Isotopes, Atomic Mass, and Atomic Structure: Study Notes for General Chemistry

Isotopes and Atomic Mass

Atomic Symbol and Mass Number

The atomic symbol provides essential information about an element, including its mass number and atomic number.

• Atomic Symbol: Shows the mass number in the upper left corner and the atomic number in the lower corner.

• Mass Number (A): The total number of protons and neutrons in the nucleus.

• Atomic Number (Z): The number of protons in the nucleus, which defines the element.

Isotopes

Isotopes are atoms of the same element that have different mass numbers due to varying numbers of neutrons.

• Isotopes have the same number of protons but different numbers of neutrons.

• Isotopes are often represented with the element symbol and mass number (e.g., 13C, 14C).

• Isotopic abundance refers to the relative amount of each isotope in a naturally occurring sample.

Calculating Atomic Mass

The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes.

• Atomic mass is calculated using the formula:

$\text{Atomic mass} = \sum (\text{fractional abundance} \times \text{isotope mass})$

• Example: For chlorine, with two isotopes 35Cl and 37Cl:

$\text{Atomic mass of Cl} = (0.7576 \times 34.97) + (0.2424 \times 36.97) = 35.45~\text{amu}$

Example Table: Weighted Average Calculation

Item	Weight (lb)	Percentage Abundance	Weight from Each Type
14-lb bowling ball	14	20	11.2 lb
8-lb bowling ball	8	80	1.6 lb
Weighted average mass	12.8 lb

Additional info: This analogy helps illustrate how atomic mass is a weighted average based on isotope abundance.

Table: Atomic Mass of Chlorine Isotopes

Atomic Symbol	Mass (amu)	Percentage Abundance	Contribution to the Atomic Mass
35Cl	34.97	75.76%	26.49 amu
37Cl	36.97	24.24%	8.957 amu
Weighted average mass of Cl			35.45 amu

Atomic Structure and Electron Configuration

Atomic Spectra and Photons

• When light from the sun or a lightbulb passes through a prism or raindrops, a continuous spectrum is produced.

• Heated atoms of an element emit light of specific wavelengths, producing a line spectrum.

• The light emitted consists of photons, which are packets of energy.

• The energy of a photon is directly proportional to its frequency:

$E = h\nu$

• Where $E$ is energy, $h$ is Planck's constant, and $\nu$ is frequency.

Electron Energy Levels

• Each electron in an atom has a specific energy level, described by quantum numbers ($n$, $l$, $m_l$, $m_s$).

• Electrons in lower energy levels are closer to the nucleus.

• When electrons change energy levels, they absorb or emit energy as photons.

Sublevels and Orbitals

• Each energy level contains one or more sublevels, labeled s, p, d, and f.

• Each sublevel contains a specific number of orbitals:

Sublevel	Number of Orbitals	Maximum Electrons
s	1	2
p	3	6
d	5	10
f	7	14

• Order of increasing energy in sublevels: $s < p < d < f$

Electron Configurations and Orbital Diagrams

• Electron configurations show the arrangement of electrons in an atom.

• Orbital diagrams use boxes and arrows to represent orbitals and electrons, respectively.

• Hund's rule: Electrons fill orbitals singly before pairing up.

Valence Electrons and Periodic Trends

Valence Electrons

• Valence electrons are the electrons in the outermost energy level (highest n value).

• They determine the chemical properties and reactivity of an element.

• Group trends:

  • Group 1: 1 valence electron

  • Group 2: 2 valence electrons

  • Group 13: 3 valence electrons

  • Group 14: 4 valence electrons

  • Group 15: 5 valence electrons

  • Group 16: 6 valence electrons

  • Group 17: 7 valence electrons

  • Group 18: 8 valence electrons (except He, which has 2)

Metals, Nonmetals, and Noble Gases

• Metals: 1-3 valence electrons, lower ionization energies.

• Nonmetals: 5-7 valence electrons, higher ionization energies.

• Noble gases: Complete octets (except He), very stable.

Summary Table: Naturally Occurring Isotopes and Atomic Masses

Element	Naturally Occurring Isotopes	Atomic Mass (weighted average)
Lithium	6Li, 7Li	6.941 amu
Carbon	12C, 13C, 14C	12.011 amu
Oxygen	16O, 17O, 18O	16.00 amu
Fluorine	19F	19.00 amu
Sulfur	32S, 33S, 34S, 36S	32.10 amu
Manganese	55Mn	54.94 amu
Copper	63Cu, 65Cu	63.55 amu

Key Points to Remember

• Atomic mass is a weighted average based on isotope abundance.

• Electron configurations and energy levels determine chemical properties.

• Valence electrons are crucial for understanding reactivity and periodic trends.

• Know how to calculate atomic mass and interpret isotope notation.