Question

The mass of an atom (atomic mass) is equal to a) the sum of the masses of the protons. b) the sum of the masses of protons and neutrons. c) the sum of the masses of protons, neutrons, and electrons. d) the sum of the masses of protons, neutrons, and electrons minus the atom's binding energy.

Short answer

Answer: Atomic mass is equal to the sum of the masses of protons and neutrons.

Step-by-step solution

Understand the composition of an atom

An atom consists of a nucleus and electrons orbiting around the nucleus. The nucleus is made up of protons and neutrons. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge.

Masses of protons, neutrons, and electrons

The mass of a proton is approximately 1 atomic mass unit (amu), the mass of a neutron is also approximately 1 amu, and the mass of an electron is much smaller (about \( \frac{1}{1836} \) amu). The mass of an electron is negligible compared to the masses of protons and neutrons.

Choose the correct answer

We can now analyze the given options and select the correct answer. a) the sum of the masses of the protons: This is incorrect because it doesn't include the mass of the neutrons. b) the sum of the masses of protons and neutrons: This is correct because the mass of electrons is negligible compared to the masses of protons and neutrons. c) the sum of the masses of protons, neutrons, and electrons: This is incorrect because it includes the mass of electrons, which is negligible. d) the sum of the masses of protons, neutrons, and electrons minus the atom's binding energy: This is incorrect because the binding energy isn't relevant to determining the atomic mass. Therefore, the correct answer is (b) the sum of the masses of protons and neutrons.

Key concepts

Composition of an Atom

Understanding the basic structure of an atom is crucial in grasping the concept of atomic mass. An atom is the smallest unit of matter that retains the properties of an element. It consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The nucleus contains positively charged protons and neutrally charged neutrons.

The electrons orbit the nucleus in various energy levels, but they have much less mass compared to protons and neutrons and do not significantly contribute to the mass of an atom. The number of protons (atomic number) determines the type of element, and the total number of protons and neutrons gives the atomic mass number.

Proton and Neutron Masses

Protons and neutrons are collectively referred to as nucleons, and they are very close in mass. However, it is a common misconception that they have identical masses. In reality, neutrons are slightly heavier than protons. The mass of a proton is about 1.007 atomic mass units (amu), while a neutron's mass is approximately 1.008 amu.

It's important to note that the electron's mass, roughly \( \frac{1}{1836} \) amu, is quite insignificant in comparison, making it negligible when calculating an atom's mass. Understanding these masses is key to many calculations and concepts in atomic physics.

Mass Units in Atomic Physics

In atomic physics, mass is measured in atomic mass units (amu), which are also referred to as Daltons (Da). One amu is defined as one-twelfth the mass of a carbon-12 atom, and this standard is used because it provides a balance between the most common isotopes of carbon.

The use of amu makes it easier to work with individual atom and molecule masses at the microscopic scale, facilitating calculations and understanding of atomic and molecular behavior. Because the scale is based on carbon-12, it creates a direct relationship for scientists to compare the masses of different atoms and molecules.