AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The trends in atomic radius, first ionisation energy and melting point of…
Full specification wording
Group 2, the alkaline earth metals: The trends in atomic radius, first ionisation energy and melting point of the elements Mg–Ba Students should be able to: explain the trends in atomic radius and first ionisation energy explain the melting point of the elements in terms of their structure and bonding.
Down Group 2 from Mg to Ba each element adds a complete electron shell, so atomic radius increases.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Down Group 2 from Mg to Ba each element adds a complete electron shell, so atomic radius increases. First ionisation energy falls: although the nuclear charge is larger, the outer pair of electrons lies further from the nucleus and is shielded by more inner shells, and those two effects outweigh the extra protons. The second ionisation energy of any of these metals is always larger than the first, because that electron is being pulled from a positively charged ion which is smaller and holds its remaining electrons more tightly, with the same number of protons now attracting fewer electrons. Melting point generally falls from calcium to barium, since the larger 2⁺ ions sit further from the delocalised electrons and the metallic bonding weakens; magnesium breaks the pattern because its crystal packing differs from the others.
What earns the marks
- Atomic radius increasing down the group because each element has one more occupied electron shell.
- First ionisation energy decreasing because the outer electron is further from the nucleus and better shielded.
- Stating that the increase in nuclear charge is outweighed by distance and shielding.
- Melting point falling from calcium to barium as the larger 2⁺ ions attract the delocalised electrons less strongly.
- Identifying magnesium as an exception, explained by its different crystal packing.
Common mistakes and how to put them right
Saying first ionisation energy increases down the group because nuclear charge increases, ignoring distance and shielding.
Predicting a perfectly smooth melting point trend from magnesium to barium.
Describing metallic bonding as an attraction between atoms rather than between positive ions and delocalised electrons.
Assuming the second ionisation energy is similar to the first, when it is much larger because the electron leaves a smaller, positive ion.
Claiming the charge on the metal ion changes down the group.
Exam tips
- Marks for ionisation energy trends require the words shielding and distance, not just nuclear charge.
- When comparing successive ionisation energies of the same element, say the electron is removed from a positive ion.
Try the lesson
The start of the Group 2, the alkaline earth metals: The trends in atomic radius, first ionisation energy and melting point of the elements Mg–Ba Students should be able to: explain the trends in atomic radius and first ionisation energy explain the melting point of the elements in terms of their structure and bonding. lesson, made from checked teaching notes for this specification point.
Have a go first
Going down Group 2 from Mg to Ba, what happens to the atomic radius of the elements?
Listen
Down Group 2 from Mg to Ba each element adds a complete electron shell, so atomic radius increases. First ionisation energy falls: although the nuclear charge is larger, the outer pair of electrons lies further from the nucleus and is shielded by more inner shells, and those two effects outweigh the extra protons. The second ionisation energy of any of these metals is always larger than the first, because that electron is being pulled from a positively charged ion which is smaller and holds its remaining electrons more tightly, with the same number of protons now attracting fewer electrons. Melting point generally falls from calcium to barium, since the larger 2⁺ ions sit further from the delocalised electrons and the metallic bonding weakens; magnesium breaks the pattern because its crystal packing differs from the others. A strong answer earns marks for these points. atomic radius increasing down the group because each element has one more occupied electron shell. first ionisation energy decreasing because the outer electron is further from the nucleus and better shielded. stating that the increase in nuclear charge is outweighed by distance and shielding. melting point falling from calcium to barium as the larger 2⁺ ions attract the delocalised electrons less strongly. Exam tip: Marks for ionisation energy trends require the words shielding and distance, not just nuclear charge.
See it
Key words
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atomic radius down a group
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shielding and distance
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successive ionisation energies
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metallic bonding and melting point
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See every definition
- atomic radius down a group
- Each element adds a complete electron shell, so the atomic radius increases down the group.
- shielding and distance
- The outer pair of electrons lies further from the nucleus and is shielded by more inner shells, and these effects outweigh the extra protons.
- successive ionisation energies
- The second ionisation energy is always larger than the first because the electron is pulled from a positively charged ion that is smaller and holds its remaining electrons more tightly.
- metallic bonding and melting point
- Melting point generally falls from calcium to barium because the larger 2⁺ ions sit further from the delocalised electrons and the metallic bonding weakens.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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Test your understanding of Group 2, the alkaline earth metals: The trends in atomic radius, first ionisation energy and melting point of the elements Mg–Ba Students should be able to: explain the trends in atomic radius and first ionisation energy explain the melting point of the elements in terms of their structure and bonding.
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