The Periodic Table: Groups and Trends
The modern periodic table arranges elements in order of increasing atomic number, in groups whose members share the same number of outer-shell electrons and therefore similar chemical properties. Group 1, the alkali metals, become more reactive down the group, while group 7, the halogens, become less reactive down the group, because the outer shell moves further from the nucleus and is shielded by more inner shells.
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Method
- Work out the group from the electronic structure, and the period from the number of shells: an element with structure 2,8,7 is in group 7 and period 3.
- Explain any group 1 trend in three linked steps: down the group the outer electron is in a shell further from the nucleus, there is more shielding by inner shells, so the outer electron is lost more easily and reactivity increases.
- Explain any group 7 trend by reversing the same logic: down the group the outer shell is further from the nucleus and more shielded, so an incoming electron is attracted less strongly, it is gained less easily and reactivity decreases.
- Learn the group 7 physical trend alongside the chemical one: down the group melting point and boiling point increase (fluorine and chlorine are gases, bromine a liquid, iodine a solid at room temperature) and relative molecular mass increases.
- Predict displacement reactions from the reactivity order: a more reactive halogen displaces a less reactive one from a solution of its salt, so chlorine displaces bromine from potassium bromide, but iodine does not displace chlorine.
- Explain the noble gases from their structure: a full outer shell (2 for helium, 8 for the others) means they have no tendency to lose, gain or share electrons, so they are unreactive, and boiling point increases down the group.
Worked example
Predict whether a reaction occurs when chlorine gas is bubbled through potassium iodide solution. Write the word and balanced symbol equations, and explain the colour change observed.
- Compare reactivity: chlorine is above iodine in group 7, so chlorine is the more reactive halogen.
- A more reactive halogen displaces a less reactive one from a solution of its salt, so a reaction does occur.
- Word equation: chlorine + potassium iodide gives potassium chloride + iodine.
- Symbol equation: Cl2 + 2KI gives 2KCl + I2, which balances with 2 potassium, 2 iodine and 2 chlorine atoms on each side.
- Colour change: the colourless solution turns brown or orange-brown as iodine is formed.
- Explain the electron transfer for a higher-tier mark: each chlorine atom gains an electron to form a chloride ion and is reduced, while iodide ions lose electrons to form iodine and are oxidised.
Practice questions
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Q1On what property are elements ordered in the modern periodic table?Show answer
Answer: Increasing atomic number (number of protons).
Q2Why do elements in the same group have similar chemical properties?Show answer
Answer: They have the same number of electrons in their outer shell, which is what determines how they react.
Q3Give two observations when lithium is added to water.Show answer
Answer: It floats and moves on the surface, fizzes as hydrogen is released, and the resulting solution turns universal indicator purple because an alkali is formed.
Q4Explain why potassium is more reactive than sodium.Show answer
Answer: Potassium's outer electron is in a shell further from the nucleus and is shielded by more inner shells, so it is attracted less strongly and lost more easily.
Q5State the trend in reactivity down group 7 and name the most reactive halogen listed on the periodic table.Show answer
Answer: Reactivity decreases down the group; fluorine is the most reactive.
Q6Why are the noble gases unreactive?Show answer
Answer: They have a full outer shell of electrons, so they have no tendency to lose, gain or share electrons.
Q7Why did Mendeleev leave gaps in his table?Show answer
Answer: He predicted that elements with the properties needed to fit those positions had not yet been discovered, and left space for them rather than forcing a wrong element into the pattern.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
Explain, in terms of electronic structure, why the alkali metals become more reactive down group 1 while the halogens become less reactive down group 7.
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Bromine water is added to a solution of potassium chloride and to a solution of potassium iodide. Predict what happens in each case and justify your answers.
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