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Atomic Mass is the mass of an atom. SI unit of mass is the kilogram, atomic mass is often expressed in non-SI unit atomic mass unit or unified mass or dalton. Atomic Weight, also called relative atomic mass, ratio of the average mass of a chemical element's atoms to some standard. Atomic theory was first proposed by John Dalton, who claimed that every substance is made up of essentially indivisible atoms, and that all atoms of any substance have the same weight, despite the fact that the atoms of one substance differ from those of another.
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Key Terms: Atomic Mass, Atomic Weight, Atomic Mass Unit, Atomic Mass of Elements, Atomic Mass of Oxygen
Read More: Number of Moles Formula
What is Atomic Mass?
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An atom has three particles: electron, proton, and neutron. Atomic mass is the sum of the masses of these three particles. The proton has a positive charge, the electron has a negative charge, and the neutrons have no charge. Protons and neutrons are the largest and have the most mass of these three particles, and both are present in the nucleus.
Atomic Mass
Total number of neutrons and protons in an atom constitute the atomic mass (m a) . It is the sum of the neutrons and protons of a single atom. Electrons are of very small size and are thus not counted.The unit of Atomic Mass is Dalton and is represented as Da. Da is the standard unit that indicates the mass of an atom.The atomic mass of an element varies depending on the number of neutrons and protons.
What is Atomic Weight ?
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Based on the abundance of isotopes, atomic weight is a weighted average of the mass of all the atoms of an element. The masses of the isotopes need to be considered while calculating the atomic weight of an atom. Atomic weight is also known as relative atomic mass. It is defined as the ratio of the mean mass of the atoms of a chemical element to a certain standard. Atomic weight is measured in units of atomic mass , abbreviated to AMU. It is also known as Daltons.
Read More: Dalton’s atomic theory
Difference Between Atomic Mass and Atomic Weight
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The table below provides the difference between the atomic mass and atomic weight;
| Atomic Mass | Atomic Weight |
|---|---|
| Sum of protons and neutrons that exist in an atom. | Average ratio of all the atoms present in the element |
| In the calculation of Atomic mass, isotopes are not included. | In the calculation of atomic weight, isotopes are included. |
| Final value is a whole number | Final value may or may not be a whole number |
| Analysis is carried out by mass spectroscopy | Analysis may be carried out by mass spectroscopy. |
Things to Remember
- Atomic radius is measured in nanometres.
1/10 9 m = 1 nm 1 m = 109 nm
- Atomic mass of some important elements are:
| Element | Atomic mass |
|---|---|
| Hydrogen | 1 |
| Carbon | 12 |
| Nitrogen | 14 |
| Oxygen | 16 |
| Sodium | 23 |
| Magnesium | 24 |
| sulphur | 32 |
| Chlorine | 35.5 |
| Calcium | 40 |
- Law of conservation of mass- The sum of the masses of the reactants and products remains unchanged during a chemical reaction This is called law of conservation of mass.
- Law of Definite Proportions - In a pure chemical compound, elements are always present in a definite proportion by mass. This is known as the Law of Definite Proportions.
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Sample Questions
Ques.Calculate the mass of 0.5 mole of N2 gas (mass from mole of molecule). (1 Mark)
Ans. Mass = molar mass × number of moles ⇒ = × = × = m M n 28 0 5 14. G
Ques.Calculate the number of moles for the 52 g of He (finding mole from mass). (2 Marks)
Ans. No. of moles = n, Given mass = m ,Molar mass = M, Given number of particles = N, Avogadro number of particles = NA
Atomic mass of He = 4 u
Molar mass of He = 4g
Thus, the number of moles = given mass molar mass ⇒ = = = n m M 52 4 13
Ques.Calculate the relative molecular mass of water (H2O). (2 Marks)
Ans. Atomic mass of hydrogen = 1u, oxygen = 16 u
So the molecular mass of water, which contains two atoms of hydrogen and one atom of oxygen is
= 2 × 1+ 1×16
= 18 u
Ques. Calculate the molecular mass of HNO3. (2 Marks)
Ans. The molecular mass of HNO3
= the atomic mass of H + the atomic mass of N+ 3 × the atomic mass of O
= 1 + 14 + 48 = 63 u
Ques. What is another name for atomic weight? (2 Marks)
Ans. Atomic weight, also called relative atomic mass, is the ratio of the total weight of the atoms of a chemical substance to a norm. The normal unit of atomic mass has been one-twelfth of the atomic mass of the carbon-12 isotope since 1961
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. (3 Marks)
Ans. 90.92% abundance with 19.99 amu
0.26% abundance with 20.99 amu
8.82% abundance with 21.99 amu
Required: Average Atomic Mass
Solution -
(90.92%/100) (19.99 amu) + (0.26%/100) (20.99 amu) + (8.82%/100) (21.99 amu)
= 20.16 amu
Ques.Which of the following represents the least number of molecules?
20.0 g of H2O (18.02 g/mol)
77.0 g of CH4 (16.06 g/mol)
68.0 g of CaH2 (42.09 g/mol)
100.0 g of N2O (44.02 g/mol)
84.0 g of HF (20.01 g/mol) (2 Marks)
Ans. The smallest number of molecules present in 20.0 g of H2O, which has the minimum number of moles. Because 1 mole equals 6.022 *1023 molecules (or atoms), the smallest number of moles equals the smallest number of molecules.
Ques. Which contains the greatest mass of oxygen: 0.75 mol of ethanol (C2H5OH), 0.60 mol of formic acid (HCO2H), or 1.0 mol of water (H2O)? Explain why. (2 Marks)
Ans. Formic acid - It has twice as many oxygen atoms in its formula as the other two compounds (one each). As a result, 0.60 mol of formic acid would be comparable to 1.20 mol of a single-oxygen-atom molecule.
Ques. The 18O : 16O abundance ratio in some meteorites is greater than that used to calculate the average atomic mass of oxygen on earth. Is the average atomic mass of an oxygen atom in these meteorites greater than, less than, or equal to a terrestrial oxygen atom? (1 Mark)
Ans. Greater, since the contribution to the average atomic mass of 18O is greater, that will raise the average atomic mass in meteorites compared to on earth.
Ques. An element X reacts with oxygen to form a compound X2O3. If the atomic mass of X is 91.5 the equivalent mass of X? (2 Marks)
Ans. An element X reacts with oxygen to form a compound X2O3.From this we can calculate the valency of X by criss-cross method.
So, valency of X is 3.
Equivalent mass = Atomic mass/ valency
= 91.5/3
Equivalent mass = 30.5
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