Atomic Weight and Mass Number: Mass Spectrometer and amu

Atomic Weight

The atomic weight of an element is a number that shows how many times the weight of one atom of that element is 1/12 heavier than that of an atom of carbon-12. Therefore, it is clear from the above definition that –

From the above definition it is clear that:-

1. Atomic weight is a number and has no units.

2. Atomic weight of carbon-12 = 12

3. The mass of carbon is 12. Hence, the atomic mass and carbon mass of carbon-12 are the same. Similarly, it can be demonstrated that the atomic weights of other elements are also approximately equal to their mass numbers. Therefore

Atomic weight = Mass Number

Isotopes have different mass numbers. Hence their atomic weights are also different. The atomic weights of isotopes are also called Isotopic Masses.

Hydrogen has three isotopes ₁H¹ ₁H² and ₁H³ known. An atom of ₁H¹ is 1.007825 times heavier than 1/12 of an atom of carbon-12, hence the atomic weight of ₁H¹ is 1.007825.

Similarly, atomic weight of ₁H² is 2.0140 and atomic weight of ₁H³ is 3.01605. In other words the isotopic masses of ₁H¹ ₁H² and ₁H³ are 1.007825 2.0140 and 3.01605 respectively.

In the above definition, the use of the term ‘average’ shows that all atoms of any element have the same mass.

So far 112 elements have been discovered. Isotopes of all elements are known. Different isotopes of a single element have different mass and their atomic weights.

Therefore, to find the atomic weight of an element, the average weight of all the atoms of that element must be known.

Example:

The atomic functions of nitrogen-14 and nitrogen-15 found in nature are in the ratio of 99.63 and 0.37%.

The atomic weight of nitrogen-14 is 14.00307, which means that one atom of nitrogen-14 is 1/12 to 14.00307 times heavier than one atom of carbon-12.

Similarly, the atomic weight of nitrogen-15 is 15.00011 which means that one atom of nitrogen-15 is 15.00011 times heavier than 1/12 of one atom of carbon-12.

Atomic Weight of nitrogen-14 = 14.00307

Atomic Weight of nitrogen-15 = 15.00011

Carbon found in nature, the ratio of carbon isotopes carbon-12 and carbon-13 is 98.89% and 1.11%.

According to the definition of atomic weight, the atomic weight of carbon-12 is 12. The weight of one atom of carbon-13 is 13.00335 times that of 1/12 of an atomic weight of carbon-12.

Atomic Weight of Carbon-12 = 12

Atomic Weight of Carbon-13 = 13.00335

To find the atomic weight of an element, the average weight of 1 atom of that element is compared to 1/12 of the weight of one atom of carbon-12. It should be noted that it is comparable to the isotopic mass of 12 mass numbers of carbon (Carbon-12) atom and not natural carbon atom (mixture of Carbon-12 and carbon-13).

In this definition, it is assumed that the atomic weight of carbon-12 is 12. In the old definition of atomic weight it was compared to an atom of hydrogen.

According to that definition, the atomic weight of hydrogen was assumed to be 1.

By that definition, the atomic weights of many elements differed significantly from the whole number. According to the current and modified definitions, the atomic mass of elements is relatively close to absolute numbers.

Hence, the definition of atomic mass in which the average weight of one atom of an element was compared to the weight of one atom of hydrogen has been changed and according to the current modified definition, the weight of one atom of carbon-12 is 1/12. The atomic weight of hydrogen is 1.008 according to the current modified definition.

Concentration of the Ore

According to the definition of atomic weight, in order to find the atomic weight of an element, the ratio of the weight of one atom of that element to that of an atom of carbon-12 must be found.

This ratio is determined with the help of mass spectrometer. The mass spectrometer was first built by Aston 1919. The brief description of mass Spectrometer is as follows.

The element whose atomic weight has to be determined is taken in gaseous state. In the gaseous state, that element flows through a higher electric field.

The electrons undergo ionization and their cations are obtained by hitting the atoms of the gas. These cations pass through such an electric field that their speed is accelerated, after which they pass through a magnetic field.

Under the influence of the magnetic field, there is a deflection in their path. The lighter the cation, the greater the deflection in its path. After the deflection, the hypotenuse collides with a photographic plate.

On the photographic plate, based on the signs obtained from different curtains, their therapeutic ratio can be determined. On the basis of the sizes of the signs, the natural abundance of the institutes of an element can also be found.

Example

Bellow results are obtained by analysing a sample of natural neon with a modern mass spectrometer.

Gram-atomic Weight

If the atomic mass of an element is expressed in grams, it is called the gram-atomic mass of that element.

Example

The atomic weight of nitrogen is 14.00676, so 14.00676 grams is the gram-atomic weight of nitrogen.

Atomic Mass unit(amu)

The unit of mass is gram. It is not convenient to dissolve the actual mass of small particles like electron protons, neutrons, atoms, molecules and ions into grams. “Gram” is a large unit for these small particles.

So in 1961, a suitable set up to show the actual mass of these particles was established. This unit is called atomic mass unit(amu).

An atom of carbon-12 has an actual mass of 12 amu.

On this basis, the actual mass in the amu of an atom of any element is equal to its atomic weight. Similarly, the actual mass in the amu of 1 molecule of any element or additive is equal to its molecular weight.