The valency of an element depends upon the
- A.total number of protons in an atom
- B.mass number of an atom
- C.total number of neutrons in an atom
- D.total number of electrons in the outermost shell of an atom
Correct answer
Explanation
The correct answer is D, total number of electrons in the outermost shell of an atom. Valency is the combining capacity of an atom, and it is decided by the valence electrons it gains, loses or shares to complete its outer shell.
Atoms tend to reach a stable octet (or a duplet for hydrogen and helium). An atom with 1, 2 or 3 outer electrons usually loses them, so its valency equals that number: sodium (2, 8, 1) has valency 1 and magnesium (2, 8, 2) has valency 2. An atom with 5, 6 or 7 outer electrons gains electrons, so its valency is 8 minus that number: oxygen (2, 6) has valency 2 and chlorine (2, 8, 7) has valency 1. Carbon, with four outer electrons, shares them and has valency 4.
A is wrong because the number of protons is the atomic number, which fixes the identity of the element, not directly its combining capacity. B is wrong because the mass number is protons plus neutrons and has no role in bonding. C is wrong because neutrons carry no charge and isotopes with different neutron numbers have the same valency.
Exam tip: Valency = outer electrons (if 1–4) or 8 minus outer electrons (if 5–8); isotopes share it.
Practice Questions
View allThe fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by
- A.Galileo
- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
Read the full explanation →Which one of the following statements is correct?
- A.Urea is produced in liver.
- B.Urea is produced in blood.
- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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- A.Condenser-Objective lens-Eyepiece-Body tube
- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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Read the full explanation →Which one of the following statements regarding haemoglobin is correct?
- A.Haemoglobin present in RBC can carry only oxygen but not carbon dioxide.
- B.Haemoglobin of RBC can carry both oxygen and carbon dioxide.
- C.Haemoglobin of RBC can carry only carbon dioxide.
- D.Haemoglobin is only used for blood clotting and not for carrying gases.
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Read the full explanation →Which one of the following statements regarding histone proteins is correct?
- A.Histones are proteins that are present in mitochondrial membrane.
- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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They are the proteins around which DNA winds to form chromatin in the nucleus of eukaryotic cells. Histones are small basic proteins, rich in the positively charged amino acids lysine and arginine, which attract the negatively charged phosphate groups of DNA.
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