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Relation Between Mass Number and Atomic Number

  • Last Updated : 21 Jul, 2021

Atoms are the fundamental building blocks of matter, and they may combine in a variety of ways to produce various substances. You must be aware that all atoms are made up of electrons, protons, and neutrons. But did you know that the presence of these subatomic particles may explain an element’s properties? In this post, we will look in-depth at the Atomic Mass and Atomic Number, as well as their significance. In this article, we will discuss the relation between the mass number and the atomic number. Before that, let’s understand what is the mass number and atomic number?

Atomic Number

It is the number of protons in an atom. Protons are known to exist in an atom’s nucleus. The atomic number of an atom is determined by the number of protons in the atom. As a result, we may define an atomic number as:

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The atomic number of an element is the number of protons in one atom. An element’s atomic number equals the number of protons in one atom of the element is termed as Atomic Number.

The atomic number of sodium, for example, is 11, since one atom of this element has 11 protons. A carbon atom, for example, has six protons, giving it an atomic number of sixes, whereas a hydrogen atom has just one proton in its nucleus. As a result, hydrogen has an atomic number of ones. The letter Z represents an element’s atomic number.

  • All atoms of the same element have the same number of protons and hence the same atomic number.
  • The amount of protons in the nuclei of various elements varies.
  • The atomic number of two elements cannot be the same.
  • Because each element has a unique atomic number, the atomic number may be used to identify it.

For example, if the atomic number is 6, it indicates that the element is carbon. There is no other element with an atomic number of 6. Thus, the atomic number differentiates one element’s atoms from the atoms of another element. Because different elements have distinct atomic numbers, their atoms differ. The number of protons in a typical atom is equal to the number of electrons. As a result, we may state-

Atomic number of an element = Number of electrons in one neutral atom

Only a neutral atom has an equal number of protons and electrons; an element’s atomic number is equal to the number of electrons in the neutral atom, not the ion. An ion, on the other hand, is formed by either removing or adding electrons to a conventional atom, and therefore contains either fewer or more electrons than protons. Chemical processes solely involve the electrons of atoms, not their protons. The number of electrons can change throughout a chemical reaction, while the amount of protons does not. As a result, the atomic number of an element does not change throughout a chemical reaction and stays constant.

Mass Number

Protons, neutrons, and electrons make up an atom. Because electrons have a tiny mass in comparison to protons and neutrons, the mass of an atom is determined solely by protons and neutrons. The mass number of an element is the total number of protons and neutrons contained in one atom.

The mass number is equal to the sum of the number of protons and neutrons or, 

Mass number = Number of protons + Number of neutrons



One atom of sodium, for example, contains 12 neutrons and 11 protons, therefore the mass number of sodium is 11+12=23. Similarly, because a carbon atom contains 6 protons and 6 neutrons, its mass number is 6+6=12.

A hydrogen atom has one proton but no neutrons. As a result, the mass number of hydrogen is only one. The letter A represents an element’s mass number. Because the mass of the sodium atom is 23, we may state that A=23 for sodium.

Notation of the Atomic Number and Mass Number 

If the element is X, then the atomic number Z and mass number A are written in chemical notation as-

Symbolic representation of an atom

Atomic number

Mass number

AXZ

          X ⇒ Element

          A ⇒ Mass no. of an element

          Z ⇒ Atomic no. of an element



Z = e = p

Z ⇒ Atomic no.

e ⇒ No. of electrons

p ⇒ No. of protons

A = p + n

A ⇒ Mass no. of an atom

n ⇒ no. of neutrons

p ⇒ No. of protons.

Relationship Between Mass Number and Atomic Number

The Relationship Between Mass Number and Atomic Number is stated as:

Mass Number = No. of protons + No. of neutrons

We know,  

Atomic Number = No. of protons

Therefore, we can say

Mass Number = Atomic Number + No. of electrons

Symbolically this can be represented by  

A = Z + n

where A = Mass number, Z = Atomic number and n= no. of neutrons.

Difference Between Mass Number and Atomic Number

 

Mass Number

Atomic Number

1. Atomic mass is proportional to the number of neutrons and protons contained in an element’s nucleus. The number of protons in an element’s nucleus is generally represented by its atomic number. 
2. It is an element’s average weight. It is the total number of protons in the nucleus of an atom. 
3.Atomic mass is denoted by AThe letter Z is used to represent an atomic number.
3.Atomic mass cannot be used to determine the element’s kind. The atomic number is primarily used in the categorization and identification of elements. 
4.Atomic mass is also used to differentiate between various isotopes of the same element. Isotopes only have one atomic number in common. 
5.The atomic mass unit is commonly used to measure atomic mass (amu).An atomic number is just a digit used to arrange elements in a periodic chart.

Sample Problems

Problem 1: The mass no. of chlorine atom is 35 and the atomic no. is 17. Find the number of neutrons present in the chlorine atom.



Solution:

Mass No. (A) = 35

Atomic No (Z) =17

We know  

Mass No = Atomic No+ no. of neutrons present 

n = Mass no – Atomic no

n= 35-17

n = 18

Therefore, the no. of neutrons present in the chlorine atom is 18.

Problem 2: The number of electrons presents in the calcium atom is 20 and the no. of neutrons present is 20. Find the atomic no. and the mass no of the calcium atom.

Solution:

We know, an atom is electrically neutral. Thus, the no. of electrons present in an atom is equal to the no. of protons present in the atom.  

So, the no. protons present in the atom = 20.  

We know, that the Atomic no. of an atom of an element = no. of protons present.

Thus, atomic no. = 20

The mass no = Atomic no + No. of neutrons

Mass no = 20+20

Mass no = 40

Problem 3: The atomic nucleus of an atom has a mass number of 23 and has 12 neutrons inside its nucleus. Calculate its atomic number.

Solution:

Mass Number = 23

No. of neutrons =12

Mass no = No. of protons + No. of neutrons

No. of protons = Mass no. – No. of neutrons

No. of protons = 23-12

 No. of protons = 11

We know

Atomic no of an element = No. of protons present in the element = 11

Atomic No. of the element = 11

Problem 4: Two elements A and B have no. of neutrons have 20 and 13 and no. of protons have 18 and 20. Which will have more mass number?

Solution:

For element A:

No. of neutrons = 20

No. of protons=18

Mass no. of element A = 20+18

                                     =38

For element B:

No. of neutrons = 13

No. of protons = 20

Mass no. of element B = 13+20

                                    = 33

Therefore, the Element B has more mass no. than element A.

Problem 5: It is said that the Atomic no. of an Element M is 12 and the no. of electrons present in the element is 13”. Is it a valid statement?

Solution:

No, it is not a valid statement. This is because an atom of an element is electrically neutral , thus the atomic no. of an element ( No. of protons ) should be equal to the no. of electrons present inside an atom. But here, it is said that the Atomic no ( or no. of protons) = 12 and no. of electrons is 13. Thus, they are not equal and the statement is invalid.




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