Which of the following sets of elements has the same valency?
- A.Na, Mg, Ca
- B.Na, Mg, Al
- C.Mg, Ca, K
- D.Mg, Ca, Ba
Correct answer
Explanation
The correct answer is D, Mg, Ca, Ba. Magnesium, calcium and barium are all alkaline earth metals of group 2, and each has a valency of 2.
Elements in the same group of the periodic table have the same number of electrons in their outermost shell, and so the same valency. Group 2 elements, beryllium, magnesium, calcium, strontium, barium and radium, each have two valence electrons, which they lose to form ions with a 2+ charge, such as Mg²⁺, Ca²⁺ and Ba²⁺. That is why their oxides share the formula MO, as in MgO, CaO and BaO, and their chlorides the formula MCl2.
A is wrong because sodium has valency 1 while magnesium and calcium have valency 2.
B is wrong because sodium, magnesium and aluminium sit in groups 1, 2 and 13 and have valencies 1, 2 and 3.
C is wrong because potassium is a group 1 alkali metal with valency 1.
Exam tip: group 1 (Li, Na, K), valency 1; group 2 (Mg, Ca, Sr, Ba), valency 2; group 13 (B, Al), valency 3.
Practice Questions
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- B.Si < N < Mg < Ne
- C.Ne < N < Si < Mg
- D.Mg < Ne < N < Si
Show answer
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