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Atoms and Molecules
Atomic Mass
Mass of atom is called atomic mass. Since, atoms are very small consequently actual mass of an atom is very small. For example the actual mass of one atom of hydrogen is equal to 1.673 x 10-24 g. This is equal to 0.000000000000000000000001673 gram. To deal with such small nuber is very difficult. Thus for convenience relative atomic mass is used.
Carbon-12 is considered as unit to calculate atomic mass. Carbon-12 is an isotope of carbon. The relative mass of all atoms are found with respect to C-12.
One atomic mass = 1/12 of the mass of one atom of C-12.
Thus atomic mass is the relative atomic mass of an atom with respect to 1/12th of the mass of carbon-12 atom. ‘amu’ is the abbreviation of Atomic mass unit, but now it is denoted just by ‘u’.
The atomic mass of hydrogen atom = 1u.
This means one hydrogen atom is 1 times heavier than 1/12th of the carbon atom.
The atomic mass of oxygen is 16u, this means one atom of oxygen is 16 times heavier than 1/12th of carbon atom.
Absolute mass or Actual atomic mass:
It is found that, the actual atomic mass of a carbon-12 atom is equal to 1.9926 X 10-23g.
Thus by multiplying the relative atomic mass with 1.6605 × 10-24 g we can get the absolute or actual mass of an atom.
Example -1 - Find the absolute mass oxygen.
Example – 2 – Find the absolute mass of Sodium.
Example – 3 – Calculate the absolute mass of hydrogen atom.
Example – 4 - Calculate the absolute or actual mass of Nitrogen atom.
FAQs on Atomic Mass - Atoms & Molecules, Class 9 Science
| 1. What is atomic mass and how is it calculated? |  |
Ans. Atomic mass refers to the mass of an atom, which is determined by the number of protons and neutrons present in its nucleus. It is calculated by summing the masses of all the protons and neutrons in the atom. The mass of an atom is measured in atomic mass units (amu) or unified atomic mass units (u), where 1 amu is approximately equal to the mass of a proton or neutron.
| 2. What is the difference between atomic mass and atomic number? | |
Ans. The atomic mass of an atom is the sum of the masses of its protons and neutrons, whereas the atomic number refers to the number of protons present in the nucleus of an atom. Atomic mass is a measure of the total mass of an atom, while atomic number is a unique identifier for each element. In the periodic table, atomic number is represented by the symbol 'Z' and atomic mass is represented by the symbol 'A'.
| 3. Why is atomic mass usually a decimal number? | |
Ans. Atomic mass is usually a decimal number because it takes into account the isotopic abundance of different isotopes of an element. Isotopes are atoms of the same element that have different numbers of neutrons, resulting in different masses. Since the atomic mass is an average of the masses of all naturally occurring isotopes, the presence of different isotopes and their relative abundances leads to a decimal value for atomic mass.
| 4. How is atomic mass useful in determining chemical formulas? | |
Ans. Atomic mass is useful in determining chemical formulas as it provides information about the relative masses of different elements in a compound. By knowing the atomic mass of each element in a compound, one can calculate the mass ratio of the elements and determine the empirical formula. This helps in understanding the composition of the compound and predicting its chemical behavior.
| 5. Can atomic mass be changed? | |
Ans. Atomic mass cannot be changed under normal circumstances. The atomic mass of an element is determined by the number of protons and neutrons in its nucleus, which cannot be altered without changing the identity of the element. However, in certain nuclear reactions, such as radioactive decay or nuclear fission/fusion, the atomic mass can change due to the loss or gain of nucleons (protons and neutrons) by the atom.
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