Atomic Number - Know Atomic Mass Number, Isotopes, Isobars & Understand with Examples

Atomic Number is the identity of an atom. The atomic number is the most fundamental property of an atom. In this article, we will study the definition of atomic number, Mass Number, Isotopes and Isobars difference, Isotopes of hydrogen and their characteristics with solved examples.

Concept of Atomic Number

The value of the positive charge present on the nucleus of an atom was determined by Henry Mosely in 1513. Mosely used anodes of different metals in the discharge tube and subjected them to attack of cathode rays. Mosely found that when cathode rays struck anodes of different metals characteristic x-rays were emitted. The wavelengths of these rays were found to decrease in a regular manner on changing the metal of the anode in the order of its position in the periodic table. Mosely found that the square root of the frequency of x-rays thus emitted is directly proportional to the number of positive charges present on the nucleus of the atom of the metal used as the anode. From these experiments, he concluded that the number of positive charges present on the nucleus of an atom is the most fundamental property of an atom.

What is an Atomic Number?

The term Atomic Number was introduced to describe this most fundamental property of an atom. The atomic number of an element can be defined as follows.

The number of unit positive charges present on the nucleus of an atom of a particular element is called the atomic number of that element.

Since positive charge present on the nucleus of an atom is due to proton and each proton has a unit positive charge, the atomic number of an element can also be defined as,

The number of protons present in the atom of a particular element is called the atomic number of the element.

In a neutral atom, the number of protons is exactly equal to the number of electrons present in it. Hence the nucleus of the atomic number of an atom is also equal to the number of electrons present in it. The atomic number is represented by Z.

Atomic Number (Z) = Number of protons = Number of electrons

Scientist Mosley further proved that the atomic number Z of an element is related to frequency v of x rays produced by equation,

\sqrt{v}=a (Z-k)

where a and k are constants. With the help of the equation given above, Mosley determined the atomic numbers of several elements. He also corrected the wrong atomic numbers of some of the elements.

What is Atomic Mass Number?

The entire mass of an atom is concentrated in its nucleus. The electrons present in the space around the nucleus possess negligible mass and do not contribute much to the mass of an atom. Hence in a true sense, the entire mass of an atom may be supposed to be present in the nucleus. The nucleus of an atom contains protons and neutrons which are collectively called nucleons. Since the mass of each of these particles is nearly 1 on an atomic mass unit scale, the sum of numbers of protons and neutrons refers to the approximate mass of an atom in amu. This number is called a Mass number. It may be defined as follows:

The sum of the numbers of protons and neutrons present in the nucleus of an atom is called the mass number of that atom. 

The mass number is represented by A and hence,

Mass number (A) = Number of protons + Number of neutrons = Total number of nucleons

Thus for a neutral atom ^A_Z{X}, with mass number A and atomic number Z,

• Number of electrons = Z
• Number of protons = Z
• Number of neutrons = A-Z

Solved Examples of Atomic Number

1. Find the Number of electrons, protons and neutrons in ^{35}_{17}Cl
2. For neutral atom ^A_Z{X}, with mass number A and atomic number Z,
   • Number of electrons = Z
   • Number of protons = Z
   • Number of neutrons = A-Z

Thus in the case of chlorine ^{35}_{17}Cl, with atomic number (Z) equal to 17 and mass number (A) equal to 35.

Number of extranuclear electrons = 17 Number of protons = 17

Number of neutrons = 35 – 17 =18

Generally, every atom X is represented as x where X is the symbol of the atom. The mass number is written towards the top left, while the atomic number is towards the left, the bottom of the symbol of the element.

Difference Between Atomic Number and Atomic Mass Number.

Atomic Number	Atomic Mass Number
The number of unit positive charges present on the nucleus of an atom of a particular element is called the atomic number of that element.	The sum of the numbers of protons and neutrons present in the nucleus of an atom is called the mass number of that atom.
Atomic number essentially helps in the grouping and distinguishing proof of a component.	Atomic mass can’t be utilized to characterize the kind of element.
Atomic number is only a digit that is utilized to put components in an occasional table.	Atomic mass is generally estimated utilizing atomic mass units (amu).
Atomic number is typically the number of protons present in a component’s core.	Atomic mass is related to the number of neutrons and protons that are available in a specific core of an element.
The letter Z is utilized to address an atomic number.	Atomic mass is signified by A
It is the absolute number of nucleons in the molecule’s core. It is the normal load of an element.	Atomic mass is additionally used to arrange various isotopes of a similar element. Isotopes just offer a similar atomic number.

Isotopes and Isobars

Isotopes and Isobars are considered as species of atoms. Here’s what they mean

Isotopes

The atomic species of an element possessing the same atomic number but different mass numbers are called isotopes. It may be defined as:

The atoms of an element that have the same number of protons but different numbers of neutrons are called isotopes of that element.

Isotopes of Hydrogen

There are three isotopes of hydrogen. These are named as protium or proton, deuterium and tritium.

Similarly, oxygen has three isotopes,

Chlorine has two isotopes: ^{35}_{17}Cl and ^{37}_{17}Cl

Isotopes of some elements

Characteristics of Isotopes

The basic characteristics of isotopes are as follows:

• Since the isotopes of an element have the same atomic number, but different mass numbers, the nuclei of isotopes contain the same number of protons but a different number of neutrons.
• Since chemical properties are mainly determined by the number of protons in the nucleus and the number of electrons in the atom. The different isotopes of an element exhibit similar properties. For example, all the isotopes of carbon on burning give carbon dioxide.

Isobars

The atoms of different neighbouring elements have the same mass number but different atomic numbers are called isobars.

Due to different atomic numbers, two isobars possess a different number of protons and electrons. Due to similar mass numbers, the sum of protons and neutrons is the same in them.

For example, ^{40}_{18}Ar, ^{40}_{19}K and ^{40}_{20}Ca are isobars. The characteristics of these isobars is summarised in the table given below:

Isobars possess different chemical properties. They occupy different positions in the periodic table. They are different atomic species and possess the same mass number.

Difference between Isotopes and Isobars

Isotopes	Isobars
Isotopes are atoms of the same element.	Isobars are atoms of different elements.
They have the same atomic number but different atomic mass numbers.	They have the same atomic mass number but different atomic numbers.
They have the same number of protons but different numbers of neutrons.	They have different numbers of protons and neutrons.
They have the same number of electrons.	They have different numbers of electrons.
They occupy the same position in the modern periodic table	They occupy different positions in the modern periodic table.
They have similar chemical properties.	They have different chemical properties.
Eg: ^{12}_{6}C, ^{13}_{6}C, ^{14}_{6}C are isotopes	Eg: ^{40}_{18}Ar, ^{40}_{19}K and ^{40}_{20}Ca are isobars

So, this is all about the Atomic Number. Get some practice of the same on our free Testbook App. Download Now!