# Atomic Number, Mass Number, and Isotopes

– In this subject, we will discuss the Atomic Number, Mass Number, and Isotopes: Definition, Formula, and Calculation

## Atomic number

– All atoms can be identified by the number of protons and neutrons they contain.

– The number of protons in the nucleus of each atom of an element is called the atomic number (Z).

– In a neutral atom, the number of protons is equal to the number of electrons, so the atomic number also indicates the number of electrons present in the atom.

– The chemical identity of an atom can be determined solely by its atomic number.

– For example, the atomic number of nitrogen is 7; this means that each neutral nitrogen atom has 7 protons and 7 electrons.

– Or viewed another way, every atom in the universe that contains 7 protons is correctly named “nitrogen.”

## Mass number

– The mass number (A) is the total number of neutrons and protons present in the nucleus of an atom of an element.

– Except for the most common form of hydrogen, which has one proton and no neutrons, all atomic nuclei contain both protons and neutrons.

– In general, the mass number is given by:

The number of neutrons in an atom is equal to the difference between the mass number and the atomic number, or (A – Z).

– For example, if the mass number of a particular boron atom is 12 and the atomic number is 5 (indicating 5 protons in the nucleus), then the number of neutrons is 12 – 5 = 7.

Note that all three quantities (atomic number, number of neutrons, and mass number) must be positive integers or whole numbers.

– The accepted way to denote the atomic number and mass number of an atom of element X is as follows:

## Isotopes

– In most cases, atoms of a given element do not all have the same mass.

– Atoms that have the same atomic number but different mass numbers are called isotopes.

– For example, there are three isotopes of hydrogen. One, simply known as hydrogen, has one proton and no neutrons.

– The deuterium isotope has one proton and one neutron, and tritium has one proton and two neutrons.

– Thus, for the isotopes of hydrogen, we write:

As another example, consider two common isotopes of uranium with mass numbers of 235 and 238, respectively:

– The first isotope is used in nuclear reactors and atomic bombs, whereas the second isotope lacks the properties necessary for these applications.

– Except for hydrogen, isotopes of elements are identified by their mass numbers.

– Thus, these two isotopes are called uranium-235 (pronounced “uranium two thirty-five”) and uranium-238 (pronounced “uranium two thirty-eight”).

– The chemical properties of an element are determined primarily by the protons and electrons in its atoms; neutrons do not take part in chemical changes under normal conditions.

– Therefore, isotopes of the same element have similar chemistries, forming the same types of compounds and displaying similar reactivities.

## Solved problem on Atomic number

Give the number of protons, neutrons, and electrons in each of the following species:

(a) 179Au95  ,  (b) 179Au97  ,  (c) 18F   ,  (d) carbon-13.

Solution:

(a) The atomic number of Au (gold) is 79, so there are 79 protons.

– The mass number is 195, so the number of neutrons is 195 – 79 = 116.

– The number of electrons is the same as the number of protons; that is, 79.

(b) Here the number of protons is the same as in (a), or 79.

– The mass number is 197, so the number of neutrons is 197 – 79 = 118.

– The number of electrons is also the same as in (a), 79.

– The species in (a) and (b) are chemically similar isotopes of gold.

(c) The atomic number of F (fluorine) is 9, so there are 9 protons.

– The mass number is 18, so the number of neutrons is 18 – 9 = 9.

– The number of electrons is the same as the number of protons; that is, 9.

(d) Carbon-13 can also be represented as 13C.

– The atomic number of carbon is 6, so there are 13 – 6 = 7 neutrons.

– The number of electrons is 6.

Reference: General Chemistry – The Essential Concepts / Raymond Chang, Jason Overby. ( sixth edition) .