# How did different scientists contribute to atomic theory?

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How did different scientists contribute to atomic theory?

Democritus He asked this question: If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? Democritus thought that it ended at some point, a smallest possible bit of matter. He called these basic matter particles, atoms.

Benjamin Franklin His famous stormy kite flight in June of 1752 led him to develop the idea that matter had charges which he represented using plus(+) and minus (-).

JJ Thompson In 1897, Thompson discovered the first component part of the atom: the electron, a particle with a negative electric charge.  In 1904, he proposed an initial model of an atom, since nicknamed "Thompsons pudding".  He imagined the atom as a sphere full of an electrically positive substance mixed with negative electrons "like the raisins in a cake".

Robert Millikan What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. He determined the charge on an electron

Ernest Rutherford In 1912, Rutherford (New Zealand physicist) discovered the atomic nucleus.

Neils Bohr The planetary model was proposed by Bohr after studying a hydrogen atom in 1915.

Electron Cloud Theory

Which element has the atomic # 10? Which element has a mass of 4 amu?
Which element is in period 3 and is a Noble Gas? 64 29 Cu +2 How many protons? How many electrons? How many neutrons?

How do we determine average atomic mass?
An element can exist in a number of forms, called isotopes.  What is an isotope? Isotopes are forms of the same atom that vary in mass (same # of protons different # of neutrons) To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type.

Let’s determine the average atomic mass of carbon!
Carbon exists naturally as two isotopes; Carbon-12 and Carbon-13 The % abundance (fractional abundance) for C-12 is 98.93% and for C-13 is 1.07% (12) x (0.9893) = amu (13) X (0.0107) = amu amu

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