Historical Development of Atomic Theory

• Ancient Greek philosophers believed all matter was made of four elements (air, earth, fire, water), but Democritus proposed matter is composed of indivisible particles called 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 always contain the same elements in the same proportions by mass.

• John Dalton's atomic theory (1808) stated: all matter consists of solid, indivisible atoms; atoms retain identity in reactions; atoms of an element are identical; different elements have different atoms; compounds are formed from elements in small whole-number ratios.

Structure of the Atom

• Atoms are made of three subatomic particles: protons (positive charge), neutrons (neutral), and electrons (negative charge).

• Most of the atom's mass is in the tiny, dense nucleus (protons and neutrons); electrons occupy the surrounding space.

• The overall charge of an atom is given by: Q=p−e where p = number of protons, e = number of electrons.

Defining Elements and Isotopes

• Each atom has an atomic number (Z, number of protons) and a mass number (A, total number of protons and neutrons).

• An element is defined by its atomic number; changing the number of protons changes the element.

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

Applications and Importance of Isotopes

• Isotope ratios are used in fields like 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: A=∑in(fi×mi) where fi is the fractional abundance and mi is the mass of each isotope.