Exam-style CBSE practice questions on Valency (Journey Inside the Atom). Try each one first, then reveal the correct answer and a step-by-step explanation. Free, from EduLevel — the AI teacher for CBSE.
The valency of an element is determined by the number of electrons present in its:
Explanation: The electrons in the outermost shell, called valence electrons, decide how an atom combines with other atoms. Therefore, the valency of an element is determined by the number of electrons in its outermost shell.
What is the valency of hydrogen?
Explanation: A hydrogen atom has only one electron in its outermost shell and needs one more electron to become stable. Since it can combine with one other atom, the valency of hydrogen is 1.
What is the valency of oxygen?
Explanation: An oxygen atom has 6 electrons in its outermost shell and needs 2 more to complete its octet of 8. Since it needs 2 electrons, the valency of oxygen is 2.
An atom has the electron distribution 2, 8, 8. What is its valency?
Explanation: An atom with the arrangement 2, 8, 8 has a completely filled outermost shell of 8 electrons, which is a stable octet. Since it does not need to gain, lose or share electrons, its valency is 0. Such elements are the noble gases, like argon.
The electronic configuration of chlorine (atomic number 17) is 2, 8, 7. What is its valency?
Explanation: Chlorine has 7 electrons in its outermost shell and needs only 1 more to complete its octet of 8. Since it needs 1 electron, the valency of chlorine is 1.
An element X has the electronic configuration 2, 8, 3. What is the valency of X?
Explanation: Element X has 3 electrons in its outermost shell. It loses these 3 electrons to achieve a stable configuration, so its valency is 3. This element is aluminium.
What is the correct chemical formula of magnesium chloride?
Explanation: Magnesium has a valency of 2 and chlorine has a valency of 1. To balance the valencies, one magnesium atom combines with two chlorine atoms, giving the formula MgCl₂.
Nitrogen has 5 electrons in its outermost shell. What is its valency?
Explanation: Although nitrogen has 5 electrons in its outermost shell, it needs 3 more electrons to complete its octet of 8. Since gaining 3 electrons is easier than losing 5, its valency is 3 (8 − 5 = 3).
Which of the following pairs of elements will combine in a 1:1 ratio to form a compound?
Explanation: Sodium has a valency of 1 and chlorine also has a valency of 1. Since both have the same valency, they combine in a 1:1 ratio to form NaCl (sodium chloride).
The formula of aluminium oxide is Al₂O₃. The valencies of aluminium and oxygen are respectively:
Explanation: Aluminium has a valency of 3 and oxygen has a valency of 2. By the criss-cross method, the valency of aluminium (3) becomes the subscript of oxygen and the valency of oxygen (2) becomes the subscript of aluminium, giving Al₂O₃.
Both sodium (Na) and magnesium (Mg) lose electrons to become stable. Why does magnesium have a higher valency than sodium?
Explanation: Sodium (2, 8, 1) has 1 electron in its outermost shell to lose, giving it a valency of 1. Magnesium (2, 8, 2) has 2 outer electrons to lose, giving it a valency of 2. Losing a greater number of outer electrons gives magnesium the higher valency.
Two elements P (electronic configuration 2, 8, 2) and Q (electronic configuration 2, 8, 7) combine to form a compound. What is its formula?
Explanation: P has 2 electrons in its outermost shell, so its valency is 2. Q has 7 outer electrons and needs 1 more, so its valency is 1. Balancing the valencies 2 and 1 by the criss-cross method gives the formula PQ₂.
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