Atomic Number and Mass Number: Notations, Solved Examples

Atoms are the building block of matter, and they combine in numerous forms to form different substances. You must be well aware that all the atoms consist of electrons, protons, and neutrons. But did you know how the presence of these subatomic particles can describe the characteristics of an element? The atomic number and the mass number differ from each other.

The atomic number is used to represent the number of protons in an atom. The Mass number is used to represent the number of protons and neutrons present in the atoms. In this article, we will study in detail about atomic and mass numbers. Scroll down to learn more!

What is Atomic Number?

It refers to the number of protons in an atom. Protons are known to exist in the nucleus of an atom. It is the number of protons in an atom that determines its atomic number.

The atomic number is defined as the number of protons in one atom of an element. The atomic number of an element \(=\) Number of protons in one atom of the element

For example, the atomic number of sodium is \(11\), because one atom of this element has \(11\) protons. Similarly, one carbon atom has six protons, giving it an atomic number of six, while a hydrogen atom has just one proton in its nucleus. Therefore, the atomic number of hydrogen is \(1\).

The atomic number of an element is denoted by the letter \(\rm{Z}\).

The nuclei of all atoms of the same element have the same number of protons, and so have the same atomic number.

The number of protons in the nuclei of different elements differs.

Two elements cannot have the same atomic number.

The atomic number can be used to identify an element because each element has its own fixed atomic number.

For example, if the atomic number is \(6\), it tells us that it is a carbon element. No other element can have an atomic number of \(6\).

Thus, it is the atomic number that distinguishes the atoms of one element from the atoms of the other element. The atoms of various elements are different because they have different atomic numbers.

In a normal atom, the number of protons is equal to the number of electrons in it. So, we can say that, “The atomic number of an element is equal to the number of electrons in a neutral atom of that element”.

The atomic number of an element \(=\) Number of electrons in one neutral atom

Only a neutral atom possesses an equal number of protons and electrons, the atomic number of an element is equal to the number of electrons in the neutral atom, not in the ion. An ion, on the other hand, is generated by removing electrons from a regular atom or adding electrons to a normal atom and hence has either fewer or more electrons than protons.

Chemical reactions involve only an atom’s electron, not its protons. The number of electrons may vary throughout a chemical reaction, whereas the number of protons does not. As a result, an element’s atomic number does not change during a chemical reaction and remains unchanged.

Below given is the list of atomic numbers of some common elements-

Notation of the Atomic Number

If the element is \(\rm{X},\) then the atomic number \(\rm{Z}\) is written in chemical notation as-

The atomic number is always denoted as the subscript.

Importance of Atomic Numbers

The importance of atomic numbers are listed below:

1. The element’s chemical properties are determined by the number of electrons in the element, which is determined by the number of protons.
2. The periodic table is arranged as per the atomic number of the elements.
3. Each element has a unique number of protons. So atomic number help in the identification of elements.

What is Mass Number?

An atom consists of protons, neutrons, and electrons. Since the mass of electrons is negligible as compared to protons and neutrons, the mass of an atom is determined by protons and neutrons only. The total number of protons and neutrons present in one atom of an element is known as its mass number.

Mass number \(=\) Number of protons \(+\) Number of neutrons

For example, one atom of a sodium element contains \(12\) neutrons and \(11\) protons, so the mass number of sodium is \(11 + 12 = 23.\) Similarly, a carbon atom has \(6\) protons and \(6\) neutrons, so the mass number of carbon is \(6 + 6 = 12.\) An atom of hydrogen has \(1\) proton but \(0\) neutrons. Therefore, the mass number of hydrogen is just \(1.\)

The mass number of an element is denoted by the letter \(\rm{A}.\) The mass of the sodium atom is \(23,\) so we can say that for sodium, \(\rm{A} = 23.\)

Notation of Mass Number

If the element is \(\rm{X},\) then the atomic mass number \(\rm{A}\) is written in chemical notation as-

The mass number is always denoted as the superscript.

The mass number of an atom also gives its atomic mass. This is because the atomic mass of an atom is numerically equal to the mass number. For example, if the mass number of an atom is \(23,\) and then its atomic mass will be \(23\,\rm{u}.\)

Relationship Between Mass Number and Atomic Number

Mass number \(=\) No. of protons \(+\) No. of neutrons

Since the number of protons in an atom is equal to the atomic number of the element, we can rewrite the above relation by putting “Atomic Number” in place of “No. of protons”. Thus,

Mass Number \(=\) Atomic Number \(+\) No. of neutrons

\(\rm{A} = \rm{Z} + \rm{n}\)

The atomic number and the mass number can be indicated on the symbol of an element. The atomic number is written on the left-hand side subscript, whereas the mass number can be written on the right-hand side superscript.

Solved Examples on Atomic and Mass Number

Example 1. Calculate the atomic number of an element whose atomic nucleus has mass number \(23\) and neutron number \(12.\) What is the symbol of the element?
Solution: Mass number \(=\) Atomic number \(+\) No. of neutrons
So, \(23 =\) Atomic number \(+ 12\)
Atomic number \(= 23 – 12 = 11\)
The element having atomic number \(11\) is sodium, and its symbol is \(\rm{Na}.\) It can be indicated as-

Example 2. The number of electrons in an atom is \(8,\) and the number of protons is also \(8.\)
a. What is the atomic number of the atom?
b. What is the charge on the atom?
Solution:
a. The atomic number is equal to the number of protons in one atom. Since the atom contains \(8\) protons, the atomic number is \(8.\)
b. This atom contains an equal number of protons and neutrons, so it has no overall charge.

Example3. Helium atom has an atomic mass of \(4\,\rm{u}\) and two protons in the nucleus. How many neutrons does it have?
Solution: The atomic mass is numerically equal to the mass number of an atom. Since the helium atom has an atomic mass of \(4\,\rm{u},\) therefore, the mass number of the helium atom will be \(4,\) and the number of protons is given as \(2.\)
Therefore,
Mass number \(=\) No. of protons \(+\) No. of neutrons
\(4 = 2 +\) No. of neutrons
No. of neutrons \(= 4 – 2 = 2\)
Thus, the helium atom has \(2\) neutrons.

Periodic Table Arrangement

During the arrangement of elements in the periodic table, there were three tables designed that depended on the atoms’ mass number and failed due to many reasons. These three periodic tables are- Dobereiner Triads, Newland Law of Octaves, and Mendeleev’s periodic table.

The accepted periodic table is the Modern Periodic Table. Elements in this periodic table are arranged from left to right and top to bottom in the increasing order of their atomic number.

Summary

The atomic number is defined as the number of protons in one atom of an element. The mass number refers to as the total number of protons and neutrons present in an atom. The letter Z is used to denote the atomic number of an element. The mass number of an element is denoted by the letter A. Furthermore, the atomic number is denoted as subscript whereas the mass number is denoted as superscript. Therefore, one can say that atomic and mass numbers are very different from each other.

FAQs on Atomic and Mass Number

Following are the frequently asked questions on atomic number and mass number:

Q.1. Are atomic mass and atomic number the same?
Ans: No, the atomic number and mass number are very different from each other. The atomic number denotes the number of electrons or protons in a neutral atom, whereas the mass number denotes the total number of protons and neutrons present in an atom.

Q.2. How can atomic number and mass number be written?
Ans: The atomic number and the mass number can be indicated on the symbol of an element. The atomic number is written on the left-hand side subscript, whereas the mass number can be written on the right-hand side superscript.
For example, the atomic number of sodium is \(11,\) and the mass number is \(23.\) It can be represented as-

Q.3. How can atomic number and mass number be calculated?
Ans: Atomic number \(=\) number of protons in an element
Mass number \(=\) No. of protons \(+\) No. of neutrons
Since the number of protons in an atom is equal to the atomic number of the element, we can rewrite the above relation by putting “Atomic Number” in place of “No. of protons”. Thus,
Mass Number \(=\) Atomic Number \(+\) No. of neutrons
\(\rm{A} = \rm{Z} + \rm{n}\)

Q.4. What is the difference between atomic number and mass number?
Ans: The major difference between an atomic number and the mass number is that the atomic number indicates the number of protons present in an atom, whereas the mass number indicates the total number of protons and neutrons present in an atom.

Q.5. What is the arrangement of atomic number and mass number on periodic table?
Ans: Elements in the modern periodic table are arranged from left to right and top to bottom in the increasing order of their atomic number.