Atomic mass of an element is equal to the sum of number of
- A.electrons and protons only
- B.protons and neutrons only
- C.electrons and neutrons only
- D.electrons, protons and neutrons
Correct answer
Explanation
The correct answer is B, protons and neutrons only. Almost all the mass of an atom is in its nucleus, so the mass number, which the question calls atomic mass, is the total of protons and neutrons, the two kinds of nucleons. A proton and a neutron each have a mass of about one atomic mass unit, while an electron is about 1,836 times lighter than a proton, so electrons add almost nothing. Carbon-12, for example, has 6 protons and 6 neutrons and a mass number of 12, while chlorine-35 has 17 protons and 18 neutrons. The number of protons alone is the atomic number, and the number of neutrons is the mass number minus the atomic number. Option A is wrong because electrons have negligible mass, and leaving out the neutrons would undercount the mass. Option C is wrong because it leaves out the protons, which are half or more of the nucleus. Option D is wrong because counting electrons adds nothing meaningful; the mass number counts only nucleons. Exam tip: mass number (A) = protons + neutrons; atomic number (Z) = protons; neutrons = A − Z.
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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Liver cells turn the toxic ammonia released from the breakdown of amino acids into urea through the ornithine cycle, also called the urea cycle.
Read the full explanation →Which one of the following is the correct sequence of passage of light in a compound microscope?
- 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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Light passes through the condenser to the specimen, is picked up by the objective lens, travels up the body tube and reaches the eye through the eyepiece.
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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It binds oxygen at its iron-containing haem groups and carries some carbon dioxide on its protein chains. Each haemoglobin molecule has four polypeptide chains and four haem groups, and each haem iron can hold one oxygen molecule, forming oxyhaemoglobin in the lungs.
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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