CHEM 121

2. 3

Learning Objectives

• Write and interpret symbols for atomic number, mass number, and charge of atoms/ions.

• Define atomic mass unit (u) and average atomic mass.

• Calculate average atomic mass and isotopic abundance.

Structure of Atoms

• Atoms consist of a nucleus (protons, neutrons) and electrons.

• Nucleus: small, dense, contains most of the mass.

• Diameter: atom ~ $10^{-10} m$, nucleus ~ $10^{-15} m$.

• Protons have a mass of $1.0073 ext{ amu}$, neutrons $1.0087 ext{ amu}$, electrons $0.00055 ext{ amu}$.

Atomic Number and Mass Number

• Atomic Number (Z): number of protons = number of electrons in neutral atoms.

• Mass Number (A): total of protons and neutrons, $A = Z + ext{(number of neutrons)}$.

• Charge of ion: $ext{Atomic charge} = ext{protons} - ext{electrons}$.

Ions and Isotopes

• Ions: charged atoms (anions: gain electrons, cations: lose electrons).

• Isotopes: same element, different number of neutrons.

• Example: Iodine isotopes with different neutron content represented as $^{127} ext{I}$.

Chemical Symbols

• Chemical symbols: one or two letters representing elements (e.g., C for carbon, Mg for magnesium).

• Standard format ensures clarity and avoids confusion.

Average Atomic Mass

• Average atomic mass is weighted based on isotopic abundance.

• Formula: $ext{average mass} = \sum (\text{fractional abundance} \times \text{isotopic mass})$.

• Example Calculation: Approximate average mass for boron is $10.81 ext{ amu}$ using isotope masses and abundances.

Mass Spectrometry

• Mass spectrometry (MS): used to analyze isotopes and determine their abundances through mass-charge ratios.

• Process: samples are vaporized, ionized, and their paths are analyzed in a detector.

Applications of Isotopes

• Isotopes like Oxygen-18 are important for climate studies and metabolic research.

• Mass spectrometry aids in various scientific fields for substance identification and analysis.