The History of Atomic Theory Mr Nelson

Democritus 400 BC The Greek philosopher Democritus began the search for a description of matter more than 2400 years ago. He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

Atomos This piece would be indivisible. He named the smallest piece of matter “atomos,” meaning “not to be cut.”

Why? The eminent philosophers of the time, Aristotle and Plato, had a more respected, theory. Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter.

Dalton’s Model In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

Dalton’s Model 1803 Dalton’s Model was that atoms are indivisible particles.

Dalton’s Theory He deduced that all elements are composed of atoms. Atoms of the same element are exactly alike. Atoms of different elements are different. Compounds are formed by the joining of atoms of two or more elements.

J. J. Thomson In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.

Thomson Model Thomson studied the passage of an electric current through a gas. Using a CRT. As the current passed through the gas, it gave off rays of negatively charged particles.

Thomson Model the atoms of the gas were uncharged. Where did they come from? the atoms of the gas were uncharged. Where had the negative charges come from?

Thomson Thomson concluded that the negative charges came from within the atom. Thomson called the negatively charged “corpuscles,” today known as electrons. Since the gas was known to be neutral, he reasoned that there must be positively charged particles in the atom. But he could never find them.

Thomson Model “Plum Pudding” model. Atoms were made from a positively charged substance with negatively charged electrons scattered around

Rutherford’s Gold Foil Experiment In 1908 English physicist Ernest Rutherford, began work on his gold foil experiment.

Rutherford’s Hypothesis Rutherford was trying to verify Thomson’s model. He expected positively charged alpha particles to go straight through a piece of very thin gold.

What Happened Most alpha particles passed straight through the gold foil A small percentage (1/8000) were deflected at large angles or returned to the source http://micro.magnet.fsu.edu/electromag/java/rutherford/

Rutherford’s Experiment There are 2 reasons alpha particles deflected Density of the nucleus Repelling charges

Rutherford’s Conclusion An atom has a small, dense, positively charged center that repelled the positively charged alpha particles. Named the center of the atom the “nucleus” The nucleus is tiny compared to the atom as a whole. This could only mean that the gold atoms in the sheet were mostly open space.

Rutherford’s Nuclear Model Rutherford reasoned that all of an atom’s positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom’s edge.

Neils Bohr Was a Jewish Scientist in Copenhagen during the onset of WWII Hitler was interested in his research of the atom. He was moved to the US to protect his knowledge.

Bohr’s Explanation Bohr thought that an electron travelled in a specific orbit at a certain distance from the nucleus called an energy level and had specific amounts of energy. Worked well for Hydrogen and Helium

Nuclear symbols In this unit we need to be familiar with this type of symbol A X Z

Hyphen Notation Includes an element name a ‘-’ and a number Example: Sulfur – 32 This sulfur atom has an atomic mass of 32 Since Sulfur has ______ protons & electrons It also has ______ neutrons