- Believed universe made of invisible units called atoms in 400 BC

- He named them Atoms

- Took 2000 years to be proved right!

- in his theory, All elements are composed of tiny indivisible particles called atoms

- Atoms of the same element are identical. Atoms of any one element are different from those of any other element.

- Atoms of different elements combine in simple

whole-number ratios to form chemical compounds

- In chemical reactions, atoms are combined, separated, or rearranged – but never changed into atoms of another element.

Atoms of the same element can have different numbers of neutrons. Thus, different mass numbers. These are called?

Parts of an Atom

- proposed by John Dalton in 1804

- this theory proposed that matter was composed of small, spherical particles

- but evidence was later gathered that matter was composed of even smaller bits

During the ______ evidence was discovered regarding charges:

atoms have positive (Rutherford’s contribution) and negative (Thomson’s contribution) parts

as a result, revisions to Dalton’s model had to be made

using available data on the atom, _______________came up with the idea of having charges embedded with Dalton’s Billiard Balls

- Also used cathode ray experiment to discover the existence of the electron

In 1897, J.J. Thomson used a _______________ to deduce the

presence of a negatively charged particle: the electron

Mass of an electron:

In 1916, ____________ determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge

- discovered a huge flaw in the previous concept of the atom during his now famous gold foil experiment

- Discovered the Nucleus and the Positive Protons

- Surmised atoms are made of mostly empty space

- Didn’t know about the Neutrons

- Particles shot through thin sheet of gold

- Most shots went straight through

- A small amount were deflected

The atoms must be made of mostly empty space with a small dense nucleus

- Discovered that electrons exist in several distinct layers or levels

- “Jimmy Neutron Model”

- Travel around nucleus like planets travel around sun

- Electrons Orbit

- Electrons can jump between levels with energy being added/released

- Found that Electrons live in fuzzy regions or “clouds” not distinct orbits

- Improved on Bohr’s findings

- Electron location can not be predicted

- Quantum Mechanical Model

Father of the Modern Periodic Table

- the current understanding of the atom is based on Quantum Mechanics

- this model sees the electrons not as individual particles, but as behaving like a cloud - the electron can be “anywhere” in a certain energy level

- Remember back to CPE with electrons behaving like bees in a beehive

- Niels Bohr proposed that electrons revolve around the central positive nucleus (like planets in the solar system)

- Bohr also suggested that the electrons can only revolve in certain orbits, or at certain energy levels (ie, the energy levels are quantized)

- If previous models were correct alpha particles would have passed straight through the gold

- Rutherford found that most (99%) of the alpha particles that he shot at the gold went straight through

- From these experiments Rutherford concluded that the atom had a dense positive core, with the rest composed of mostly empty space with the occasional negatively charged electron

The ___________ of an atom is the number of protons in its nucleus

The ____________ of an atom is the sum of the numbers of protons and neutrons in its nucleus