Historical Development of Atomic Theory

• Ancient Greek philosophers believed matter was made of four elements; Democritus proposed the existence of indivisible atoms.

• Antoine Lavoisier established the law of conservation of mass: mass is neither created nor destroyed in chemical reactions.

• Joseph Proust demonstrated the law of definite proportions: compounds have constant composition.

• John Dalton's atomic theory (1808) stated:

  • All matter consists of solid, indivisible atoms.

  • Atoms retain identity in chemical reactions and are indestructible.

  • Atoms of the same element are identical; different elements have different atoms with distinct masses.

  • Compounds are formed from elements in small whole-number ratios.

Structure of the Atom

• Atoms are composed of three subatomic particles:

  • Protons: positive charge, located in the nucleus.

  • Neutrons: neutral, located in the nucleus.

  • Electrons: negative charge, orbit the nucleus.

• Most of the atom's mass is concentrated in a tiny nucleus; most of the atom's volume is empty space.

• The overall charge of an atom is determined by the difference between the number of protons and electrons: Q=p−e where p is the number of protons and e is the number of electrons.

Defining Elements and Isotopes

• Each atom is characterized by:

  • Atomic number (Z): number of protons.

  • Mass number (A): total number of protons and neutrons (nucleons): A=p+n

• Changing the number of protons changes the element.

• Atoms of the same element with different numbers of neutrons are called isotopes.

Applications and Importance of Isotopes

• Isotope ratios are used in fields such as biology, geology, paleontology, and archaeology for tracing and dating samples.

• Forensic applications include using 14C in tooth enamel to estimate year of birth based on atmospheric nuclear testing history.

Measuring Isotopes: Mass Spectrometry

• Mass spectrometry separates isotopes based on mass, producing a spectrum that shows the proportion of each isotope in a sample.

• This technique allows determination of the isotopic composition and calculation of average atomic mass.

Average Atomic Mass

• Most elements exist as mixtures of isotopes; the atomic mass on the periodic table is a weighted average of all naturally occurring isotopes.

• The average atomic mass is calculated as: M=∑in(fi×mi) where fi is the fractional abundance and mi is the mass of isotope i.