Development of the Atomic Theory

Democritus 460 BC - Greek philosopher proposes the existence of the atom ; calls it Atomos meaning indivisible. His theory: all atoms are small hard particles made of a single material formed into different shapes and sizes always moving, and that they form different materials by joining together

Democritus Democritus model was a small sphere that was invisible

John Dalton British chemist; elements combined in specific proportions to form compounds. His theory:   all substances are made of atoms that cannot be created, divided, or destroyed   atoms join with other atoms to make new substances   atoms of the same element are exactly alike, and atoms of different elements are different in mass and size (elements)

John Dalton Dalton’s model was a solid sphere that was invisible

Edward Frankland English chemist developed the valence theory His theory:   every atom has a fixed number of bonds (chemical links) that it can form   for the atom to be stable, all of these bonds must be used.

J.J. Thomson English chemist and physicist discovered 1 st subatomic particles His theory: negatively charged particles called electrons and positively charged matter created a model to describe the atom as a sphere filled with positive matter with negative particles mixed in Referred to model as the plum pudding model

J.J. Thomson Thomson’s model was a sphere filled with positive matter and negative particles.

Ernest Rutherford New Zealand physicist discovered the nucleus His theory:   small, dense, positively charged particle present in nucleus called a proton   electrons travel around the nucleus, but their exact places cannot be described

Ernest Rutherford Rutherford’s model had a nucleus of protons surrounded by electrons.

Niels Bohr Danish physicist discovered energy levels His theory:   electrons travel around the nucleus in definite paths and fixed distances; orbits   electrons can jump from one level to a path in another level

Niels Bohr Bohr’s model had a nucleus surrounded by electrons orbits.

Erwin Shrodinger Austrian physicist developed the electron cloud model His theory:   electrons exact path cannot be predicted   regions, referred to as the electron cloud, are areas where electrons can likely be found.

Erwin Shrodinger Shrodinger's model had a nucleus surrounded by an electrons cloud.

James Chadwick English physicist discovered neutrons English physicist discovered neutrons His theory: His theory:  neutrons have no electrical charge  neutrons have a mass nearly equal to the mass of a proton  unit of measurement for subatomic particles is the atomic mass unit (amu)

James Chadwick Chadwick's model had a complete nucleus with protons and neutrons.

Modern Theory of the Atom Atoms are composed of three main subatomic particles: the electron, the proton, and the neutron. Most of the mass of the atom is concentrated in the nucleus of the atom. The protons and neutrons are located within the nucleus while the electrons exist outside of the nucleus. In stable atoms, the number of protons is equal to the number of electrons.

Modern Theory of the Atom The type of atom is determined by the number of protons it has. The number of protons in an atom is equal to the atomic number The sum of the number of protons and neutrons in a particular atom is called the atomic mass Valence electrons are the outermost electrons and are where bonding takes place

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