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Atomic mass of an element
(1) The mass of an atom is related to the number of protons, electrons, and neutrons it has. Atom of an element is extremely small and therefore it is not easy to weigh it. Even that, it is possible to determine the mass of one atom relative to another experimentally. For this, it is necessary to assign a value to the mass of one atom of a given element so that it can be taken as a standard value. An atom of carbon isotope (called carbon-12) was chosed for this purpose. Carbon-12 has six protons and six neutrons and has been assigned a mass of exactly 12 atomic mass unit (amu now known as u).
Thus one atomic mass unit (amu or u) is defined as a mass exactly equal to one twelfth (1/12th) of the mass of one carbon-12 atom.
Mass of one atom of Carbon-12 = 12 u or 12 amu
or 1 u = mass of one Carbon-12 atom / 12
Mass of every other element is determined relative to this mass. For e.g. –
(a) atomic mass of nitrogen is 14 u i.e. one atom of nitrogen is 14 times heavier than 1/12 of a carbon-12 atom.
(b) Hydrogen atom is only 0.0840 times heavier than C-12 atoms.That means, on carbon-12 scale, atomic mass of hydrogen = 0.0840 × 12.00 u = 1.008 u.
(c) An oxygen atom is, on the average, 1.3333 times heavier than C-12 atom. Therefore
atomic mass of oxygen = 1.3333 × 12.00 u = 16.0 u
(2) Atomic mass of a few elements on C-12 scale is provided in Table.
Atomic Number – Symbol – Name – Atomic mass (u)
1 H Hydrogen 1.008
2 He Helium 4
6 C Carbon 12.01
7 N Nitrogen 14
8 O Oxygen 16
11 Na Sodium 23
12 Mg Magnesium 24
13 Al Aluminium 27
15 P Phosphorus 31
16 S Sulfur 32
17 Cl Chlorine 35.5
20 Ca Calcium 40
26 Fe Iron 56
29 Cu Copper 63.5
It can be analyzed from the table that atomic mass is not a whole number. For example, atomic mass of carbon is not 12 u but 12.01 u. This is because most naturally occurring elements (including carbon) have more than one isotope. Therefore when we determine atomic mass of an element we generally measure or calculate average mass of the naturally occurring mixture of isotopes. For example – Chlorine contains two natural isotope Cl-35 and Cl-37 in the ratio of 3:1.
average atomic mass of chlorine = 35 x 3 + 37 x 1 /4
= 35.5 u
Thus, ‘atomic mass’ of an element means average atomic mass of that element.
Quiz – Isotopes and Atomic mass
(3) The actual or absolute mass of one atomic mass unit is
1.66 x 10-24 gram or 1.66 x 10-27 Kg.
1 u = 1.66 x 10-24 gram or 1.66 x 10-27 Kg
Example – The actual or absolute mass of one carbon atom is 12 u or
12 x 1.66 x 10-24 gram or 1.99 x 10-23 gram.
- Do you know how much atoms really weigh, in pounds or grams?
One gram is about 600,000,000,000,000,000,000,000 amu (that’s 600 sextillion, or a 6 followed by 23 zeros). A pound is just shy of 300 septillion amu–that is, 300,000,000,000,000,000,000,000,000.
(26 zeros there.) but we don’t normally use those units for measuring the mass of an atom.
- Atomic mass is sometimes referred to as atomic weight. Although mass and weight are not the same thing, the terms “atomic mass” and “atomic weight” do have the same meaning.
Particle Symbol Charge amu grams
electron e- -1 5.45 x 10-4 9.07 x 10-28
proton p +1 1.00 1.67 x 10-24
neutron n 0 1.00 1.67 x 10-24
An electron is so small (~0.05% of a proton) that its mass is ignored in normal chemical calculations.
Read next –
Molecules and Elements