Posted In: Chemistry

The Atomic number of an element is defined as the number of protons found in the nucleus of an atom of a chemical substance. It is the charge number of the nucleus as neutrons carry no net electric charge.The atomic number is also known as the proton number.

Atomic mass number
The atomic mass number A is the total number of protons and neutrons in an atom. This can be determined from the periodic table of elements. You need to take the atomic weight and round off to the nearest whole number. This is because protons and neutron are very close to 1 atomic mass unit and electrons are very close to 0 atomic mass unit. It should be noted that the atomic mass number is a decimal number.

Periodic tables have the full name of each of the elements and their chemical symbol along with the atomic number. The atomic number is located at the top left or top right of the element. The periodic table is arranged in order of the atomic number.

Average atomic mass
The average atomic mass of an element is defined as the sum of the masses of its individual isotopes, each multiplied by its natural abundance. Isotopic mass is the atomic mass. Abundance is determined as the number of atoms of one isotope of an element divided by the total number of atoms in a mixture of the isotopes.

The average atomic mass gives the relationship between mass and number of atoms in a typical sample of the element. As atomic mass units are very small measures, it is common to weigh samples in grams. In order to do this conversion you have to multiply the average atomic mass by 1 g / mol (the molar mass constant) to get an answer in g / mol instead.

Calculating average atomic mass
The average atomic mass of the element provides the average mass per atom in a typical sample of that element. Two isotopes of the same element differ in the number of neutrons per atom and hence have different atomic mass. You can calculate the average atomic mass of an element by following the below steps:

Step 1: First step is to refer to the mass of each isotope. The steps to determine atomic mass are given above. You will notice that isotopes that have more neutrons will have more mass.

Step 2: Now that you know the atomic mass of each isotope, the next step is to determine the abundance of each isotope. Abundance is a measure of how common the isotope is and it can be expressed as a percentage of all atoms of the element. In an element, the abundance of all isotopes must add up to 100%.

Step 3: The next step is to find the weighted average of the individual atomic masses. The average atomic mass of an element with n isotopes is calculated as

(Mass Isotope 1 * Abundance Isotope 1) + (Mass Isotope 2 * Abundance Isotope 2) + (Mass Isotope 3 * Abundance Isotope 3) + … + (Mass Isotope n * Abundance Isotope n)

Example for silver average atomic mass can be calculated as:
Average atomic mass Ag = (massAg-107 * abundanceAg-107) + (massAg-109 * abundanceAg-109)
=(106.9050 * 0.5186) + (108.9047 * 0.4814)
= 55.4410 + 52.4267
= 107.8677 amu.

In this way you can determine the average atomic mass number.


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