Study Notes

Atoms are made of three subatomic particles:

• Proton: located in the NUCLEUS, charge = +1, mass ≈ 1 atomic mass unit (amu)

• Neutron: located in the NUCLEUS, charge = 0 (neutral), mass ≈ 1 amu

• Electron: located OUTSIDE the nucleus (in electron cloud/orbitals), charge = −1, mass ≈ 0 (negligible, ~1/1836 of proton)

The nucleus is very small but contains almost all the atom's mass. The electron cloud is much larger in volume but contributes almost no mass.

Atomic Number (Z): the number of PROTONS in the nucleus. This defines what element an atom is. Every atom of an element has the same atomic number.

Mass Number (A): the number of PROTONS + NEUTRONS in the nucleus.

Number of Neutrons = Mass Number − Atomic Number

The model of the atom has evolved through history as new experiments revealed new information:

1. Solid Sphere Model (Dalton, ~1803): Atoms are tiny, indivisible solid spheres. Different elements = different spheres.

2. Plum Pudding Model (Thomson, 1904): After discovering electrons, Thomson proposed atoms are a "pudding" of positive charge with electrons embedded like plums. Cathode ray experiments showed electrons exist and have negative charge.

3. Nuclear Model (Rutherford, 1911): Gold Foil Experiment showed most of the atom is empty space, with a tiny, dense, positively charged NUCLEUS at the center. Some particles bounced back — unexpected.

4. Planetary / Bohr Model (Bohr, 1913): Electrons orbit the nucleus in fixed circular energy levels (shells), like planets around the sun. Only specific orbits are allowed.

5. Quantum / Electron Cloud Model (modern): Electrons don't have fixed paths — they exist in probability regions called orbitals. We can only say where electrons are LIKELY to be.

Isotopes are atoms of the same element with the same number of protons but DIFFERENT numbers of neutrons.

Example: Carbon-12 and Carbon-14 are both carbon (6 protons), but C-12 has 6 neutrons and C-14 has 8 neutrons.

Isotope notations (all equivalent):

• Carbon-12 → C-12 → ¹²C (where 12 is the mass number)

• Written as: symbol-mass number, or mass number superscript before the symbol

Average atomic mass: because most elements have multiple naturally occurring isotopes, the periodic table shows a weighted average based on natural abundance.

Formula: Average atomic mass = Σ (mass of each isotope × relative abundance as decimal)

Example: Chlorine has two isotopes:

Cl-35 (34.97 amu, 75.77% abundant) and Cl-37 (36.97 amu, 24.23% abundant)

Average = (34.97 × 0.7577) + (36.97 × 0.2423) = 26.50 + 8.96 = 35.45 amu

The mole is a counting unit for chemistry, just as "dozen" means 12.

Avogadro's Number: 1 mole = 6.022 × 10²³ particles (atoms, molecules, ions, etc.)

Molar mass: the mass of one mole of a substance, equal to the atomic/molecular mass in grams.

• 1 mole of carbon (atomic mass 12.01 amu) = 12.01 g

• 1 mole of H₂O (molecular mass = 2×1.01 + 16.00 = 18.02) = 18.02 g

Conversions using dimensional analysis:

• Grams → Moles: divide by molar mass (g/mol)

• Moles → Grams: multiply by molar mass

• Moles → Particles: multiply by 6.022 × 10²³

• Particles → Moles: divide by 6.022 × 10²³

Example: How many grams is 2.5 mol of NaCl?

Molar mass of NaCl = 22.99 + 35.45 = 58.44 g/mol

2.5 mol × (58.44 g / 1 mol) = 146.1 g

Always show conversion factors as fractions with units!

Scientific Notation: a way to write very large or very small numbers.

Format: M × 10ⁿ where 1 ≤ M < 10

• 6,022,000,000,000,000,000,000,000 = 6.022 × 10²³

• 0.000000001 = 1 × 10⁻⁹

• Moving decimal left → positive exponent; right → negative exponent

Significant Figures (sig figs): the digits in a measurement that are meaningful/reliable.

Rules for counting sig figs:

• All nonzero digits are significant: 1234 → 4 sig figs

• Zeros BETWEEN nonzero digits are significant: 1002 → 4 sig figs

• Leading zeros are NOT significant: 0.0045 → 2 sig figs

• Trailing zeros with a decimal point ARE significant: 2.500 → 4 sig figs

• Trailing zeros WITHOUT a decimal are ambiguous: 1200 → unclear (2, 3, or 4?)

Note: You will NOT be asked to track sig figs through calculations — only to count how many sig figs a number has as written.