As we know that atoms of all elements are extremely small. We cannot see them either by the naked eye or even by the powerful microscope. Absolute (or actual) masses of atoms of elements are very, very small.
Therefore, it is difficult to measure the absolute (i.e. actual) mass of an atom precisely. For example, one atom of the hydrogen element has a mass of 1.673 * 10-24 grams or 0.000000000000000000001673 grams. It is not convenient to use such small and complicated figure in the measurement.
However, it is possible to determine the relative mass of an atom of different elements if a very small unit of mass is taken as a standard. For this purpose, John Dalton chose an atom of hydrogen (being lightest of all atoms) as standard. The relative masses of atoms of all elements were expressed in the terms of the mass of an atom of hydrogen. Scientist, Dalton taken the mass of one hydrogen atom as 1 atomic mass unit.
Thus, Dalton has given the following the definition for the atomic mass of an element.
Definition of Atomic Mass
The atomic mass of an element is a number which indicates how many times the mass of an atom of the element is heavier compared to the mass of an atom of hydrogen.
Atomic mass of an element (A) = Mass of one atom of element / Mass of one atom of hydrogen
In 1858, an atom of oxygen was preferred as a standard because of the following reasons:
(i) As oxygen is a more reactive element than hydrogen, it is much easier to obtain compounds of elements with oxygen as compared with hydrogen.
(ii) By taking the mass of natural oxygen as 16 units, the relative atomic masses of most of the elements become approximately whole numbers, but with hydrogen as standard, the atomic masses of most of the elements are fractional. Thus,
Atomic mass of an element = Mass of one atom of the element / 1/16 th part of the mass of one atom of oxygen
After taking oxygen as a standard, the atomic mass of hydrogen comes as 1.008, sodium 22.991 and sulphur 32.066.
In 1961, the International Union of Chemists chose a new unit for expressing the atomic masses. They have taken the most stable isotope of carbon (C-12 isotope) with a mass number of 12 as the standard for comparison of the atomic masses of various elements. They took the mass of C-12 isotope as 12 atomic mass units.
Definition of Atomic Mass based on Carbon-12
Atomic mass of an element is the number which indicates how many times an atom of the element is heavier than 1/12 th part of one atom of carbon-12.
In other words, the atomic mass of an element tells us as to how many times the mass of one atom of the element is heavier compared to 1/12 th part of the mass of one atom of carbon-12 (12C). Thus,
Atomic mass of an element = Mass of one atom of the element / 1/12 th part of the mass of one atom of carbon-12
Definition of Atomic Mass Unit (amu)
Atomic mass unit is the quantity 1/12th mass of an atom of carbon-12 (12C). It is abbreviated as amu. The actual mass of one atom of carbon-12 is 1.9924 x 10-23g or 1.9924 x 10-26kg.
Thus, 1 amu = 1.9924 x 10-23 / 12 = 1.66 X 10-24 g or 1.66 x 10-27 kg
At present, ‘amu’ has been replaced by ‘u’, known as unified mass.
Atomic mass unit scale is the scale at which the relative atomic masses of different atoms are expressed.
Average Atomic Mass
Scientist found some elements occur in nature as a mixture of several isotopes. An element that possesses the same atomic number but a different mass number is called isotopes. In such cases, the atomic mass of an element is the average of relative masses of different isotopes of the element.
We can calculate the average atomic masses of various elements by multiplying the atomic mass of each isotope by its fractional abundance and adding the values thus obtained. We can determine the fractional abundance by dividing percentage abundance by hundred. Thus,
Average atomic mass = (m * a + n * b) / (m + n)
This is the equation of average isotopic mass. In this equation, a, b are the atomic masses of isotopes in the ratio of m : n.
For example, chlorine has two types of atoms having relative masses 35 u and 37 u. The relative abundance of these isotopes naturally occurs in the ratio of 3 : 1. Thus,
Atomic mass of chlorine = (35 * 3 + 37 * 1) / (3 + 1) = 35.5 amu
Examples based Average Atomic Mass
Example 1:
Boron occurs by nature in two types of isotopes, boron-1O and boron-11 whose percentage abundances are 19.6% and 80.4% respectively. What is the average atomic mass of boron?
Solution:
Contribution of boron-10 = 10.0 x 0.196 = 1.96 amu
Contribution of boron-11 = 11.0 x 0.804 = 8.844 amu
Adding both = 1.96 + 8.844 = 10.804 amu
Thus, average atomic mass of boron = 10.804 amu
Example 2:
Carbon occurs in nature as a mixture of two isotopes, such as carbon-12 and carbon-13. The average atomic mass of carbon is 12.011. What is the percentage abundance of carbon-12 in nature?
Solution:
Let x be the percentage abundance of carbon-12; then (100 – x) will be the percentage abundance of carbon-13.
Average atomic mass = (xa + yb) / 100
Here, x and y are percentage abundance of two isotopes a and b respectively. y = (100 – x). Therefore,
Average atomic mass = 12x / 100 + 13(100 – x) / 100 =
12.011 * 100 =12x + 1300 – 13x
1201.1 = -x + 1300
x = 98.9
Percentage abundance of carbon-12 is 98.9%.
Atomic Mass of First 30 Elements
We have listed the first 30 elements based on the atomic number and their corresponding atomic mass in a given below table.
| Atomic Number | Element | Atomic Mass |
|---|---|---|
| 1 | Hydrogen | 1.008 |
| 2 | Helium | 4.0026 |
| 3 | Lithium | 6.94 |
| 4 | Beryllium | 9.0122 |
| 5 | Boron | 10.81 |
| 6 | Carbon | 12.011 |
| 7 | Nitrogen | 14.007 |
| 8 | Oxygen | 15.999 |
| 9 | Fluorine | 18.998 |
| 10 | Neon | 20.180 |
| 11 | Sodium | 22.990 |
| 12 | Magnesium | 24.305 |
| 13 | Aluminium | 26.982 |
| 14 | Silicon | 28.085 |
| 15 | Phosphorus | 30.974 |
| 16 | Sulfur | 32.06 |
| 17 | Chlorine | 35.45 |
| 18 | Argon | 39.948 |
| 19 | Potassium | 39.098 |
| 20 | Calcium | 40.078 |
| 21 | Scandium | 44.956 |
| 22 | Titanium | 47.867 |
| 23 | Vanadium | 50.942 |
| 24 | Chromium | 51.996 |
| 25 | Manganese | 54.938 |
| 26 | Iron | 55.845 |
| 27 | Cobalt | 58.933 |
| 28 | Nickel | 58.693 |
| 29 | Copper | 63.546 |
| 30 | Zinc | 65.38 |
Gram Atomic Mass or Gram Atom
When we express the numerical value of atomic mass of an element in grams, the value becomes gram-atomic mass or gram atom. For example, the atomic mass of an element hydrogen is 1.008, while the gram atomic mass of hydrogen is 1.008 g. Similarly, the gram atomic masses of carbon, nitrogen, and oxygen are 12.011 g, 14.007 g, and 15.999 g respectively.
Gram atomic mass of every element comprises the same number of atoms. This number is called Avogadro’s number. The value of Avogadro’s number is 6.02 x 1023.
Absolute mass of one carbon atom = 12 amu =12 x 1.66 x 10-24 g
Therefore, the mass of 6.02 x 1023 atoms of carbon = 12 x 1.66 x 10-24 x 6.02 X 1023 = 12 g (gram atomic mass).
Thus, we can define gram atomic mass as the absolute mass in grams of 6.02 x 1023 atoms of any element.
We can calculate the number of gram atomic mass or gram atoms of any element with the help of the following formula:
No. of gram atoms = Mass of the element in grams / Atomic mass of the element in grams
In this chapter, you have known about what is atomic mass of an element, definition of average atomic mass, and atomic mass unit. Hope that you will have understood the basic points of atomic mass.
Thanks for reading!!!
