Atomic Theory Chapter 4 History Democritus 400 BC

Atomic Theory Chapter 4

History

Democritus • 400 BC • Greek philosopher • Thought all matter was made up of atoms (atomos), which are the basic, indivisible particles of matter

Aristotle • Believed all matter was continuous-did not believe in atoms • His opinion was accepted for nearly 2000 years

Late 1700’s Scientists agreed that 1. Most natural materials are mixtures of pure substances. 2. Pure substances are elements or compounds. 3. Law of constant composition-a compound always has the same composition.

John Dalton • English schoolteacher who tied all three laws together in his atomic theory • Dalton turned Democritus’s idea into a scientific theory that could be tested by experimentation.

Dalton’s Atomic Theory 1. Elements are made of tiny particles called atoms. 2. All atoms of a given element are identical. 3. The atoms of an element are different from those of any other element. 4. Atoms of one element combine with others to form compounds. 5. Atoms are indivisible in chemical processes. A chemical reaction changes the way the atoms are grouped together.

Joseph John Thomson • Cathode-ray experiment: showed that the atoms of any element can be made to emit tiny negative particles • Determined the charge ratio of electrons

William Thomson • Plum pudding model-a bunch of positive stuff with the electrons scattered throughout.

Rutherford, Geiger, Marsden-nucleus • Gold foil experiment, which led to the discovery of the nucleus. • Like bullets through a tissue

Lead block Uranium Florescent Screen Gold Foil

What he expected

Because, he thought the mass was evenly distributed in the atom.

What he got

Atomic Structure

Protons • The number of protons in an atom determines the element’s identity • Nuclear forces hold the nuclear particles together • The atomic number equals the number of protons

Electrons • Are very small. • If the nucleus is a grape, the electrons would be about one mile away. • Have a negative charge • The arrangements of electrons determines the element’s chemical properties.

Neutrons • Mass number= protons+neutrons • Neutrons=mass number-atomic number • Isotope-atoms that have the same number of protons and electrons but different numbers of neutrons (disproves point 2 of Dalton’s theory) • Nuclide-any isotope of any element

Table 2. 1 The Mass and Charge of the Electron, Proton, and Neutron

Practice. Give the protons, neutrons, and electrons for each • Mercury • Sodium • Carbon • 13 C 6

Answers • Mercury 80 p, 80 e, 121 n • Sodium 11 p, 11 e, 12 n • Carbon 6 p, 6 e, 6 n • 13 C 6 p, 6 e, 7 n 6

Ions • An ion is formed when we remove or add an electron to a neutral atom. • Cation-a positive ion • Anion-a negative ion

Ion Example Regular sodium has 11 electrons, 11 protons, and 12 neutrons. If we take away 1 electron, it would have 10 electrons (-), and 11 protons (+) so the charge would be +1. (Neutrons would stay the same).