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For parts of the free-response question that require calculations, clearly show the method used and the steps involved in arriving at your answers. You must show your work to receive credit for your answer. Examples and equations may be included in your answers where appropriate.
Answer the following questions related to K .
(a) In terms of atomic structure, explain why the atomic radius of K is larger than that of Na .
(b) In terms of atomic structure, explain why the first-ionization energy K is less than that of Ca.
(c) K forms the compound K2O, which is an ionic compound that is brittle. Identify another element, M, that is likely to form a brittle, ionic compound with the formula M2O. Justify your answer in terms of periodic trends.
(d) The compound K2O2 also exists. A chemist can determine the mass of K in a sample of known mass that consists of either pure K2O or pure K2O2. From this information, can the chemist answer the question of which compound is in the sample? Indicate yes or no, and explain.
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As an example, potassium (K) has a larger average atomic radius (220 pm)than sodium (Na) does (180 pm). The potassium atom has an extra electron shell compared to the sodium atom, which means its valence electrons are further from the nucleus, giving potassium a larger atomic radius.
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The answers are as follows:
(a) The atomic radius of K ( 280pm) is much larger than Na (227pm) because K belongs to the 4th period while Na belongs to the 3rd period. Due to the presence of an extra shell in K, its size is much larger.
(b) The first-ionization energy of K is less than that of Ca because K⁺ ion is much more stable than Ca⁺ ion.
The electronic configuration of K is 1s² 2s² 2p⁶ 3s¹. On removing an electron, the cation attains the stable configuration of a noble gas.
(c) Na₂O is also a brittle, ionic compound with the formula M₂O.
This is because both sodium and potassium belong to the same group of the periodic table. They have similar chemical properties because both have valency +1.
(d) Yes, the chemist can know which compound is in the sample.
Since the chemist knows the mass of the given sample, he can determine the mass of K in it using elemental analysis.
He can then calculate the ratio of K and O in the sample and match it with ratios of K₂O or K₂O₂.