Chapter U1.13, Problem 2E

Interpretation Introduction

Interpretation:

Differences between average atomic mass given on a periodic table and mass of an atom is to be explained.

Concept introduction:

An atom is made of particles called electrons, protons and neutrons. Atoms of each element are different. Isotopes are atoms of same element with different number of neutrons.

Explanation of Solution

Average atomic mass - It is the weighted average mass of an element with respect to all its existing isotopes and their relative abundance. It is calculated by taking average of masses of isotopes of an element in a sample. Since each isotope has different number of neutrons, it has different atomic masses. Average atomic mass is often expressed in decimal number. Masses and abundance of isotopes are used in calculation.

Mass of an atom - The mass of an atom is the mass of a single atom for a single isotope of an element. To calculate mass of an atom, mass of protons, neutrons and electrons is taken into consideration. Since most of the mass of an atom is concentrated in the nucleus of an atom, the atomic mass is calculated by considering mass of protons and mass of neutrons. Each isotope of an element has a different mass. Mass of an atom is generally a whole number.

Example:

Most of the carbon atoms found in nature has a mass of 12 amu. For every 100 carbon atoms, there is a probability of finding one carbon atom with atomic mass of 13 amu and very rarely one carbon atom of mass 14 amu. All these three carbon atoms are isotopes of carbon. So the average atomic mass of carbon is calculated as 12.01 taking into consideration masses of all three isotopes and their percentage abundance.

Hence, masses of isotopes of carbon atom are 12 amu, 13 amu and 14 amu.

Averageatomic mass of carbon is 12.01amu.

Conclusion

Average atomic mass is the average of the masses of the isotopes of an element. Mass of an atom is the sum of number of protons and neutrons in an atom.