# Which of the following statements about the mass number of an atom is incorrect ? a. It has a unit defined as a dalton. b. On Earth, it equals the atomic weight. c. Unlike the atomic weight of an atom, it does not change when gravitational forces change. d. It equals the number of electrons in an atom. e. It is the sum of the protons and neutrons in the atomic nucleus.

### Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
Publisher: Cengage Learning
ISBN: 9781305389892

### Biology: The Dynamic Science (Mind...

4th Edition
Peter J. Russell + 2 others
Publisher: Cengage Learning
ISBN: 9781305389892

#### Solutions

Chapter
Section
Chapter 2, Problem 1TYK
Textbook Problem

## Which of the following statements about the mass number of an atom is incorrect?a. It has a unit defined as a dalton.b. On Earth, it equals the atomic weight.c. Unlike the atomic weight of an atom, it does not change when gravitational forces change.d. It equals the number of electrons in an atom.e. It is the sum of the protons and neutrons in the atomic nucleus.

Expert Solution
Summary Introduction

Introduction:

The elements are made up one type of atoms which contain electrons, protons, and neutrons. The atoms contain a nucleus, which contains protons and neutrons in it. The electrons revolve around the nucleus in their orbital paths. The atomic number of an element is the total number of protons present in the nucleus.

It equals the number of electrons in an atom.

### Explanation of Solution

Option (d) is that it equals the number of electrons in an atom. The mass number is represented by the sum of protons and neutrons. It is taken so because the mass of a proton and a neutron is almost equal, which is 1.66×1024 g and the mass of an electron is about 1800 times lesser than this. So, it is ignored. The mass number is not equal to the number of electrons in an atom. Hence, option (d) is correct.

Option (a) is that its unit is Dalton. The mass of proton and neutron is given by a unit named Dalton (Da). So, it is an incorrect option.

Option (b) is that on earth, it equals its atomic weight. The atomic mass is called Atomic weight if the gravitational force is considered. The mass number equals atomic weight when it is affected by the Earth’s gravitational force. So, it is an incorrect option.

Option (c) is that unlike the atomic weight of an atom, it does not change when gravitational forces change. The weight changes with a change in gravitational force but the mass is not changed. So, it is an incorrect option.

Option (e) is that it is the sum of the protons and neutrons in the atomic nucleus. The atomic nucleus contains neutron and protons, whose summation gives the mass number. So, it is an incorrect option.

Hence, options (a), (b), (c) and (e) are incorrect.

Conclusion

Thus, it can be concluded that the mass number is independent of the number of electrons contained in an atom. It is equal to the sum of the number of protons and neutrons in the atomic nucleus.

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