1. Carbon Atom Mass Number
2. The Mass Number Of An Atom Is The Number Of
3. Mass Number Of The Element

Mass number is an integer (whole number) equal to the sum of the number of protons and neutrons of an atomic nucleus. In other words, it is the sum of the number of nucleons in an atom. Mass number is often denoted using a capital letter A. Contrast this with the atomic number, which is simply the number of protons. The number of nucleons present in an atom is called mass number. Mass number, as from the name, is responsible for the mass of an atom or atomic mass. Since protons and neutrons are much heavier than electrons, the mass of an atom is estimated by the number of protons and the number of neutrons or in other words, mass number. The mass number of the atom (M) is equal to the sum of the number of protons and neutrons in the nucleus. The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z). Examples: Let's determine the number of protons, neutrons, and electrons in the following isotopes.

The students study were discussing the mass number, which is a way of describing the massive. And Adam Now on Adam consists of an exact number of protons, neutrons and electrons. But the mass number, which it can also be abbreviated as a, is exactly the number of protons and the number of neutrons oven adam.

Contents

• Atomic number and Mass number
• Isotopes

Atom

An atom is the smallest particle of an element which can take part in chemical reaction. Atom consists of three fundamental particles i.e. proton, neutron and electron. Atoms of same elements are similar in properties whereas atoms of different elements are different in properties. Example:- ‘H’ represent the atom of hydrogen.

Proton is positively charged and electron is negatively charged particle. In an atom, number of protons = number of electrons. Hence, the net charge present in an atom is zero i.e. a free atom is chargeless.

Atomic number and Mass number

Atomic number :

• Atomic number is the number of protons present in an atom.
• The modern periodic table is arranged in order of increasing atomic number.

Mass number and Atomic mass :

• Mass number is the sum of the number of protons and the number of neutrons present in an atom. It is a whole number.

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

• Atomic mass is the average mass of the all of the isotopes of that element. It is a decimal number.
• For example: Hydrogen has three isotopes – ₁H¹, ₁H² and ₁H³ having mass number 1, 2 and 3 respectively. Naturally occurring hydrogen contains about 99.985% of protium, 0.014% of deuterium and 0.001 % of tritium. Therefore the atomic mass of hydrogen is 1.00784 amu.
• The atomic mass of an element element is measured in atomic mass unit (amu, also known as Daltons ‘ D’or unified atomic mass unit ‘u’).
• 1amu = 1.66 x 10^-24 grams. 1gm = 6.022 x 10²³ amu ( i.e. Avogadro’s number).

Here,

• Atomic number = Number of protons = Number of electrons = 13
• Mass number = No. of protons + No. of neutrons
• No. of neutrons = Mass number – No. of protons = 27-13 = 14.

Atomic mass of first 20 elements

Isotopes

Atoms of the same element having same atomic number but different mass number (atomic mass/weight) are called isotopes. For example:

Isotopes of hydrogen :

There are three isotopes of hydrogen:

1. Protium or ordinary hydrogen
2. Deuterium or heavy hydrogen
3. Tritium or radioactive hydrogen.

Naturally occurring hydrogen contains about 99.985% of protium, 0.014% of deuterium and 0.001 % of tritium.

Isotopes have different physical properties since they differ in their mass number.

They have same chemical properties since their electronic configuration is same. However, they differ in the rate of chemical reaction. For example, D₂ reacts with Cl₂ about 13 times slower than H₂ does. The different in rate of reaction due to difference in mass of the atoms of the same element is called isotope effect.

Some other examples of isotopic elements :

Isobars

Atoms of different elements having different atomic number but same mass number are called isobars. For example :

₁₈Ar⁴⁰, ₁₉K⁴⁰ and ₂₀Ca⁴⁰

Isotones

Atoms of different elements having different atomic number and mass number but same number of neutrons are called isotones. For example :

₆C¹⁴, ₇N¹⁵ and ₈O¹⁶

Objective questions and their answers

1. Which of the following is known as heavy hydrogen?

a. Protium c. Tritium

b. Deuterium d. Para hydrogen

2. Which of the following is known as radioactive hydrogen?

a. Protium c. Tritium

b. Deuterium d. Para hydrogen

3. Least abundant isotope of hydrogen is:

a. Protium c. Tritium

b. Deuterium d. Heavy hydrogen

4. Diamond and graphite are :

a. Isotopes c. Isotones

b. Isobars d. Allotropes

5. ₆C¹⁴ and ₈O¹⁶ are :

a. Isotopes c. Isotones

b. Isobars d. Allotropes

6. ₆C¹⁴ and ₇N¹⁴ are :

a. Isotopes c. Isotones

b. Isobars d. Allotropes

7. All particles residing inside the nucleus of an atom are termed as:

a. Protons c. Electrons

Carbon Atom Mass Number

b. Neutrons d. Nucleons

8. What makes the atomic mass fractional ?

a.Prerence of isotopes

b. Number of unpaired electrons

c. Spherical shape

d. Quantum number.

9. Which of the following are not isotopes:

a. ₁H¹ and ₁H³

b. ₁₈K⁴⁰ and ₂₀Ca⁴⁰

c. ₆C¹⁴ and ₇N¹⁴

d. Both b and c.

10. Charge present in the nucleus of an atom is :

a. Positive c. Chargeless

b. Negative d. Both +Ve and -Ve

11. Molecular weight of heavy water is :

a. 16 c. 20

b. 18 d. 22

Answers :

1. b 2. c 3. c

4. d [Note : different forms of same element having different properties are called allotropes]

5. c 6. b 7. d

8. a 9. d 10. a

11. c Note :Heavy water– Deuterium oxide (D₂O) is called heavy water. It’s molecular weight is 20 and boiling paint is 101.5⁰C and melting point is 3.8⁰C.

References

• Sthapit, M.K., Pradhananga, R.R., Foundations of Chemistry, Vol 1 and 2, Fourth edition, Taleju Prakashan, 2005.

20th Jun 2019 @ 13 min read

An atom consists of electrons, protons, and neutrons. Protons and neutrons reside in the nucleus of an atom, and they, in together, are called nucleons. The number of nucleons present in an atom is called mass number. Mass number, as from the name, is responsible for the mass of an atom or atomic mass. Since protons and neutrons are much heavier than electrons, the mass of an atom is estimated by the number of protons and the number of neutrons or in other words, mass number. Mass number is always a whole number because the number of protons and neutrons present in an atom is always a whole number.

Definition

Mass number is the number of nucleons (the sum of protons and neutrons) present in an atom. Mass number is also called as atomic number or nucleon number.

Notation and Formula

Mass number is denoted as A. The value of A is calculated by summing the number of protons and neutron in the nucleus of an atom.

where: n_p⁺ is the number of protons and n_n⁰ is the number of neutrons.

Representation

The mass number is represented as the left superscript to the symbol of an element. Consider an example of sodium, which has a mass number of 23. The number is superscripted to the left of the symbol of sodium, which is Na (see the below figure). When using the full name of an element, not its symbol, mass number come after the name of an element with a hyphen in between, for example, sodium-23, carbon-12, iron-56.

Mass Number and Atomic Number

Mass number and atomic number are often confusing for beginners, but they both are different. The below table distinguishes them.

Mass Number, Atomic Number and Neutron Number

Mass number (A), atomic number (Z), and neutron number (N) relate to each other as simple mathematically equation, which is mentioned below.

Mass Number and Atomic Mass

Mass number is nearly equal to atomic mass of an element. Mass number is a whole number while atomic number occurs in decimals except in the case of carbon-12, which has the exact atomic mass of 12 u. Mass number is a dimensionless quantity while atomic mass is expressed in the atomic mass unit (amu or u). For most of common elements, the difference is very small in the range of (1 u). The difference between them is called mass excess. The table below list some of elements with their mass number and atomic number.

As we can observe from the above table, the ratio of atomic mass to mass number is close to one except in the case of carbon, where it exactly equals one.

Mass Number and Isotopes

Mass number is very useful in distinguishing the isotopes of an element. Isotopes are atoms of the same element which have the same atomic number but a different mass number. Consider an example of uranium, which has two naturally occurring isotopes: uranium-235 (²³⁵
₉₂U) and uranium-238 (²³⁸
₉₂U). Both the isotopes have the same proton number or atomic number. we can differentiate these two by mass number or atomic mass. In fact, we can separate these isotopes with the help of this difference. In uranium enrichment plants, (²³⁵
₉₂U), which is fissile, is separated from (²³⁸
₉₂U) using centrifugation. Centrifugation is a technique in which the constituents are separated using the difference in their masses.

The Mass Number Of An Atom Is The Number Of

Mass Number in Nuclear Reactions

Nucleons of an atom do not take part in a chemical reaction, only electrons do. Thus, mass number of an element is unaltered in a chemical reaction. But this is not a case with nuclear reactions. In nuclear reactions, mass number may be or may not be altered. Consider the famous uranium fission reaction.

As we can see, uranium accepts a neutron to form barium and krypton. The mass number of the reactant and the products are different. But in overall reaction the mass number is conserved.

Consider one more example of the natural decay of carbon-14. The carbon-14 is a naturally occurring radioactive isotope, which decays by emitting β-rays. The reaction is as follows:

In the above reaction, carbon-14 decays to nitrogen-14 by transmitting an electron and an antineutrino. There is a change in atomic number, but mass number is the same in both carbon and nitrogen.

Examples

Example 1: Mass Number of Hydrogen

Statement: Hydrogen atom contain only one proton and no neutron. What is the mass number of the hydrogen atom?

Solution: Since there is only one proton and no neutron, the number of nucleons is one. Thus, mass number is also one.

Example 2: Mass Number of Carbon

Statement: The atomic number of carbon is 6. For an atom of carbon, neutron number also equals 6. What is mass number of the carbon atom?

Solution: The atomic number (Z) is 6 and neutron number (N) is also 6. The mass number (A) is calculated as:

Example 3: Mass Number of Chlorine

Statement: The atomic number of chlorine is 17. For an atom of chlorine, neutron number equals 18. What is mass number of the chlorine atom?

Solution: The atomic number (Z) is 17 and neutron number (N) is 18. The mass number (A) is calculated as:

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