Namrata Das Exams Prep Master
Exams Prep Master
The difference between the Atomic Mass and Atomic Number is that the former is nothing but the total number of protons and neutrons present in the atom of an element whereas the later is determined just by looking at the number of protons inside the atom. Atomic mass and atomic number are the two fundamental properties of an element which account for various characteristics of elements that distinguish them from one another. An atom is a fundamental unit of any element that can be described as matter. Let’s learn more about the difference between atomic mass and atomic number.
| Table of Contents |
Key Terms: Neutrons, Protons, Electrons, Atoms, Element, Atomic Mass, Atomic Number
What is Atomic Mass?
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Atomic Mass refers to the sum of neutrons and protons present in the nucleus of an atom. We all know that the charge on protons is positive and equal to the charge on a single electron in an atom whereas the neutrons are neutral in nature with no charge on them. Moreover, the rest mass of a single proton in the atom is 1.67262 × 10−27 kg, which is 1,836 times the mass of a single electron in the atom. Atomic mass is closely related to the mass number of an atom. It is represented by 'A' and determined in amu (atomic mass unit). For example, we know that there are 6 protons and 6 neutrons in a single carbon atom. The atomic mass of carbon can hence be determined by taking the sum of neutrons and protons which is equal to 12. Hence, the atomic mass of a single carbon atom is 12 amu. Mathematically,
Mass Number = Number of Protons + Number of Neutrons
Also read: Atomic number and Mass number
What is Atomic Number?
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Atomic number refers to the total number of protons present inside the nucleus of an atom. It is represented by Z. All the atoms of an element have the same number of protons whereas the number of neutrons may vary. This leads to the concept of isotopes which further leads to the various properties of elements that helps in differentiating them from one another on the basis of several physical and chemical characteristics. Isotopes can be described as the family of a particular element with the same number of protons and varying number of neutrons. Hence, the atomic number of an element remains fixed in case of isotopes whereas the atomic mass of an element may vary depending upon the number of neutrons in it. For instance, there are 6 protons in the atom of Carbon and 8 protons in the atom of Oxygen. We can hence conclude that the atomic number of Carbon is 6 whereas the atomic number of Oxygen is 8. Mathematically,
Atomic Number = Total Number of Protons
Also read: Planck's Quantum Theory and Black Body Radiation
Difference between Atomic Mass and Atomic Number
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Tabulated below are the key differences between Atomic Mass and Atomic Number:
| Atomic Mass | Atomic Number |
|---|---|
| Atomic Mass is determined by taking the sum of the number of neutrons and the number of protons in the atom of an element. | Atomic Number is determined just by looking at the total number of protons in the atom of an element. |
| It is represented by A. | It is represented by Z. |
| Atomic Mass varies according to the number of varying neutrons. | Atomic Number does not depend upon the number of neutrons. |
| Atomic Mass of different Isotopes of the same element vary. | Atomic Number of different Isotopes of the same element does not vary. |
| Atomic Mass of Isobars can be the same. | Atomic Number of Isobars cannot be the same. |
| The value of Atomic Mass of an element is always greater in number as compared to the value of atomic number of an element. | Atomic Number is always lesser in value than Atomic Mass. |
| Atomic Mass determines the average weight of an element. | Atomic Number is the total number of protons. |
| Atomic Mass does not help in identifying the element. | Atomic Number helps in identification, characterization and classification of an element. |
| Atomic Mass is represented in terms of amu. | Atomic Number is a unitless quantity. |
| Atomic Mass of two elements can be the same. | Atomic Number of two elements cannot be the same. |
| The atomic mass of hydrogen is 1 amu, helium has an atomic mass of 4 amu, oxygen has an atomic mass of 16 amu and chlorine has an atomic mass of 35.5 amu. | The atomic number of hydrogen is 1, helium has an atomic number 2, oxygen has an atomic number 8 and chlorine has the atomic number 17. |
Things to Remember
- Atomic Mass depends upon the total number of protons and neutrons whereas Atomic Number is independent of the number of neutrons.
- The protons and neutrons of an atom together are called nucleons.
- Atomic Number is not affected by the number of neutrons in the atom.
- Isotopes and Isobars are the result of varying mass number and atomic number of an element.
- Isotopes can be defined as the atoms of the same element with varying mass number and fixed atomic number.
- Isobars can be defined as the atoms of different elements with the same number of protons and protons, i.e, nucleons.
- The number of neutrons in an element can be determined by deducting the atomic number of the element from its mass number, i.e, Number of Neutrons = A-Z, where A represents the mass number and Z represents the atomic number, provided the number of protons in the element are known.
Sample Questions
Ques. Calculate the number of protons, electrons and Neutrons in Phosphorus, Magnesium and Chlorine. (4 marks)
Ans. i) We know that,
Atomic Number of Phosphorus (Z) = 15
Mass Number of Phosphorus (A) = 31
Number of Protons = Atomic Number = 15
Number of Electrons = Atomic Number = 15
Number of Neutrons = A-Z = 31-15 = 16
ii) Atomic Number of Magnesium (Z) = 12
Mass Number of Magnesium (A) = 24
Number of Protons = Atomic Number = 12
Number of Electrons = Atomic Number = 12
Number of Neutrons = A-Z = 24-12 = 12
iii) Atomic Number of Chlorine (Z) = 17
Mass Number of Chlorine (A) = 37
Number of Protons = Atomic Number = 17
Number of Electrons = Atomic Number = 17
Number of Neutrons = A-Z = 37-17 = 20
Ques. : The atomic number of aluminium is 13 and atomic mass is 27u. Calculate the number of protons and neutrons. (2 marks)
Ans. Given, Atomic Number of Aluminium = 13
We know that, Z= Atomic Number = Number of protons
Therefore, number of proton=13
Also, Mass Number of an atom = Number of protons + Number of neutrons
Mass Number = atomic mass = 27 u
27 u = 13 + n Or, n = 27-13 = 14
Thus, Number of proton =13 and number of neutron = 14
Ques. What is valency? Find the valency of Chlorine and Sulphur. (3 marks)
Ans. Valency can be defined as the number of electrons present in the outermost shell of an atom which combine with another atom to form a bond. In other words, valency is the number of electrons required by an element in order to complete its octet and gain a stable electronic configuration.
Calculation of valency of Chlorine and Sulphur:
We know that the electronic configuration of chlorine is 2, 8, 7. Hence, chlorine has a total of 7 electrons in its outermost shell. Also, valency of chlorine = 8 - number of electrons in outermost = 8−7 = 1.
The electronic configuration of sulphur is 2, 8, 6. Hence, sulphur has a total of 6 electrons in its outermost shell. Also, valency of chlorine = 8 - number of electrons in outermost = 8−6 = 2.
Ques. An element has the following natural abundances and isotopic masses: 90.92% abundance with 19.99 amu, 0.26% abundance with 20.99 amu and 8.82% abundance with 21.99 amu. Calculate the average atomic mass of this element. (2 marks)
Ans. Average atomic mass is given by:
(atomic mass × %abundance)1 + (atomic mass × %abundance)2 + (atomic mass × %abundance)3 / 100 = ((19.99 × 90.92) + (20.99 × 0.26) + (21.99 × 8.82)) / 100 = 20.16 amu.
Ques. Calculate the atomic number of the element that has 12 neutrons and a mass number of 23. (1 mark)
Ans. We know that,
Atomic number = Mass number - number of neutrons, i.e, 23 −12=11. Hence, 11 is the atomic number and sodium is the element.
Ques. Calculate the mass number and atomic number of an atom having 6 electrons, 6 protons and 6 neutrons in its atom. (2 marks)
Ans. Given that,
Number of Electrons = 6
Number of Protons = 6
Number of Neutrons = 6
We know that,
Mass Number of an Element = Number of Protons + Number of Neutrons
Substituting the values, we get:
= 6 + 6 = 12
So, Mass Number is 12
Since, Atomic Number = Number of Protons
So, Atomic Number = 6
Ques. Find the element Mass number whose atomic number is 15 and the number of neutrons present is 15. (2 marks)
Ans. Given that,
Atomic Number = 15
Number of Neutrons = 15
Using the formula for Molar Mass which is
Atomic Number = Number of Protons
Mass Number = Number of Protons + Number of Neutrons = 15+15 = 30
Ques. Bromine atoms are available in two isotopes, 35/79 Br (49.7%) and 35/81 Br (50.3%), the average atomic mass of a bromine atom is? (2 marks)
Ans. Percentage of 35/79 Br = 49.7%
Percentage of 35/81 Br = 50.3%
Therefore,Average atomic mass = 49.7 × 79 + 50.3 × 81 / 100 = 3926.3 + 4074.3 / 100 = 8000.6 / 100 = 80.006
Ques. Chlorine has two isotopes of atomic mass units 34.97 and 36.97. The relative abundance of an isotope is 0.755 and 0.245 respectively. Find the average atomic mass of chlorine. (1 mark)
Ans. Average atomic mass = (34.97 × 0.735 + 36.97 × 0.245) / (0.735 + 0.245) = (25.70295 + 9.05765) / 0.98 = 35.47u
Ques. Lithium exists in nature in the form of two isotopes, Li−6 and Li−7 with atomic masses 6.0151u and 7.0160u and the percentage 8.24 and 91.76 respectively.
Calculate average atomic mass. (3 marks)
Ans. Given: The atomic mass of Li6 = 6.0151μ
The atomic mass of Li7 = 7.0160μ
Percentage abundance of Li6 = 8.24% = 0.0824
Percentage abundance of Li7 = 92.70 = 0.9270
Mass of each isotope can be calculated as
Average atomic mass = Percentage abundance of isotope × atomic mass of isotope 1+ Percentage abundance of isotope 2 × atomic mass of isotope 2
Average mass of lithium = 0.0824 × 6.0151μ + 0.9270 × 7.0160μ = 0.4956μ + 6.5038 = 6.9994μ
The average atomic mass of lithium is 6.9994μ
Ques. The O18 / O16 ratio in some meteorites is greater than that used to calculate the average atomic mass of oxygen one earth. The average mass of an atom of oxygen in these meteorites is ______ that of a terrestrial oxygen atom? (4 marks)
Ans. Given, O18/O16 ratio is greater than that which is used for calculation on earth numerically, on earth. Let O16% be x%. Therefore, O18% be (100−x)%
Average mass on earth = 16x +(100−x)18 / 100 = 16x−14x+1800/100 = -2x+1800/100
On meteorites O18/O16 is greater. Let O18 (100−x+2), i.e., 100−cn−2% and O16% be x−2
Average at mass = 16(x−2)18(100−x+2) / 100 = 16x−32+1800−18x+36 /100 = −2x+180/100
Comparing both atomic mass
−2x+1800/100 < −2x+180^4/100
Therefore, Average atomic mass of O on meteorites will be greater.
Ques. What is the value of 1 amu? (1 mark)
Ans. One amu is equal to 1/12 of the mass of one carbon-12 atom. It is equal to 1/NA, ie, 1/6.022×1023 = 1.66×10-24 g.
Ques. The number of atoms of the He in 104 amu is: (1 mark)
Ans. We know that the mass of one atom of He = 4 a.m.u. Therefore the number of atoms of the He in 104 amu is 104/4 =26.
| Class 11 & 12 Physical Chemistry Related Topics | ||
|---|---|---|
| Huckel Rule of Aromaticity | Hydrogen Spectrum | Positron |
| Bomb Calorimeter | What is an Ideal Solution | Ideal Gases Law |
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