Timeline of the AtomSarah Pomerantz and Jessica Nekritz

What was Democritus’s theory?

Democritus’s theory was that there are minuscule things called atomos present in everything in the atmosphere, liquid or solid. Atoms cannot be split. Atoms have no specific size and can vary depending on factors.

What was Aristotle’s theory?

Aristotle’s theory proposed that there are only four

elements: earth, fire, air and water. Those elements

smallest unit, atoms, made up everything on earth

What was Dalton’s Theory?

Dalton’s theory proposed multiple ideas. He said that

all matter was created with atoms, which cannot be

made or destroyed. All atoms of the same element

are identical, for example all water atoms are the

same, but different elements are made of different

atoms. When atoms are rearranged, chemical

reactions occur. Compounds are made of atoms of

the constitute elements.

What was JJ Thomson’s theory?

JJ Thomson discovered the electron, the negatively

charged particle in an atom. He proposed that an

atom is a sphere that is positively charged.

Embedded within that sphere are the negatively

charged electrons. Thomson and his assistant Francis

Aston also discovered isotopes. Isotopes are atoms

with different atomic weights.

What was Rutherford’s Theory?

Rutherford created the gold foil experiment to confirm

Thomson’s plum pudding model. However the

experiment proved that an atoms weight is not

consistent throughout the whole thing but rather

contained mainly in the nucleus. Based off of this

discovery he developed the planetary model of the

atom, with protons placed in the center of the atom

with electrons rotating around the nucleus. This model

has electrons rotating around the nucleus like planets

rotating around the sun.

What was Bohr’s Theory?

Bohr proposed that electrons had a fixed orbit around

the nucleus. They move with a fixed size and energy.

The energy depends on the size of the orbit and the

smaller the orbit the lesser the energy. The atom is

completely stable in the state of the smallest orbit.

That lowest state is called the ground state. Electrons

can jump from orbit to orbit by emitting or absorbing

energy.

What was de Broglie’s Theory?

De Broglie suggested that rather than being in set

orbits, electrons rotate around the nucleus in waves.

He said that electrons could act as both particles and

waves. An electron confined in an orbit around the

nucleus has a specific wavelength, energy and

frequency.

What was Heisenberg’s Theory?

Heisenberg created the uncertainty principle. This

stated that if someone wants to see an electron in

orbit, one must shine a wavelength of light on it that is

smaller then the electrons wavelength. The small

wavelength of light has a very high energy which the

electron will absorb. That energy will change the

electrons position. He said that we can never know

the momentum and the position of an electron at the

same time.

What Was Shrodinger’s Theory?

Shrodinger’s theory stated that electrons were in orbitals that were distributed throughout electron clouds. An orbital was defined by “the space that surrounds the nucleus in which two electrons can randomly move.” He stated that electrons cannot be given a direct position withing the atom and instead can be given probable regions. He also described four types of orbitals: the s, p, d, and f-orbitals.

Orbitals and Energy Levels

What was Chadwick’s Theory?

Chadwick proved the existence of neutrons, uncharged particles in the nucleus of an atom.