THE ATOMIC THEORY

DEMOCRITUS

• Born around the year 470 B.C. and died around 360 B.C.
• He theorized that everything is composed of atoms
• These atoms are indivisible, and between each atom lies empty space

JOHN DALTON

• Born in 1766 and died on 1844
• He agreed that atoms were indivisible and indestructable
• He imagined them as tiny spheres
• He theorized that all atoms of an element were chemically and physically identical
• He also theorized that atoms combine in single, whole number ratios to form compounds

JOHN DALTON

• Dalton was also able to calculate the relative mass of an atom
• He based his scale on Hydrogen equaling 1 amu, or atomic mass unit
• 1 amu is approximately 1.66 × 10^-24 grams

J.J. THOMSON

• Born in 1856 and died in 1940
• Conducted experiments by passing electric currents through gases
• Conducted his experiments in glass tubes with electrodes fitted at both ends
• He then passed te currents through the gases at a low pressure
• One electrode became positively charged, while the other became negatively charged

J.J. THOMSON

• He concluded that atoms are breakable
• He also concluded that an atom was a series of electrons
• These electrons are suspended in a magnetic field
• His model is known as the plum pudding model
• However, a positive charge is needed to balance out the negative charge

GOLDSTEIN AND CHADWICK

• Eugen Goldstein also performed the cathode ray experiment
• He found that the positive charge came from protons, also found in the atom
• James Chadwick discovered that neutrons were also found in an atom
• They had no charge and weighed about the same as a proton

ERNEST RUTHERFORD

• Rutherford wanted to test Thomson's theory
• He passed a beam of particles through gold atoms
• Rutherford found that something in the atoms were deflecting some of the beams
• He concluded that the positive charge was located in a nucleus, found at the center of the atom
• His atomic model is known as the nuclear atom

NIELS BOHR

• Bohr proposed that electrons were found in orbits around the nucleus
• Each orbit has a fixed energy level
• An exact amount ove energy is needed to move from one orbit to the next
• In 1914, he proposed a new model atomic model

ERWIN SCHRODINGER

• Schrödinger found an equation he could use to predict the relative location of an atom
• His equation combined with Bohr's model is known as the quantum mechanical model
• This is the model we use today
• This model, however, cannot determine the exact orbits of electrons
• Instead, it perdicts the probability of an atom being in a certain location