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Atomic Mass Unit

The scale in which the relative atomic masses of different atoms are expressed is called the atomic mass unit scale. Let's see what atomic mass means. An atom is made up of neutrons, protons and electrons and the sum of the mass of these constituent particles of an element is called its atomic mass. Thus,
Atomic mass of an element = Mass of neutrons + Mass of protons + Mass of electrons.
Mass of an electron is almost nil (0.0005449), the atomic mass of an element is taken as:
Atomic mass of an element = Mass of neutrons + Mass of protons
Atomic mass number is defined as the total sum of the number of protons and neutrons present in the nucleus of an atom. Each proton and neutron carries a unit mass on the atomic weight scale. The atomic mass of an element is approximately equal to its mass number. The mass number is designated as ‘A’. So, from the mass number and atomic number Z, the number of neutrons present in a nucleus can be calculated.

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Atomic Mass Unit Definition

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The fundamental unit of mass is defined as$\frac{1}{12th}$ the mass of the 12C atom. The mass defect is the difference between the measured atomic mass of an atom and the mass if the atom were considered as an ensemble of Z hydrogen atoms and N neutrons.

Masses of fundamental atomic particles are often expressed in atomic mass units (u). One atomic mass unit 1u is one twelfth of the mass of an atom of carbon with six protons and six neutrons.

Relative Atomic Mass

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The atomic mass of an element is a relative quantity and is the mass of one atom of the element relative to or compared to $\frac{1}{12}$ the mass of one carbon-12 atom. Thus,
Relative atomic mass = $\frac{Mass\ of\ 1\ atom\ of\ the\ element}{\frac{1}{12} \times\ mass\ of\ one\ C\ - 12\ atom}$

$\frac{1}{12}$ the mass of one C - 12 atom = 1a.m.u, 1 a.m.u = 1.66 x 10 -24 g


Atomic Mass Number

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All atoms of a given element will have the same number of protons in the nucleus, Hydrogen being the simplest element with one nuclear proton. Helium atoms have two protons, all lithium atoms have three protons and all beryllium atoms have four protons. The number of protons in the nucleus of an element is given by its atomic number, which is generally indicated by the symbol Z.
  • It represents a specific element.
  • It represents a specific number of nucleons in an element.

Gram Atomic Mass

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The atomic mass of an element expressed in grams is called the gram atomic mass of the element. It is also termed as gram atom.
Gram atom = $\frac{Mass\ of\ the\ element\ in\ grams}{Atomic\ mass\ of\ the\ element}$Example: The gram atoms present in 46 grams of sodium

= $\frac{46}{atomic\ mass\ of\ Na}$

= $\frac{46}{23}$ = 2

Finding Atomic Mass

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Calculating atomic mass

To find the atomic mass of any element, the number of protons and number of neutrons is added.

Example

Lithium has 3 electrons, 3 protons and 4 neutrons.
Atomic mass = Number of protons + number of neutrons
= 3 + 4 = 7

Use the below widget to find the atomic mass of an element.

Calculating Average Atomic Mass

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Atoms of the same element having a different neutron number leading to a different mass number or atomic mass are better known as Isotopes of the element involved.

For example : Carbon has two forms, one with atomic mass of 12 and another with atomic mass 13. Both have the same atomic number: 6
The sum of the masses of these constituent particles in a given isotope is known as its atomic mass.
Atomic mass of an isotope = Mass of neutrons + mass of electrons.
The atomic mass of an element is the average of the atomic masses of all its isotopes. The value of atomic weight depends upon the relative proportions of the various isotopes. The actual atomic weight of neon which consists of:

Ne – 20, Ne – 21 and Ne – 22
in percentage is - Ne -20 = 90.92% , Ne – 21 = 0.26% and Ne – 22 = 8.82 %.

So, if the percentage of the isotopes is known, the average atomic mass of an element can be calculated.

To calculate the average atomic mass of an isotope, the percentage of all isotopes are taken, and divided by the sum of masses of both isotopes.

Example: Calculate the atomic mass of chlorine from the following data:
Cl : 37 = 24.6 % , Cl : 35 = 75.4 %.

Average atomic mass of chlorine = $\frac{ 75.4 \times 35 + 24.6 \times 37 }{ 75.4 + 24.6}$ = 35.492.

Thus, the average atomic mass of chlorine from this data is 35.492 amu.
Electron : The first discovered subatomic particle was a negative particle. It has a unit negative charge (-1) and it is present outside the nucleus in the three dimensional spaces, called as orbitals.

Protons : These are positive particles. They have a unit positive charge (+1). Protons are present inside the nucleus of an atom.

It is understood that in an atom, there will be equal number of positive and negative charges. So, there will be equal number of protons and neutrons.

Neutrons : These are neutral particles present inside the nucleus with the protons. Neutrons with protons provide mass to an atom. So, these three subatomic particles make up the atom of any element.
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Atomic Mass Scales

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A single atom is, of course, a very small thing and it has very little weight, but it has been possible in modern times to calculate the actual weight of a single atom of any element. A single hydrogen atom, for example, weighs: 0,000,000,000,000,000, 000, 000, 0017 grams.

The weight of an atom is the sums of weights of all the particles present in it. As the weight of an atom is very small, the weight is always quoted relative to a fixed standard. Thus,
“The atomic mass of an element is the mass of one atom of the element compared with the weight of one atom of a standard element”.

The Atomic mass scales used to calculate the relative atomic mass are as follows:
  1. Hydrogen mass scale: Hydrogen is selected as the standard element. The atomic masses of other elements are compared with Hydrogen atom's mass.
  2. Oxygen atomic scale: Oxygen used to be taken as the standard and was in vogue before the discovery of isotopes. In this scale the atomic mass of oxygen (16) is compared with the rest of the elements. Based on this scale, hydrogen was found to have an atomic mass of 1.008.
  3. Carbon atomic mass scale: Due to the difficulties of finding the whole numbers for a lot of elements, carbon – 12 was selected as the standard element. Carbon is also selected due to its abundance in nature. According to the carbon scale, atomic mass unit, amu, is defined as :
“One twelfth (1/12) of the mass of a single atom of C-12 isotope of carbon."

Atomic Mass Table

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Atomic Mass of Different Table has given Below:-

Elements
Symbol Atomic number
Number of electrons or protons Mass number
Neutrons
(A – Z)
Hydrogen H 1 1 1 NIL
Lithium Li 3 3 7 7 - 3 =4
Boron B 5 511 11 - 5 = 6
Silicon Si 14 14 28 28 -14 = 14
Sulphur S 16 16 32 32 -16 = 16

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