Democritus proposed that atoms are minuscule particles that make up everything. Dalton proposed that atoms are the smallest units that cannot be created or destroyed, and that different elements are made of different atoms. Rutherford discovered that an atom's mass is concentrated in a nucleus rather than being evenly distributed.

1. Timeline of the Atom
Sarah Pomerantz and Jessica Nekritz

2. 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.

3. 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

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

11. Orbitals and Energy Levels

12. What was Chadwick’s Theory?
Chadwick proved the existence of neutrons,
uncharged particles in the nucleus of an atom.