3.1: History/Origins of Atomic Theories

Question 1

Who was John Dalton? What did he propose?

Answer 1

• Billard Ball Model
• proposed that all matter is made up of indivisible atoms, each of which was a fundamental, unchangeable unit
• when elements combine, they do it in ratios of small whole numbers; supports the idea of atoms combining in fixed ratios to form compounds
• he assigned relative weight to atoms (known as atomic mass) & stated that atoms of different elements have different weights

Question 2

Who was JJ Thomson? What did he discover?

Answer 2

• Plum Pudding Model
• discovered the electron by use of cathode rays which he determined were streams of negatively charged particles
• created the plum pudding model in which matter contained evenly distributed positive charges
• he called the negatively charged particles “corpuscles” (aka electrons)

Question 3

Who was Ernst Rutherford? What did he discover?

Answer 3

• Planetary model
• conducted a study on alpha particles being aimed at thin gold sheets (alpha particles were positively charged)
• most of them passed through without changing course at all, some of them bounced of as if they hit something solid
• he knew positive charges repel positive charges
• concluded that an atom had a small, dense, positively charged center that repelled positively charged “bullets”
• all of an atom’s positively charged particles were contained in the nucleus & the negatively charged particles were scattered outside around the atom’s edge

Question 4

Who was Niels Bohr? What did he discover?

Answer 4

• Bohr Atomic Model
• looked at the emission spectrum of atoms & theorized that the orbits were energy levels
• placed each electron in a specific energy level
• according to Bohr’s atomic model, electrons move in definite orbits around the nucleus
• these orbits, or energy levels, are located at certain distances from the nucleus

Question 5

Who was Schrodinger? What did he discover?

Answer 5

• Quantam Mechanical Model
• combined equation of the behaviour of waves with the de Broglie equation to generate a model for the atom that accurately calculated energy levels of electrons
• his model and equation suggested that electrons behave as both particles and waves
• proposed a wave equation which defines the shape and energy level of the orbitals