Quantifying Atoms and Compounds

Significant Figures

  • Reflect the precision of measurements.

  • More digits indicate a more precise measurement.

  • Used to maintain reliability in calculations and prevent overstating accuracy.

Counting Significant Figures

  • All non-zero numbers are significant.

  • Leading zeros are not significant.

  • Zeros between non-zero numbers are significant.

  • Trailing zeros are significant.

Significant Figures: Addition and Subtraction

  • Look at the digits to the right of the decimal point ONLY.

  • Round the answer to the fewest number of significant digits to the right of the decimal.

Significant Figures: Multiplication and Division

  • Express the answer with the fewest total significant figures from the inputs.

Atomic Number and Mass Number

  • Atomic number (Z) = number of protons.

  • Mass number (A) = sum of protons and neutrons.

  • Element representation: ZAE^{A}_{Z}E

Relative Mass

  • Relative mass is proportional to Carbon-12, which has exactly 12 unified atomic mass units (u).

Relative Isotopic Mass

  • Isotopes: atoms of the same element with different numbers of neutrons and mass numbers.

  • Relative Isotopic Mass: mass of an isotope relative to the mass of Carbon-12 (12 u).

  • Relative isotopic mass=mass of an atom of the isotopemass of an atom carbon−12X12Relative\,isotopic\,mass = \frac{mass\,of\,an\,atom\,of\,the\,isotope}{mass\,of\,an\,atom\,carbon - 12} X 12

Mass Spectrometry

  • Used to measure atoms by:

    1. Ionization.

    2. Acceleration.

    3. Deflection (separates isotopes by mass).

    4. Detection (records mass and amount of each isotope).

Mass Spectrometry Data

  • Mass spectrum: graph showing number of isotopes and their relative isotopic mass (x-axis) and abundance (y-axis).

Mass Number vs Relative Isotopic Mass

  • Mass Number:

    • Always a whole number.

    • protons+neutronsprotons + neutrons.

  • Relative Isotopic Mass:

    • Rarely whole numbers.

    • Relative mass compared to carbon-12.

Relative Atomic Mass

  • Relative Atomic Mass (Ar): mass of naturally occurring isotopes of an element on the relative atomic mass scale.

  • Relative Isotopic Abundance: percentage of an isotope in the natural environment, used for calculating Ar.

  • Ar=(RIM of first isotope×% abundance)+(RIM of second isotope×% abundance)+…100A_r = \frac{(RIM\,of\,first\,isotope \times \%\,abundance) + (RIM\,of\,second\,isotope \times \%\,abundance) + …}{100}

Relative Molecular/Formula Mass

  • Relative Molecular Mass (Mr): sum of relative atomic masses of atoms in a molecule (covalent compounds).

  • Relative Formula Mass (Mr): sum of relative atomic masses of atoms in the formula of an ionic compound.

The Mole

  • Mole: amount of substance containing the same number of particles as atoms in 12g of Carbon-12.

  • Symbol: n, Unit: mol

Specifying Moles

  • Indicate the particle when referring to moles of a substance (e.g., 1 mole of water molecules).

Avogadro’s Constant

  • NA=6.02×1023mol−1N_A = 6.02 \times 10^{23} mol^{-1}

Using Avogadro's Constant

  • Three quantities:

    • Mole (n, mol).

    • Avogadro’s constant (NAN_A, 6.02×10236.02 \times 10^{23}).

    • Actual number of particles (N).

Law of Conservation of Mass

  • Mass cannot be created or destroyed; used to calculate the number of particles.

Molar Mass

  • Molar Mass (M): mass of one mole of a substance.

  • Symbol: M, Unit: g/mol

  • n=mMn = \frac{m}{M}

Compounds and Formulas

  • Compounds: substances with two or more elements chemically bonded in a fixed ratio.

Percentage Composition

  • Percentage Composition: proportion of the masses of elements within the total mass of the compound.

Types of Formulas

  • Empirical Formula: simplest whole number ratio of elements.

  • Molecular Formula: actual number and type of atoms in a molecule.

  • Condensed Formula: represents the structural formula of a compound.

  • Structural Formula: 3D arrangement of atoms showing all bonds.

  • Skeletal Formula: shorthand omitting carbon and hydrogen atoms (organic chemicals).

Determining Empirical Formula:

  1. Obtain the mass (m) of each element in a compound – use 100g of the compound if percentage is given

  2. Calculate the amount in mol, of each element present

  3. Convert the mole of each element calculated in the previous step to a whole number ratio

  4. Write the empirical formula

Determining Molecular Formula:

  • Scaling factor = molar mass of the compoundmolar mass of one empirical formula unit\frac{molar\,mass\,of\,the\,compound}{molar\,mass\,of\,one\,empirical\,formula\,unit}