CHEM 110 - Chapter 2 - Early Ideas & Evolution of Atomic Theory

Dalton’s Atomic Theory

1. matter is comprised of tiny particles called atoms

2. the atoms of a given element are identical

3. the atoms of different elements differ from each other

4. chemical compounds are formed when atoms of different elements combine with each other

• a given compound always has the same relative numbers and types of atoms

5. chemical reactions involve reorganization of the atoms

• atoms themselves are not changed in a chemical reaction

• atoms are neither created nor destroyed during a chemical reaction

Law of Conservation of Mass

matter cannot be created nor destroyed

Law of Definite Proportion

compounds always contains the same proportion of elements by mass

J.J. Thomson

studied electrical discharges in cathode-ray tubes

• concluded that the particles were negatively charged components of atoms

• determined the charge to mass ratio of electrons

Robert Milikan

determined the charge and mass of electrons

Radioactivity

in the late 1800s, scientists found that some elements produce high-energy radiation

types of radioactive emission:

• gamma rays - high-energy “light”

• beta particles - high speed electrons

• alpha particles - possess a 2+ charge

Ernest Rutherford

rutherford performed experiments to test thomson’s plum pudding model

conclusions:

• the volume occupied by an atom is largely empty space

• atoms have a small highly concentrated center with a positive charge (the nucleus)

Atoms

atoms are the smallest particle of an element which have the properties of that element

Molecules

consist of two or more atoms joined by forces called chemical bonds

Composition of the Atom

subatomic particles

• protons in the nucleus (+1 charge)

• neutrons in the nucleus (uncharged)

• electrons outside the nucleus (-1 charge)

protons and electrons have opposite charges with equal magnitude

protons and neutrons have virtually the same mass

Isotopes

atoms with the same number of protons and electrons, but different numbers of neutrons

Ions

all atoms are electrically neutral

• same number of protons and electrons

atoms can develop a charge by gaining or losing electrons

ions are charged atoms