name | Valence e⁻ | Key Feature |
|---|---|---|---|
| Group I | Alkali Metals | 1 | Very reactive; react violently with water |
| Group II | Alkaline Earth Metals | 2 | Reactive; form +2 ions |
| Group VII | Halogens | 7 | Reactive non-metals; form −1 ions |
| Group 0 (VIII) | Noble Gases | 8 (full shell) | Chemically unreactive; full outer shell |
| Direction | Metallic Nature | Atomic Radius | Non-metallic Nature |
|---|---|---|---|
| ↓ Down a group | Increases ↑ | Increases ↑ | Decreases ↓ |
| → Left to right across period | Decreases ↓ | Decreases ↓ | Increases ↑ |
Group II: Be, Mg, Ca, Sr, Ba, Ra. All have 2 valence electrons and form +2 ions.
| Reaction | Magnesium (Mg) | Calcium (Ca) | Barium (Ba) |
|---|---|---|---|
| With O₂ | Burns with blinding white flame: 2Mg + O₂ → 2MgO | Brick-red flame: 2Ca + O₂ → 2CaO | Apple-green flame: 2Ba + O₂ → 2BaO |
| With H₂O | Very slowly; Mg(OH)₂ + H₂↑ | Vigorously; Ca(OH)₂ + H₂↑ | Very vigorously; Ba(OH)₂ + H₂↑ |
| With HCl | Vigorously; MgCl₂ + H₂↑ | Very vigorously; CaCl₂ + H₂↑ | Violently; BaCl₂ + H₂↑ |
| Element | Formula | State (25°C) | Colour | Valence e⁻ |
|---|---|---|---|---|
| Fluorine | F₂ | Gas | Pale yellow | 7 |
| Chlorine | Cl₂ | Gas | Yellow-green | 7 |
| Bromine | Br₂ | Liquid | Red-brown | 7 |
| Iodine | I₂ | Solid | Grey-black (purple vapour) | 7 |
A more reactive halogen displaces a less reactive halogen from its salt solution.
| Halogen Added | Salt Solution | Reaction? | Observation | Equation |
|---|---|---|---|---|
| Cl₂ | KBr (aq) | ✅ Yes | Turns red-brown (Br₂ produced) | Cl₂ + 2KBr → 2KCl + Br₂ |
| Cl₂ | KI (aq) | ✅ Yes | Turns brown (I₂ produced) | Cl₂ + 2KI → 2KCl + I₂ |
| Br₂ | KI (aq) | ✅ Yes | Turns brown (I₂ produced) | Br₂ + 2KI → 2KBr + I₂ |
| Br₂ | KCl (aq) | ❌ No | No colour change | No reaction |
| I₂ | KBr (aq) | ❌ No | No colour change | No reaction |
Period 3: Na, Mg, Al, Si, P, S, Cl, Ar — all have 3 occupied electron shells.
| Element | Config | Type | Reactivity | Atomic Radius trend |
|---|---|---|---|---|
| Na | (2,8,1) | Metal | Most reactive metal (loses 1 e⁻ easily) | Decreases → (more protons pull electrons closer) |
| Mg | (2,8,2) | Metal | Less reactive than Na | |
| Al | (2,8,3) | Metal | Least reactive Period 3 metal | |
| Si | (2,8,4) | Metalloid | Semiconductor; shares electrons | |
| P | (2,8,5) | Non-metal | Least reactive Period 3 non-metal | |
| S | (2,8,6) | Non-metal | More reactive non-metal | |
| Cl | (2,8,7) | Non-metal | Most reactive Period 3 non-metal | |
| Ar | (2,8,8) | Noble gas | Unreactive — full outer shell |
Click any element to see its details. Colour-coded by element type.
👆 Click an element above to see its details
Click each test tube to see what happens when a halogen is added to a salt solution!
👆 Click a test tube to see the reaction
Select a trend to visualise. Charts animate when switching.
Click the card to flip it!
Problem: Element X has 20 protons. Determine: its electronic configuration, group, period, the number of valence electrons, and whether it is a metal or non-metal.
Type each answer (or type show to reveal):
Atomic number = 20 means 20 electrons. Fill shells: 2, 8, 8, then how many remain? Write it as: 2,8,8,2
The group number equals the number of valence electrons (outermost shell). From (2,8,8,2), how many electrons are in the outermost shell? What group is this?
The period number equals the number of occupied electron shells. Count the shells in (2,8,8,2).
Group II elements are always metals. They tend to LOSE electrons to form +2 ions. Is element X (Calcium) a metal or non-metal? Type one word.
Click a term, then click its matching description. Green = correct!
1 (a) Group V (5 valence electrons ✓); Period 3 (3 occupied shells ✓)
2 (b) Non-metal ✓ — it is in Group V on the right side of the periodic table, where non-metals are found. It tends to gain electrons rather than lose them. ✓
3 (c) Gains electrons ✓; gains 3 electrons ✓ (to complete its outer shell to 8; 8 − 5 = 3)
Element X is Phosphorus (P).
1 Calcium is below magnesium in Group II. ✓
2 Calcium has more electron shells than magnesium → larger atomic radius. ✓
3 The valence electrons in calcium are further from the nucleus and more shielded by inner shells. ✓
4 Therefore it is easier to lose the 2 valence electrons → calcium is more reactive. ✓
Note: Must include all 4 points for full marks — "further from nucleus", "more shielding", "easier to lose electrons", "more reactive".
1 (a) Chlorine is more reactive (stronger oxidising agent) than bromine. ✓
2 Chlorine displaces bromine from the KBr solution, forming free bromine (Br₂). ✓ The orange-brown colour is due to Br₂ being produced. ✓
3 (b) Cl₂(g) + 2KBr(aq) → 2KCl(aq) + Br₂(aq) ✓ (balanced, state symbols ✓)
4 (c) Bromine is less reactive than chlorine (lower in Group VII, weaker oxidising agent). A less reactive element cannot displace a more reactive element. ✓
Any three of:
1 Sodium is a solid at room temperature; chlorine is a gas. ✓
2 Sodium is shiny/has metallic lustre; chlorine is dull/yellow-green. ✓
3 Sodium conducts electricity; chlorine does not (poor conductor). ✓
4 Sodium has a high melting/boiling point; chlorine has a low melting/boiling point. ✓
5 Sodium has a high density; chlorine has a low density. ✓
1 Mendeleev arranged elements in order of increasing atomic mass ✓; the modern table uses atomic number (proton number) ✓.
2 Using atomic mass caused some elements to be out of order (e.g. tellurium and iodine were swapped) ✓.
3 Moseley (1914) showed that atomic number is the fundamental property that determines an element's identity. Using atomic number removes all the anomalies. ✓
Group = valence electrons
Period = occupied shells
Reactivity ↑ down group
Be < Mg < Ca < Sr < Ba
Reactivity ↑ up group
I < Br < Cl < F
More reactive displaces less reactive
Cl₂ + 2KBr → 2KCl + Br₂
Metal react: Na > Mg > Al
Non-metal: Cl > S > P
Döbereiner → Newlands
→ Mendeleev → Moseley