The atomic number of Ca is 20 thus its electronic configuration is 2, 8, 8, 2
(i) It is a metal because it has two electrons in its outermost shell and it easily loses these two electrons to acquire inert gas configuration and form +2 ion.
(ii) Potassium (K) with atomic number 19 comes before Calcium (Ca) having an atomic number 20; in the same period (IV period).
The following reasons explain why the atomic radius will be larger or smaller than that of potassium with atomic number 19:
(a) Thus, size of Ca is smaller than K since on moving from left to right in a period of the periodic table, the size of the atoms decreases.
(b) Because the atomic number of the elements increases which means that the number of protons and electrons in the atoms increases (the extra electrons being added to the same shell).
(c) Due to large positive charge on the nucleus, the outermost shell is pulled with more force by the nucleus, it moves closer to the nucleus and the size of the atom decreases.
(iii) The oxide of calcium is calcium oxide (CaO).
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