1.2 Atomic Mass vs. Atomic Weight

Atomic Mass vs. Atomic Weight

Learning Objectives

  • Describe atomic mass and atomic weight.
  • Recall the units of molar mass.
  • Predict the number of protons, neutrons, and electrons in a given isotope.

Key Terminology

  • Atomic Mass: Mass of a specific isotope (varies).
  • Mass Number: synonymous with atomic mass
  • Atomic Weight: Weighted average of isotopes of an element (constant, found on the periodic table).

Atomic Mass

  • The mass of one proton is approximately 1 amu (atomic mass unit).
  • 1 amu is defined as exactly 1/12 the mass of a carbon-12 atom, approximately 1.66×10−241.66 \times 10^{-24} grams.
  • Carbon-12 has 6 protons and 6 neutrons.
  • The difference in mass between protons and neutrons is very small, approximately the mass of an electron.
Key Concepts:
  • Atomic number (Z) = number of protons.
  • Mass number (A) = number of protons + number of neutrons.
  • In a neutral atom: number of protons = number of electrons.
  • Electrons are not included in mass calculations due to their significantly smaller mass.
  • The atomic mass of an atom (in amu) is nearly equal to its mass number.
  • Some mass is lost as binding energy (refer to MCAT Physics and Math Review).
Isotopes
  • Atoms of the same element with different mass numbers are isotopes.
  • Isotopes differ in the number of neutrons.
  • Notation: Element name followed by the mass number (e.g., Carbon-12, Iodine-131).
  • Hydrogen isotopes have unique names:
    • Protium: 1 proton, atomic mass of 1 amu.
    • Deuterium: 1 proton, 1 neutron, atomic mass of 2 amu.
    • Tritium: 1 proton, 2 neutrons, atomic mass of 3 amu.
  • Isotopes have similar chemical properties due to the same number of protons and electrons.

Atomic Weight

  • Most elements exist as multiple isotopes in nature.
  • Atomic weight is the weighted average of these isotopes.
  • It is the number reported on the periodic table.
Example: Chlorine
  • Chlorine-35 and Chlorine-37 are the two main naturally occurring isotopes.
  • Chlorine-35 is about three times more abundant than Chlorine-37.
  • Therefore, the atomic weight of chlorine is closer to 35 than 37 and is listed as 35.5 amu on the periodic table.
Half-Life and Stability
  • Half-life indicates the stability of an isotope.
  • Longer-lasting isotopes are generally more abundant.
Key Concept
  • No single isotope of an element will have a mass exactly equal to its atomic weight.
Example: Bromine
  • Bromine has an atomic weight of 79.9 amu on the periodic table.
  • This is an average of Bromine-79 and Bromine-81, which occur in almost equal proportions.
  • There are no bromine atoms with a mass of exactly 79.9 amu.
Utility of Atomic Weight
  • Represents the mass of the average atom of that element in amu.
  • Represents the mass of one mole of the element in grams.
Mole and Avogadro's Number
  • Mole (n) = 6.02×10236.02 \times 10^{23} (Avogadro's number, NAN_A) of atoms, ions, or molecules.
Example: Carbon
  • The atomic weight of carbon is 12 amu.
  • The average carbon atom has a mass of 12 amu.
  • 6.02×10236.02 \times 10^{23} carbon atoms have a combined mass of 12 grams.
Mnemonic
  • Atomic mass is nearly synonymous with mass number.
  • Atomic weight is the weighted average of naturally occurring isotopes of that element.

Example Problem: Element Q

  • Element Q has three isotopes: A, B, and C.
    • Isotope A: mass = 40 amu, abundance = 60% (0.60)
    • Isotope B: mass = 44 amu, abundance = 25% (0.25)
    • Isotope C: mass = 41 amu, abundance = 15% (0.15)
  • Calculate the atomic weight of element Q.
Solution
  • Atomic weight = (0.60×40)+(0.25×44)+(0.15×41)(0.60 \times 40) + (0.25 \times 44) + (0.15 \times 41).
  • Atomic weight = 24+11+6.15=41.1524 + 11 + 6.15 = 41.15 amu.

MCAT Concept Check 1.2

  1. What are the definitions of atomic mass and atomic weight?
    • Atomic Mass: The mass of a specific isotope.
    • Atomic Weight: The weighted average of the masses of naturally occurring isotopes of an element.
  2. While molar mass is typically written in grams per mole, is the ratio moles per gram also acceptable?
    • Yes, the ratio of moles per gram is acceptable.
  3. Calculate and compare the subatomic particles that make up the following atoms.
    • Isotope | Protons | Neutrons | Electrons
      • 19O | 8 | 11 | 8
      • 16O | 8 | 8 | 8
      • 17O | 8 | 9 | 8
      • 19F | 9 | 10 | 9
      • 16F | 9 | 7 | 9
      • 238U | 92 | 146 | 92
      • 240U | 92 | 148 | 92