ATOMIC THEORY TIMELINE

DEMOCRITUS

• Greek philosopher
• First to discover the atom (around 460 BC)
• Belived that atoms were tiny indivisible spheres
• Other philosophers disagreed with this theory

JOHN DALTON

• English chemist
• Created his own Atomic theory in the early 1800s
• Said that all atoms of an element are identical
• Belived that atoms combine in simple, whole number ratios
• Also belived that atoms were tiny indestructible spheres

DALTON (CONTINUED)

• Used the composition of elements to determine relative atom mass
• Based his scale on H = 1 amu
• Now based on C-12 = 12 amu exactly
• Today units of atom mass are referred to as amu or daltons

J.J. THOMSON

• English physicist
• Discovered the electron in 1887
• Said that the mass of an atom is due to the mass of electrons
• Said that there must be a lot of empty space in the atom
• Said that the atom is breakable

J.J. THOMSOM

• Atom's structure has electrons suspended in a positively charged electric field
• Aimed electricity through a cathode to an anode
• Found that small amount of electricity passed through the anode
• Discovered that the beam of electricity could be bent
• Discoveries replaced Dalton's

ERNEST RUTHERFORD

• British physicist known as "father of nuclear physics"
• Discovered the nucleus im 1911
• Shot high energy alpha particles at a thin sheet of gold
• Discovered that 0.01% of the particles he shot at the foil bounched back
• Experiment disproved the plum pudding model

RUTHERFORD (CONTINUED)

• Proposed that there was something else- the neutron
• Nucleus is essentially the entire mass of an atom
• Nucleus is positively charged
• Discovered the proton

NIEL BOHR

• Said that electrons travel in orbitals around nucleus
• Electrons far away from nucleus have more energy
• Each orbit has a specific amount of energy
• Atoms gain energy and move further away
• Atoms lose eneergy and move closer to nucleus

ERWIN SHRODINGER

• Discovered how to find the location of an electron
• Must have a certain energy and location