C2: The Periodic Table
Development of the Periodic Table and properties of groups
C2 The Periodic Table
📋 Key Definitions
Periodic table: A table of elements arranged in order of atomic number, so that elements with similar chemical properties appear in the same vertical column (group).
Group: A vertical column in the periodic table. Elements in the same group have the same number of electrons in their outer shell and similar chemical properties.
Period: A horizontal row in the periodic table. Elements in the same period have the same number of electron shells.
Alkali metals: The elements in Group 1 of the periodic table. They are very reactive soft metals.
Halogens: The elements in Group 7 of the periodic table. They are reactive non-metals that exist as diatomic molecules.
Noble gases: The elements in Group 0 of the periodic table. They are unreactive because they have full outer electron shells.
📜 Development of the Periodic Table
Mendeleev's Periodic Table (1869)
Dmitri Mendeleev arranged the known elements in order of increasing atomic mass. He made two key decisions that made his table successful:
Mendeleev's key decisions: He left gaps for undiscovered elements and predicted their properties. When these elements were later discovered, their properties matched Mendeleev's predictions closely, which supported his table. He also swapped the order of some elements so they were in the right group, even though this went against the atomic mass order.
Mendeleev's Prediction
Mendeleev left a gap below silicon for an undiscovered element he called "eka-silicon". He predicted it would be a grey metal with a density of 5.5 g/cm³. When germanium was discovered in 1886, it had properties very close to Mendeleev's predictions.
The Modern Periodic Table
The modern periodic table is arranged in order of atomic number (number of protons), not atomic mass. This was made possible after the discovery of protons. Elements are arranged so those with similar properties fall into the same group.
| Mendeleev's Table | Modern Table | |
|---|---|---|
| Arranged by | Atomic mass | Atomic number |
| Gaps left | Yes - for undiscovered elements | No - all known elements placed |
| Swapped elements | Yes (e.g. iodine and tellurium) | No - atomic number order resolves this |
| Predictions | Made predictions for undiscovered elements | Not needed |
🔋 Group 1: Alkali Metals
The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and caesium (Cs). They all have one electron in their outer shell.
Reactivity trend: Reactivity increases down Group 1. As you go down, the outer electron is further from the nucleus and is more shielded by inner electrons, so it is lost more easily.
Properties of Alkali Metals
- Soft solids that can be cut with a knife (softness increases down the group)
- Low density (lithium, sodium and potassium float on water)
- Low melting and boiling points that decrease down the group
- Very reactive - stored in oil to prevent reaction with oxygen and moisture
Reaction with Water
All alkali metals react vigorously with water, producing hydrogen gas and a metal hydroxide (an alkali).
2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
Observations of Alkali Metals in Water
- Lithium: Fizzes steadily, moves slowly on the water surface
- Sodium: Melts into a ball, fizzes rapidly, moves quickly on the surface
- Potassium: Burns with a lilac flame, moves very fast, may spark
The reactions get more vigorous down the group, showing increasing reactivity.
🟢 Group 7: Halogens
The halogens are fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). They all have seven electrons in their outer shell and exist as diatomic molecules (F₂, Cl₂, Br₂, I₂).
Reactivity trend: Reactivity decreases down Group 7. As you go down, the outer shell is further from the nucleus and more shielded, so the atom finds it harder to attract an extra electron.
| Halogen | Colour at Room Temp | State at Room Temp | Relative Reactivity |
|---|---|---|---|
| Fluorine (F₂) | Pale yellow | Gas | Most reactive |
| Chlorine (Cl₂) | Green-yellow | Gas | |
| Bromine (Br₂) | Red-brown | Liquid | |
| Iodine (I₂) | Dark grey/violet | Solid | Least reactive |
Displacement Reactions
A more reactive halogen will displace a less reactive halogen from an aqueous solution of its salt.
Worked Example - Displacement Reactions
Will chlorine displace bromine from potassium bromide solution?
Chlorine is more reactive than bromine, so YES.
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
The solution turns orange (colour of bromine) as bromine is released.
Will iodine displace chlorine from potassium chloride solution?
Iodine is less reactive than chlorine, so NO. No reaction occurs.
Predicting Displacement Reactions
- Chlorine + sodium iodide → chlorine displaces iodine (solution turns brown)
- Bromine + sodium chloride → no reaction (bromine is less reactive)
- Chlorine + sodium bromide → chlorine displaces bromine (solution turns orange)
- Bromine + sodium iodide → bromine displaces iodine (solution turns brown/violet)
⚪ Group 0: Noble Gases
The noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe). They all have a full outer electron shell (8 electrons, except helium which has 2).
Inertness: Noble gases are unreactive (inert) because they have full outer electron shells. This means they do not need to gain, lose or share electrons, so they rarely form compounds.
Properties and Uses
| Noble Gas | Use | Why it is suitable |
|---|---|---|
| Helium | Balloons and airships | Very low density - lighter than air; non-flammable |
| Neon | Advertising signs | Gives a bright red-orange glow when electricity passes through |
| Argon | Welding (provides inert atmosphere) | Inert - prevents the metal reacting with oxygen |
| Argon | Filling double glazing | Low thermal conductivity - reduces heat loss |
| Krypton | Laser eye surgery | Produces specific light wavelengths |
Boiling points of noble gases increase down the group as the atoms get larger and the intermolecular forces increase.
🟤 Transition Metals
Transition metals are found in the central block of the periodic table (between Groups 2 and 3). Common examples include iron, copper, zinc, silver and gold.
Transition metals have typical metallic properties but are much less reactive than Group 1 metals. They form coloured compounds and many are good catalysts.
| Property | Alkali Metals (Group 1) | Transition Metals |
|---|---|---|
| Reactivity | Very reactive | Much less reactive |
| Melting points | Low (decrease down group) | High |
| Hardness | Soft (can be cut with knife) | Hard and strong |
| Density | Low | High |
| Coloured compounds | White compounds | Coloured compounds |
| Catalytic activity | Not typically catalysts | Many are good catalysts |
Examples of Transition Metal Properties
- Copper(II) sulfate is blue, iron(II) sulfate is pale green, iron(III) sulfate is yellow/brown
- Iron is a catalyst in the Haber process (making ammonia)
- Nickel is a catalyst in the hydrogenation of alkenes
- Vanadium(V) oxide is a catalyst in the Contact process (making sulfuric acid)
- Platinum is a catalyst in catalytic converters in cars
❓ Practice Questions
Q1: Foundation Describe two ways Mendeleev arranged elements in his periodic table and explain why his table was accepted by other scientists.
Q2: Foundation Explain why alkali metals become more reactive as you go down the group.
Q3: Foundation Write a word equation and a symbol equation for the reaction of sodium with water. What observations would you make?
Q4: Higher Chlorine is added to a solution of potassium iodide. State what happens and explain why. Write a symbol equation for the reaction.
Q5: Foundation Explain why noble gases are unreactive and give one use of argon.
Q6: Higher Compare the properties of alkali metals with transition metals. Give at least three differences.
✅ Answers
- Mendeleev arranged elements in order of atomic mass and put elements with similar properties in the same group. He left gaps for undiscovered elements and predicted their properties. His table was accepted because when the missing elements were discovered, their properties matched his predictions.
- As you go down Group 1, the outer electron is further from the nucleus and there are more inner electron shells providing shielding. This means the outer electron is less strongly attracted to the nucleus and is lost more easily, making the atom more reactive.
- Word equation: sodium + water → sodium hydroxide + hydrogen. Symbol equation: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Observations: sodium melts into a ball, fizzes rapidly, moves quickly on the water surface.
- Chlorine displaces iodine from potassium iodide because chlorine is more reactive than iodine (it is higher in Group 7). The solution turns brown/violet as iodine is released. Equation: Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq).
- Noble gases are unreactive because they have full outer electron shells, so they do not need to gain, lose or share electrons. Argon is used in welding to provide an inert atmosphere that prevents the hot metal reacting with oxygen in the air.
- Three differences: (1) Alkali metals are very reactive, transition metals are much less reactive. (2) Alkali metals have low melting points, transition metals have high melting points. (3) Alkali metals are soft (can be cut with a knife), transition metals are hard and strong. (4) Transition metals form coloured compounds, alkali metals form white compounds. (5) Many transition metals are good catalysts; alkali metals are not.
🎯 Exam Tips
- When explaining reactivity trends, always mention both distance from the nucleus AND shielding by inner electrons
- For displacement reactions, state clearly which halogen is more reactive AND what you would observe (colour change)
- "Explain why noble gases are inert" = full outer electron shell. Do NOT just say "they are stable"
- When comparing Groups 1 and transition metals, give specific examples to support your points
- Mendeleev questions: always mention gaps, predictions, and the fact that discovered elements matched his predictions
- Remember: halogens are diatomic (F₂, Cl₂, Br₂, I₂) - losing the ₂ is a common error
🔢 Maths Skills
Mathematical Skills
Interpreting group trends: You may be given data tables showing melting points, boiling points or reactivity for elements in a group. Practise identifying trends (increasing/decreasing) and calculating differences between consecutive elements to describe the pattern quantitatively.
Example: Group 1 melting points: Li 181°C, Na 98°C, K 63°C. The trend is a decrease of roughly 40–80°C per step down the group.
⚠️ Common Misconceptions
Watch Out!
Elements in the same group have identical properties. Wrong: same group = identical properties Correct: same group = similar properties (same number of outer electrons), but properties change gradually down the group
Noble gases are completely inert and never form compounds. Wrong: noble gases are completely inert Correct: noble gases are very unreactive, but some compounds of xenon and krypton have been made under extreme conditions
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the reactivity trends in Group 1 and Group 7.
Group 1 reactivity increases down the group because the outer electron is further from the nucleus and more shielded by inner electron shells, so it is lost more easily. Group 7 reactivity decreases down the group because the outer shell is further from the nucleus and more shielded, so the atom finds it harder to attract an extra electron. Both trends are explained by increasing atomic size and shielding, but the outcome differs because Group 1 loses an electron (easier further out) while Group 7 gains one (harder further out).
Mark scheme: 1 mark for Group 1 trend; 1 mark for Group 1 explanation (distance + shielding); 1 mark for Group 7 trend; 1 mark for Group 7 explanation (distance + shielding); 1 mark for linking both to outer electron; 1 mark for contrasting loss vs gain of electrons.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
The table below shows data for Group 2 elements:
| Element | Melting point (°C) | Density (g/cm³) | Reaction with water |
|---|---|---|---|
| Mg | 650 | 1.7 | Very slow |
| Ca | 842 | 1.6 | Fizzes steadily |
| Sr | 777 | 2.6 | Fizzes rapidly |
| Ba | 727 | 3.6 | Very vigorous |
Question: Use the data to predict the reactivity of an unknown Group 2 element between Ca and Sr. Justify your answer.
Answer: Reactivity increases down the group (Ca slow → Sr rapid), so an element between Ca and Sr would react with water more vigorously than Ca but less vigorously than Sr — it would fizzle moderately.
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