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Graph 1 shows the sizes of atoms and ions for elements in the third period of the Periodic Table - Scottish Highers Chemistry - Question 2 - 2016
Question 2
Graph 1 shows the sizes of atoms and ions for elements in the third period of the Periodic Table. The covalent radius is a measure of the size of an atom. (i) Expl... show full transcript
Worked Solution & Example Answer:Graph 1 shows the sizes of atoms and ions for elements in the third period of the Periodic Table - Scottish Highers Chemistry - Question 2 - 2016
Step 1
(i) Explain why covalent radius decreases across the period from sodium to chlorine.
Answer
As we move from sodium to chlorine, the increasing number of protons in the nucleus results in a greater positive charge. This stronger nuclear charge pulls the electron shells closer to the nucleus, thus reducing the covalent radius. Additionally, the addition of electrons across the period does not significantly increase shielding, meaning that the outermost electrons experience a stronger attraction to the nucleus.
Step 2
(ii) Explain fully why the covalent radius of sodium is larger than the ionic radius of sodium.
Answer
When sodium loses an electron to form a sodium ion (Na⁺), it is left with only ten electrons instead of eleven. The remaining electrons are pulled closer to the nucleus due to the increased effective nuclear charge, as there is less electron-electron repulsion with one less electron. Therefore, the ionic radius of Na⁺ is smaller than the covalent radius of neutral sodium (Na).
Step 3
(i) Explain why the first ionisation energy decreases going down Group 1.
Answer
As we descend Group 1, the outermost electron is increasingly shielded from the nuclear charge by additional electron shells. This shielding effect results in a weaker attraction between the outer electron and the nucleus, making it easier to remove the outermost electron and thereby decreasing the first ionisation energy.
Step 4
(ii) Explain fully why the second ionisation energy is much greater than the first ionisation energy for Group 1 elements.
Answer
The second ionisation energy involves removing an electron from a positively charged ion (M⁺), which is more stable than the neutral atom. The electron being removed is from an inner shell, closer to the nucleus, and experiences a greater effective nuclear charge. This requires significantly more energy compared to removing the first electron, which explains why the second ionisation energy is much greater.
Step 5
(i) Predict the lattice enthalpy, in kJ mol⁻¹, for rubidium fluoride.
Answer
Based on the trends observed in lattice enthalpies for Group 1 elements, the predicted lattice enthalpy for rubidium fluoride (RbF) can be estimated to be around 650 kJ mol⁻¹, considering the ionic radius of Rb⁺ and F⁻, which result in lower lattice enthalpy compared to LiF.
Step 6
(ii) Write a general statement linking lattice enthalpy to ionic radii.
Answer
Generally, as the ionic radii of ions in an ionic compound increase, the lattice enthalpy decreases. This is due to the increased distance between the ions, which weakens the electrostatic forces of attraction, leading to a lower energy required to separate the ions.