Easy Notes On Atomic Mass, Average Atomic Mass and Mass Defect

CHEMISTRY NOTES: BASIC CONCEPTS

1. Atomic Mass

• Every element has a characteristic atomic mass.
• Atomic mass is the mass of an atom.
• It is determined relative to the mass of a carbon-12 atom as the standard.

2. 1 amu (Atomic Mass Unit)

• Definition: One amu is defined as a mass exactly equal to one-twelfth of the mass of one carbon-12 atom.
• Recently, amu has been replaced by unified mass unit called dalton (symbol 'u' or 'Da').
• Formula / Value:
  • 1 amu = 1/12 × mass of one C-12
  • 1 amu = 1/12 × 1.992648 × 10^-23 g
  • 1 amu = 1.66056 × 10^-24 g

3. Average Atomic Mass

• Definition: Average atomic mass: Average atomic mass is the weighted average mass of all the naturally occuring isotopes of an element. (taking into account the atomic masses of isotopes and their relative abundance i.e., percent occurrence).

A. Calculation for Neon (Ne):

• Neon exists as three isotopes with the following natural abundance:
  • ²⁰Ne = 19.9924 u (90.92%)
  • ²¹Ne = 20.9940 u (0.26%)
  • ²²Ne = 21.9914 u (8.82%)
• Average atomic mass of Neon = [(19.9924 × 90.92) + (20.9940 × 0.26) + (21.9914 × 8.82)] / 100
• Average atomic mass of Neon = 20.1707 u

B. Calculation for Chlorine (Cl):

• The two natural isotopes of chlorine, viz. ³⁵Cl and ³⁷Cl exist in relative abundance of 75:25.
• This Percent abundance of ³⁵Cl is 75% and that of ³⁷Cl is 25% .
• Therefore, Average atomic mass of Chlorine = [(3 × 35) + (1 × 37)] / 4
• Average atomic mass of Chlorine = 35.5 u

4. Mass Defect (Concept & Helium Atom Example)

(Extra concept explained simply for understanding)

• Definition of Mass Defect: The actual mass of an atom is always slightly less than the total sum of the masses of its individual protons, neutrons, and electrons. This difference is called as "Mass Defect". Jevha protons ani neutrons ekatra yeun nucleus banto, tevha thoda mass energy madhe convert hoto.

Helium Atom Example:

Let's calculate the expected mass of a Helium atom (2 protons, 2 neutrons, 2 electrons):

• Mass of 1 proton = 1.00727 u
• Mass of 1 neutron = 1.00867 u
• Mass of 1 electron = 0.00055 u

Expected Total Mass:

• 2 Protons = 2 × 1.00727 = 2.01454 u
• 2 Neutrons = 2 × 1.00867 = 2.01734 u
• 2 Electrons = 2 × 0.00055 = 0.00110 u
• Total Expected Mass = 2.01454 + 2.01734 + 0.00110 = 4.03298 u

Actual Mass & Mass Defect:

• Actual measured mass of a Helium atom = 4.00260 u
• Mass Defect (Δm) = Total Expected Mass - Actual Mass
• Mass Defect = 4.03298 u - 4.00260 u = 0.03038 u

(Simple meaning: 0.03038 u of mass has "disappeared" because it was converted into binding energy to hold the nucleus together.)