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Correction from last class: H2, O2 are elements. H2O is compound
Theories of the Atom
The Evolution of Atomic Theory
-400BCE, the Greek philosopher Democritus named his proposed particle the atom. 1. Of different sizes, 2. In
constant motion, 3. Separated by empty spaces
-450BCE, the Greek philosopher Aristotle supported an earlier theory (all matter is made up of four basic
substances: earth, water, air, and fire). This theory was accepted for almost 2000 years.
-1807, the English scientist John Dalton revived Democritus’ theory. He proposed that:
1. all matter is made up of tiny, indivisible particles called atoms
2. all atoms of an element are identical
3. atoms of different elements are different
4. atoms are rearranged to form new substances in chemical reactions, but they are never created or destroyed
-1897, J.J. Thomson theorized that
1. atoms contain negatively charged electrons
2. since atoms are neutral, the rest of atom is a positively charged sphere
3. negatively charged electrons are evenly distributed throughout the atom
-1909, Ernest Rutherford supervised an experiment.
Reasoned that these large angles of deflection were caused by a collision with a small, concentrated, positively
charged central mass inside the atom.
1. the centre of the atom has a positive charge.(nucleus)
2. the nucleus is surrounded by a cloud of negatively charged electrons
3. most of atom is empty space
+1920, Rutherford also discovered proton: a positively charged particle that is found in the atom’s nucleus. (but
their mass doesn’t add up, then there must be another third particle in the nucleus about same mass as the
proton but was neutral in charge.)
-1932, James Chadwick, Rutherford’s student, discovered neutrons, a neutral particles in the atom’s nucleus.
Electron orbits
1913, a Danish scientist, Niels Bohr performed an experiment:
-electrons orbit the nucleus of the atom much like the planets orbit the Sun
-each electron in an orbit has a definite amount of energy
-the farther the electron is from nucleus, the greater its energy
-they release energy as light when they jump from higher to lower orbits
-each orbit can hold a certain maximum number of electrons.(2, 8, 8)
Explaining the Periodic Table
Atomic Number: the number of protons in the nucleus
Protons are tightly held in the nucleus of an atom. It would take a nuclear reaction (such as that inside an atomic
bomb or a nuclear reactor) to combine two nuclei into one.
Mass Number: The mass of an atom consists of the contents of its nucleus – protons and neutrons. Once you
know the mass number, you can also determine the number of neutrons.
Isotopes: Atoms with the same number of protons but different numbers of neutrons. (ex. Lithium isotope with
a mass number of 6 is called lithium-6 or Li-6)
Atomic Mass: measured in atomic mass units(u). not whole numbers. The atomic mass of an element is the
weighted average of the masses of its isotopes.
Bohr-Rutherford Diagrams of an Atom
It shows the numbers and locations of protons, neutrons, and electrons in an atom.
Patterns in the Periodic Table
-The alkali metals undergo similar reactions because they all have one electron in their outermost orbits.
From Charcoal to Diamonds
Buckminsterfullerene (or bucky-ball) is a spherical fullerene
molecule with the formula C60.
Questions: