Chemistry Notes: Atomic Masses and Chemical Formulas

Dinitrogen Trioxide

  • Chemical formula: N₂O₃

Xenon Difluoride

  • Chemical formula: XeF₂

Dioxygen Difluoride

  • Chemical formula: O₂F₂

Sulfur Hexafluoride

  • Chemical formula: SF₆

Dinitrogen Monoxide

  • Chemical formula: N₂O

Dinitrogen Tetroxide

  • Chemical formula: N₂O₄

Dinitrogen Pentoxide

  • Chemical formula: N₂O₅

Chemical Formulas

  • N₂O₅: Dinitrogen pentoxide

  • B₄C₃

  • PCl₅: Phosphorus pentachloride

  • O₂Cl₂

  • N₂S₃

  • XeF₄: Xenon tetrafluoride

  • IF₃: Iodine trifluoride

  • XeO₃: Xenon trioxide

  • S₂F₁₀

Masses of Atoms and Molecules

Learning Objective

  • Express the masses of atoms and molecules.

Key Concepts

  • Matter has mass and takes up space, thus atoms and molecules have mass.

  • Atoms and molecules are very small, so their masses are also very small.

  • Grams and kilograms are too big to measure the mass of atoms and molecules.

  • The atomic mass unit (u) is used to express the mass of atoms and molecules.

Atomic Mass Unit (u)

  • Defined as one-twelfth of the mass of a carbon-12 atom.

  • Carbon-12 has six protons and six neutrons in its nucleus.

  • Proton mass: 1.00728 u1.00728 \text{ u}

  • Neutron mass: 1.00866 u1.00866 \text{ u}

  • Electron mass: 0.000549 u0.000549 \text{ u}

  • The mass of an atom can be estimated by counting the total number of protons and neutrons in the nucleus (mass number) and ignoring the electrons.- Mass of carbon-12 ≈ 12 u12 \text{ u}

    • Mass of oxygen-16 ≈ 16 u16 \text{ u}

    • Mass of uranium-238 ≈ 238 u238 \text{ u}

  • The exact mass of uranium-238 is 238.050788 u238.050788 \text{ u}.

Atomic Mass

  • Most elements exists as a mixture of isotopes, each with its own mass.

  • The atomic mass of an element is a weighted average of the masses of its isotopes.

Weighted Average

  • To calculate the weighted average, multiply each isotope's mass by its fractional occurrence (in decimal form) and then add all the products.

Example

  • Element consists of two isotopes: 50% with mass 10 u10 \text{ u}, and 50% with mass 11 u11 \text{ u}.

  • Calculation:- 0.50×10 u=5.0 u0.50 \times 10 \text{ u} = 5.0 \text{ u}

    • 0.50×11 u=5.5 u0.50 \times 11 \text{ u} = 5.5 \text{ u}

    • Sum = 10.5 u10.5 \text{ u} = atomic mass of the element

  • No atom in the hypothetical element has a mass of 10.5 u10.5 \text{ u}; it is the average mass of the atoms.

Boron Example

  • Boron exists as about 20% boron-10 (five protons and five neutrons) and about 80% boron-11 (five protons and six neutrons).

  • Calculation:- 0.20×10 u=2.0 u0.20 \times 10 \text{ u} = 2.0 \text{ u}

    • 0.80×11 u=8.8 u0.80 \times 11 \text{ u} = 8.8 \text{ u}

    • Sum = 10.8 u10.8 \text{ u} = atomic mass of boron

  • Atomic mass of boron is 10.8 u10.8 \text{ u}.

General Information

  • Atomic masses may vary significantly from whole numbers because all elements exist as mixtures of isotopes.

  • The number of isotopes tend to increase with atomic size.

  • Tin has the record of 10 isotopes.

Selected Atomic Masses of Some Elements

  • Aluminum: 26.981 u26.981 \text{ u}

  • Argon: 39.948 u39.948 \text{ u}

  • Arsenic: 74.922 u74.922 \text{ u}

  • Barium: 137.327 u137.327 \text{ u}

  • Beryllium: 9.012 u9.012 \text{ u}

  • Bismuth: 208.980 u208.980 \text{ u}

  • Boron: 10.811 u10.811 \text{ u}

  • Bromine: 79.904 u79.904 \text{ u}

  • Calcium: 40.078 u40.078 \text{ u}

  • Carbon: 12.011 u12.011 \text{ u}

  • Chlorine: 35.453 u35.453 \text{ u}

  • Cobalt: 58.933 u58.933 \text{ u}

  • Copper: 63.546 u63.546 \text{ u}

  • Fluorine: 18.998 u18.998 \text{ u}

  • Gallium: 69.723 u69.723 \text{ u}

  • Germanium: 72.64 u72.64 \text{ u}

  • Gold: 196.967 u196.967 \text{ u}

  • Helium: 4.003 u4.003 \text{ u}

  • Hydrogen: 1.008 u1.008 \text{ u}

  • Iodine: 126.904 u126.904 \text{ u}

  • Iridium: 192.217 u192.217 \text{ u}

  • Iron: 55.845 u55.845 \text{ u}

  • Krypton: 83.798 u83.798 \text{ u}

  • Lead: 207.2 u207.2 \text{ u}

  • Lithium: 6.941 u6.941 \text{ u}

  • Molybdenum: 95.94 u95.94 \text{ u}

  • Neon: 20.180 u20.180 \text{ u}

  • Nickel: 58.693 u58.693 \text{ u}

  • Nitrogen: 14.007 u14.007 \text{ u}

  • Oxygen: 15.999 u15.999 \text{ u}

  • Palladium: 106.42 u106.42 \text{ u}

  • Phosphorus: 30.974 u30.974 \text{ u}

  • Platinum: 195.084 u195.084 \text{ u}

  • Potassium: 39.098 u39.098 \text{ u}

  • Radium: n/a

  • Radon: n/a

  • Rubidium: 85.468 u85.468 \text{ u}

  • Scandium: 44.956 u44.956 \text{ u}

  • Selenium: 78.96 u78.96 \text{ u}

  • Silicon: 28.086 u28.086 \text{ u}

  • Silver: 107.868 u107.868 \text{ u}

  • Sodium: 22.990 u22.990 \text{ u}

  • Strontium: 87.62 u87.62 \text{ u}

  • Sulfur: 32.065 u32.065 \text{ u}

  • Tantalum: 180.948 u180.948 \text{ u}

  • Tin: 118.710 u118.710 \text{ u}

  • Titanium: 47.867 u47.867 \text{ u}

  • Tungsten: $$183