C2: The Periodic Table
Development and structure of the Periodic Table, properties of groups
๐ Key Definitions
Periodic Table: A tabular arrangement of elements ordered by increasing atomic number, arranged so that elements with similar 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 therefore 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 that form ionic compounds with non-metals.
Halogens: The elements in Group 7 of the Periodic Table. They are reactive non-metals that form ionic compounds with metals and covalent molecules with other non-metals.
Noble gases: The elements in Group 0 of the Periodic Table. They are unreactive because they have full outer electron shells, making them stable.
๐ Development of the Periodic Table
Early Attempts
Before the modern Periodic Table, scientists tried to organise the elements. In the early 19th century, only about 30 elements were known. Dalton arranged them by atomic mass, and Newlands proposed his Law of Octaves in 1865, noting that every eighth element had similar properties. However, Newlands' table was criticised because it included some elements that did not fit the pattern and left no gaps for undiscovered elements.
Mendeleev's Periodic Table (1869)
Dmitri Mendeleev made the key breakthrough. He arranged the known elements in order of increasing atomic mass, but crucially he:
- Left gaps for undiscovered elements and predicted their properties
- Swapped the order of some elements so they fitted the properties of their group better
- Placed elements with similar chemical properties in the same vertical column
Mendeleev's Prediction of Gallium
Mendeleev left a gap below aluminium and called the missing element "eka-aluminium". He predicted its density, melting point and the formula of its oxide. When gallium was discovered in 1875, its properties matched Mendeleev's predictions closely, providing strong evidence for his table.
The Modern Periodic Table
Mendeleev's table was eventually adopted because his predictions were confirmed. However, the modern Periodic Table arranges elements by atomic number (number of protons) rather than atomic mass. This resolved the problems with pairs like argon and potassium, where the heavier argon (atomic mass 40) comes before the lighter potassium (atomic mass 39) because argon has 18 protons and potassium has 19.
Why atomic number? When protons were discovered, Henry Moseley showed that arranging elements by atomic number resolved the inconsistencies in Mendeleev's table. The modern table is ordered by atomic number, not atomic mass.
๐ Layout of the Modern Periodic Table
Elements are arranged in order of increasing atomic number. The table is divided into groups (columns) and periods (rows). Metals are found on the left and centre, non-metals on the right.
| Groups (Columns) | Periods (Rows) | |
|---|---|---|
| Direction | Vertical | Horizontal |
| Tells you | Number of electrons in outer shell | Number of electron shells |
| Similar properties? | Yes โ same group = similar chemistry | No โ properties change across a period |
| Number | Groups 1โ7, 0 (plus transition metals between Groups 2 and 3) | Periods 1โ7 |
๐ฅ Group 1: Alkali Metals
The alkali metals are lithium (Li), sodium (Na), potassium (K), rubidium (Rb) and caesium (Cs). They are soft metals that can be cut with a knife and must be stored in oil to prevent reaction with air and water.
| Property | Trend down Group 1 |
|---|---|
| Reactivity | Increases โ outer electron is further from nucleus, easier to lose |
| Melting point | Decreases |
| Density | Increases |
| Softness | Increases โ easier to cut |
Why reactivity increases down Group 1: Each alkali metal atom has one electron in its outer shell. Down the group, the outer electron is further from the nucleus and shielded by more inner electron shells. The attraction between the nucleus and the outer electron decreases, so the electron is more easily lost and the metal is more reactive.
Reactions of Alkali Metals with Water
All alkali metals react vigorously with water, producing hydrogen gas and a metal hydroxide:
2Li(s) + 2HโO(l) โ 2LiOH(aq) + Hโ(g)
2Na(s) + 2HโO(l) โ 2NaOH(aq) + Hโ(g)
2K(s) + 2HโO(l) โ 2KOH(aq) + Hโ(g)
Lithium: Fizzes steadily on the water surface.
Sodium: Melts into a ball and fizzes rapidly.
Potassium: Burns with a lilac flame and moves rapidly on the water surface.
๐ข Group 7: Halogens
The halogens are fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). They are all non-metals that exist as diatomic molecules (Fโ, Clโ, Brโ, Iโ).
| Property | Trend down Group 7 |
|---|---|
| Reactivity | Decreases โ outer shell is further from nucleus, harder to gain an electron |
| Melting/boiling point | Increases โ larger molecules have stronger intermolecular forces |
| State at room temp | Gas โ Gas โ Liquid โ Solid |
| Colour | Pale yellow โ Green โ Orange-brown โ Dark grey |
Why reactivity decreases down Group 7: Each halogen atom needs to gain one electron to achieve a full outer shell. Down the group, the outer shell is further from the nucleus and shielded by more inner shells. The attraction for an incoming electron decreases, so it is harder to gain an electron and the halogen is less reactive.
Displacement Reactions of Halogens
A more reactive halogen will displace a less reactive halogen from an aqueous solution of its salt. For example, chlorine is more reactive than bromine, so:
Clโ(g) + 2KBr(aq) โ 2KCl(aq) + Brโ(aq)
Chlorine displaces bromine because chlorine is higher in Group 7. However, bromine cannot displace chlorine:
Brโ(aq) + 2KCl(aq) โ No reaction
Fluorine will displace chlorine, bromine and iodine. Chlorine will displace bromine and iodine. Bromine will displace iodine only.
โช Group 0: Noble Gases
The noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe). They are all colourless, unreactive gases at room temperature.
| Property | Trend down Group 0 |
|---|---|
| Reactivity | Very low โ all have full outer shells |
| Boiling point | Increases โ larger atoms have stronger intermolecular forces |
| Density | Increases |
Why noble gases are unreactive: Noble gas atoms have full outer electron shells (He has 2 electrons, others have 8). This is a very stable arrangement, so they do not need to gain, lose or share electrons. They exist as single atoms rather than molecules.
Uses of Noble Gases
- Helium: Used in balloons and airships (less dense than air, non-flammable)
- Neon: Used in advertising signs (glows red-orange when electricity passes through)
- Argon: Used in light bulbs and welding (provides inert atmosphere to prevent oxidation)
- Krypton: Used in lasers and specialist lighting
๐ถ Transition Metals
Transition metals occupy the central block of the Periodic Table, between Groups 2 and 3. They include iron, copper, zinc, gold, silver and many other common metals.
Properties of transition metals: They are hard, strong and have high melting points. They are good conductors of heat and electricity. They form coloured compounds and can have more than one oxidation state (e.g. Feยฒโบ and Feยณโบ). Many are useful as catalysts (e.g. iron in the Haber process).
| Property | Alkali Metals | Transition Metals |
|---|---|---|
| Hardness | Soft (can cut with knife) | Hard and strong |
| Melting point | Low | High |
| Reactivity | Very reactive | Much less reactive |
| Density | Low | High |
| Compounds | White/colourless | Coloured |
| Oxidation states | +1 only | Variable (e.g. +1, +2, +3) |
โ Practice Questions
Q1: Foundation Explain why Mendeleev left gaps in his Periodic Table and how this helped validate his arrangement.
Mendeleev left gaps for undiscovered elements. He predicted the properties of these missing elements, and when they were later discovered (e.g. gallium), their properties matched his predictions. This provided strong evidence that his arrangement was correct.Q2: Foundation Explain why alkali metals become more reactive as you go down the group.
Going down Group 1, the outer electron is further from the nucleus and shielded by more inner electron shells. The attraction between the nucleus and the outer electron decreases, so the electron is more easily lost. Since losing the outer electron is how alkali metals react, they become more reactive down the group.Q3: Higher Write a word equation and a balanced symbol equation for the reaction of potassium with water. Include state symbols.
Word: potassium + water โ potassium hydroxide + hydrogen. Symbol: 2K(s) + 2HโO(l) โ 2KOH(aq) + Hโ(g). Potassium is a solid, water is a liquid, potassium hydroxide dissolves in water (aqueous), and hydrogen is a gas.Q4: Foundation Explain why chlorine can displace bromine from potassium bromide solution, but bromine cannot displace chlorine from potassium chloride solution.
Chlorine is more reactive than bromine because it is higher in Group 7 and its atoms more readily gain an electron. A more reactive halogen displaces a less reactive one from its salt. Bromine is less reactive than chlorine, so it cannot displace chlorine.Q5: Higher Explain why the noble gases are unreactive and state two uses of noble gases with reasons.
Noble gases are unreactive because their atoms have full outer electron shells, making them stable โ they do not need to gain, lose or share electrons. Uses: Helium in balloons (less dense than air, non-flammable unlike hydrogen); Argon in welding (provides an inert atmosphere to prevent the hot metal reacting with oxygen).๐ฏ Exam Tips
When explaining reactivity trends, always mention: distance of outer electrons from nucleus, shielding by inner shells, and the effect on attraction. All three are needed for full marks.
Remember: Group 1 reactivity increases DOWN the group, but Group 7 reactivity DECREASES down the group. The explanations involve the same factors but apply in opposite directions (losing vs gaining an electron).
For displacement reactions, state that "the more reactive halogen displaces the less reactive halogen" โ this phrasing always earns the mark.
Transition metals can have variable oxidation states โ this is a key difference from alkali metals that you may be asked to compare.
๐งฎ Maths Skills
Interpreting Trends in Group Data
Reading data tables: When given data for Group 1 elements (e.g. melting points: Li 180ยฐC, Na 98ยฐC, K 63ยฐC, Rb 39ยฐC), identify the trend by comparing values as you go down the group. Here, melting point decreases. Always state the direction of the trend clearly.
Calculating relative atomic mass: Use Aแตฃ = (sum of: isotope mass ร percentage abundance) รท 100. E.g. boron has two isotopes: ยนโฐB (19.9%) and ยนยนB (80.1%). Aแตฃ = (10 ร 19.9 + 11 ร 80.1) รท 100 = (199 + 881.1) รท 100 = 10.8.
Plotting and interpreting graphs: When plotting group data (e.g. boiling point vs atomic number), choose appropriate scales, label axes with units, and draw a line of best fit. Use the graph to predict values for unknown elements by extrapolation or interpolation.
Percentage change: If the reactivity of an element doubles from one period to the next, the percentage increase = (new value โ original value) รท original value ร 100.
โ Common Misconceptions
Misconceptions About Groups and Reactivity
Wrong: Elements in the same group have identical properties
Correct: Elements in the same group have similar chemical properties because they have the same number of outer electrons, but the properties are not identical. Reactivity changes down the group due to changing atomic size and shielding.
Wrong: Noble gases do not react at all โ they are completely inert
Correct: Noble gases are very unreactive because they have full outer electron shells, but some noble gas compounds have been made under extreme conditions (e.g. XeFโ and XeOโ). They are described as "very unreactive" rather than "completely inert".
Wrong: Reactivity increases down every group in the Periodic Table
Correct: Reactivity increases down Group 1 (easier to lose electrons) but decreases down Group 7 (harder to gain electrons). The trend depends on whether the atom needs to lose or gain electrons.
โ๏ธ 6-Mark Extended Question
Question
Explain the trends in reactivity in Group 1 and Group 7. Use ideas about electrons in your answer.
Group 1 โ reactivity increases down the group: Alkali metals have one electron in their outer shell. Down the group, the outer electron is further from the nucleus and there are more inner electron shells providing shielding. The attraction between the nucleus and the outer electron decreases, so the electron is more easily lost. Since losing this electron is how alkali metals react, reactivity increases down the group [3 marks].
Group 7 โ reactivity decreases down the group: Halogens need to gain one electron to achieve a full outer shell. Down the group, the outer shell is further from the nucleus and shielded by more inner shells. The attraction for an incoming electron decreases, so it is harder to gain an electron. Since gaining an electron is how halogens react, reactivity decreases down the group [3 marks].
Mark scheme: 3 marks for Group 1 (outer electron further from nucleus, increased shielding, easier to lose electron โ more reactive); 3 marks for Group 7 (outer shell further from nucleus, increased shielding, harder to gain electron โ less reactive). Must compare the two groups and explain the opposite trends.๐ AO3: Analyse and Evaluate
Analysing Group 1 Reaction Data
Scenario: A student investigates the reaction of Group 1 metals with water. They record the time taken for each metal to fully react and disappear. Results: Li = 145 s, Na = 40 s, K = 12 s, Rb = 4 s. Identify the trend and explain it using the metallic bonding model.
Analysis: The reaction time decreases down the group (Li 145 s โ Rb 4 s), meaning reactivity increases. This is because each alkali metal atom has one outer electron. Down the group, the outer electron is further from the nucleus and shielded by more inner electron shells. The electrostatic attraction between the nucleus and the outer electron weakens, so the electron is lost more easily. The metal reacts more quickly with water because it loses its outer electron more readily. The trend supports the prediction that reactivity increases down Group 1.
Evaluation: The student should repeat each measurement at least three times and calculate a mean to improve reliability. They should also control variables such as water temperature, surface area of the metal, and volume of water. The trend is consistent and strongly supports the theoretical model.
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