Atomic Structure

1. Democritus: Around 300 BC, a Greek philosopher, Democritus stated that everything is made up of tiny, invisible particles He said the particles were indivisible He called them “atomos” – which means unable to divide

2. John Dalton : Then in the early 1800’s, an English school teacher, John Dalton developed the first atomic theory His theory was successful because it was supported with scientific evidence By using experimental methods, Dalton transformed Democritus’ ideas on atoms into a scientific theory. His theory has 4 main points

4 parts of Dalton’s theory: 1. All elements are composed of tiny indivisible particles called atoms. 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction

Much of Dalton’s atomic theory is accepted today. One important change, however, is that atoms are now known to be divisible. Also, atoms of the same element are not identical.

3. JJ Thomson In the late 1800’s, an English physicist, JJ Thomson performed an experiment with a cathode ray tube His experiment led to the discovery of the electron An electron has one unit of negative charge, and its mass is 1/1840 the mass of a hydrogen atom.

Thomson’s experiment:

4. Eugen Goldstein In 1886, Eugen Goldstein discovered protons If matter has negative particles, it must also have positive particles. 5. James Chadwick In 1932, James Chadwick confirmed the existence of yet another subatomic particle: the neutron Neutrons are subatomic particles with no charge but with a mass nearly equal to that of a proton.

Chadwick’s experiment:

6. Ernest Rutherford In the early 1900’s, Ernest Rutherford performed the “Gold Foil Experiment” He observed that the alpha particles went through the gold foil

The Rutherford atomic model is known as the nuclear atom. His model inferred that the atom was almost entirely empty space Also concluded that the nucleus was extremely tiny and contained the protons and the neutrons The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

According to this model, the nucleus is tiny and densely packed compared with the atom as a whole. If an atom were the size of a football stadium, the nucleus would be about the size of a marble.

ISOTOPES

Mass number = protons + neutrons Electrons are so much less massive than protons and neutrons that their mass is not included in atomic mass The mass listed in the periodic table is the weighted average of the atomic masses of the naturally occurring isotopes

Isotopes are atoms of the same element with different masses Neutrons were actually discovered due to isotopes In an isotope the number of protons and electrons are the same – only the neutrons differ

One way to specify an isotope is to use the nuclear symbol. The nuclear symbol includes the chemical symbol, atomic number, and atomic mass.

The value shown in the periodic table is the average atomic mass. It is a weighted average For example: Chlorine has two isotopes Chlorine-35 and Chlorine-37 Cl-35 has an amu of with an abundance of % Cl-37 has an amu of with an abundance of % ( x %) + ( x %) = The atomic mass of chlorine