ATOMIC THEORY

Atomic Theory Pioneers Democritus: A Greek philosopher around the year 400 BC. concluded that matter could not be divided into smaller and smaller pieces forever. the smallest piece of matter would be found used the word "Atomos" to describe the smallest possible piece of matter.

During Dalton’s time the first empirical evidence for the existence of atoms was collected and summarized as follows: Laws of Chemical Change: 1.Law of Conservation of Matter (Matter is not lost or gained in a chemical reaction) 2.Law of Definite Composition (Elements combine in definite ratios by mass) 3.Law of Multiple Proportions (Element-masses combine in small whole number ratios) Development of an Atomic Theory

Dalton’s Atomic Theory 1 - All matter is made up of invisible particles called atoms. 2 - All atoms of one element are identical. The atoms of any one element are different from those of all other elements. 3 - Chemical change is the permanent union or separation of atoms. 4 - Atoms combine in small whole number ratios to form compounds. Dalton: reintroduced the concept of “atoms” in He proposed the underlying principles supporting the Laws of Chemical Change, which include the following postulates:

Thompson: (1897) studied cathode ray tubes and discovered that the emitted rays were made of tiny negatively charged particles, which he called electrons. (Milliken determined the charge of an electron: 1-.) The atom is now no longer considered indivisible. Thomson’s model of the atom was called the raisin bun theory, consisting of a sphere with negative electrons embedded in it. (plum pudding theory in England)

Lord Ernest Rutherford: performs his gold foil experiment, proving that the atom is mainly empty space with all the positive charge and very dense mass being located in the center or nucleus. This is a diagram of Rutherford’s experiment

This illustrates what actually happened and why the model was changed to show an atom with a nucleus.

If the atom is the Houston Astrodome Then the nucleus is a marble on the 50 yard line

Niels Bohr: The Danish scientist who, in 1913, proposed the Planetary Model of the atom: Electrons move in definite orbits around the nucleus, like planets moving around the nucleus. Bohr proposed that each electron moves in a specific energy level. Electrons can “jump” from one level to another, but cannot exist between levels. Model works wonderfully for hydrogen, but starts to fall apart for other elements - it does however, provide a good working model from which to start understanding the atom’s structure...

The BALMER SERIES is the series of emission lines of the H atom which is the result of transitions from n=3 to n=2, n=4 to n=2, etc., all ending on n=2 energy level.

Sir James Chadwick: discovered the neutron in 1932, and won the Nobel Prize for this in At this time, scientists thought that all of the subatomic particles had been discovered.. Helium atom: Atoms are made up of 3 particles protons: large, heavy, positively charged electrons: tiny, negatively charged, energy levels neutrons: large, heavy, no charge. However….. electron proton neutron

8 electrons are the MAX! Valence shell: the outermost electron shell Valence electron: electron in the outermost shell (valence shell) Atoms like to be stable. A stable element has 8 electrons in its valence shell. (*except hydrogen and helium because they only have 1 shell - filled with 2 electrons.)

Elements bond together for a simple reason! They like to have full valence shells with 8 electrons! + = H 2 0 (water) They can transfer (Ionic bond) or share (Covalent bond) electrons to obtain the same degree of stability as their nearest Noble Gas.

1 2 # of valence electrons

It is important to remember that only the valence electrons take part in any kind of bonding during a chemical reaction. Key points: Protons are not involved in a chemical reaction unless it is a nuclear reaction… we won’t be doing any of those :)

Atomic number (Z) = number of protons in nucleus Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Isotopes are atoms of the same element (X) with different numbers of neutrons in the nucleus

How many protons, neutrons, and electrons are in C 14 6 ? How many protons, neutrons, and electrons are in C 11 6 ? 6 protons, 8 (14 - 6) neutrons, 6 electrons 6 protons, 5 (11 - 6) neutrons, 6 electrons Do You Understand Isotopes? 2.3