Historical Development of Atomic Theory

• Ancient Greek philosophers believed 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 constant composition (definite proportions): compounds have consistent elemental ratios.

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

  • All matter consists of solid, indivisible atoms.

  • Atoms are indestructible and retain identity in chemical reactions.

  • Atoms of the same element are identical; atoms of different elements differ in mass and properties.

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

Structure of the Atom

• Atoms are made of three subatomic particles:

  • Protons (positive charge, in nucleus)

  • Neutrons (neutral, in nucleus)

  • Electrons (negative charge, outside nucleus)

• Most atomic volume is empty space; the nucleus is extremely small but contains most of the atom's mass.

• Atomic charge is determined by the difference between the number of protons and electrons: Charge=#protons-#electrons

Defining Elements and Isotopes

• Each atom has an atomic number (Z) equal to the number of protons, and a mass number (A) equal to the total number of protons and neutrons (nucleons):

  • A=#protons+#neutrons

  • Z=#protons

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

• Isotopes are atoms of the same element (same Z) with different mass numbers (A), due to varying numbers of neutrons.

Applications and Importance of Isotopes

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

• Forensic science uses 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: Average\ Atomic\ Mass=∑i(fractional\ abundance×isotope\ mass)

• Example: Silicon has three isotopes with different abundances and masses; the average atomic mass is calculated using their respective values.