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ChemistryEdu Logo Mole Concept | Introduction#

Basic things to remember:

  • Mass of electron = 9.1 x 10-31 Kg
  • Mass of proton = 1.672 x 10-27 Kg
  • Mass of neutron = 1.674 x 10-27 Kg
  • Charge of electron = -1.6 x 10-19 Kg
  • Charge of proton = 1.6 x 10-19 Kg
  • Charge of neutron = 0

Atomic Number and Mass Number#

Any atom can be represented by:

\[X_p^m\]

Here, p = Atomic number and m = Mass number

  • Atomic number represents the number of protons in an atom. If atomic number of two atoms are same, then they represent the same atom although they may be isotopes. For example, \(C_6^{12}\) and \(C_6^{14}\) both represent carbons and are isotopes of carbon.

  • Mass number represents the number of neutrons and protons (together called as nucleons) in an atom. Nucleons are present in the nucleus of an atom.

\[Mass\ number = Number\ of\ protons + Number\ of\ neutrons\]

  • For a neutral atom, number of protons is equal to number of electrons. For example, in \(Ca_{20}^{40}\) number of electrons = number of protons = 40-20 = 20.

  • Mass number and atomic number are dimensionless quantities since they represent numbers. So, atomic number and mass number have no unit.

  • Only nucleons account for the mass of atom. Due to very small value, the mass of electron is neglected while calculating mass of atom. In other words, mass of electron is negligible as compared to mass of proton or neutron.

\[ ∴ Mass\ of\ Cl = Mass\ of\ Cl^-\]
  • Mass of protons plus the mass of neutrons of an atom is always more than the atomic mass due to binding energy. Binding energy account for the stability of nucleus.

If Z = atomic number, mp = mass of proton, mn = mass of neutron, A = mass number, M = Mass of atom, then:

\[Zm_p + (A-Z) m_n = M + B.E \]

where, B.E = Binding energy of atom

Mass of one atom#

  • To measure mass of an atom, a unit has been defined, which is known as atomic mass unit or a.m.u.
\[1\ a.m.u. = {1 \over 12} \times Mass\ of\ one\ C^{12}\ atom\]
  • We know that:
\[Mass\ of\ 6.022\times 10^{23}\ C^{12}\ atoms = 12\ g\]
\[Mass\ of\ 1\ C^{12}\ atoms = {12\over 6.022\times 10^{23}} g\]
\[Mass\ of\ 1\ C^{12}\ atoms = 1.99 \times 10^{23} g\]
  • Calculating atomic mass unit:
\[1\ a.m.u. = {1 \over 12} \times Mass\ of one\ C^{12} atom\]
\[1\ a.m.u. = {1 \over 12} \times 1.99 \times 10^{-23} g\]
\[1\ a.m.u = 1.67 \times 10^{-24} g\]
  • Atomic mass unit (a.m.u or u) is the standard unit relative to which mass of every atom can be measured. Thus, mass number is the relative number which says how many times an atom is heavier than \({1 \over 12} \times Mass\ of\ one\ C^{12} atom\).
  • Atomic mass unit (a.m.u or u) is used to measure mass of atom or masses comparable to mass of atoms.

Remember:

\[1\ a.m.u. = {1 \over 12} \times Mass\ of one\ C^{12} atom\]
\[1\ a.m.u. = {1 \over 6.022 \times 10^{23}} g = {1 \over {N_A}} g\]
\[1\ a.m.u = 1.67 \times 10^{-24} g\]

Here, NA = Avogadro Number = 6.022 x 1023

Question#

If mass of protons and neutrons are known, why do we not write: Mass of one C12 atom = 6 x Mass of proton + 6 x Mass of neutron?

It is because the sum of masses of all nucleons is greater than the actual mass of atom due to binding energy.