A) Early Atomic Theory
1) Democritus (400 B.C.) proposed the first Atomic Theory.
a) Stated the world was made up of empty
space and atoms.
b) His theory was not widely accepted
2) Aristotle’s Theory was accepted although
wrong. He believed matter was continuous,
and made up of one substance.
3) Antoine Lavoisier showed that the total mass
Of all the reactants always equaled the total
Mass of all the products.
àDeveloped the law of the conservation of
Mass. ( Matter cannot be created or
4) John Dalton looked at work from other
Scientist and put together an atomic theory.
a) All atoms of the same element are
identical. ( This is not correct) All
Atoms of the same elements have the
same number of protons. This is correct.
b) Atoms of different elements are different.
c) Atoms unite in definite ratios to form
d) All matter is composed of atoms.
B) Early Research on Atomic particles.
1) A cathode ray tube was used to discover
a) anode – the positive terminal.
b) cathode – the negative terminal
2) J.J. Thompson discovered the rays in a
cathode ray tube were made of electrons.
àA negative ion is called an ANION
Because it is attracted to the positive
terminal- the ANODE
àA positive ion is called a CATION
because it is attracted to the negative
terminal – The CATHODE
3) Robert Millikan determined the charge of
an electron to be -1. The symbol for an
electron is “e-1”
4) J.J. Thompson discovered the proton.
àThe proton has a charge of +1 and is
1836 times the mass of an electron.
5) James Chadwick discovered the Neutron.
a) The Neutron has no Charge.
b) The neutron and proton are about
equal in mass.
6) Particles that make up the atom
Particle Charge Location
Protons +1 Makes up the nucleus
Neutrons 0 Makes up the nucleus
Electrons -1 Orbits the nucleus
II) Isotopes and Atomic Number
A) The atomic number is the number of protons
in the nucleus of the atom. The symbol used
is “ Z”
àTHE NUMBER OF PROTONS INDICATES
B) Isotopes contain the same number of protons,
But a different number of neutrons.
àThe number of neutrons determines the
particular isotopes of the element.
C) Nucleons are particles that make up the nucleus of the atom can be either a proton or a neutron.
D) Mass Number or Atomic Mass – The sum of the protons and neutrons in an atom. The symbol is “A”.
E) Nuclide – is an atom with a specific number of protons and neutrons
àAn example is Uranium 235. (Notice the
element with a number after it) This Isotope
of Uranium contains 92 Protons and 143
F) How to find the number of neutrons in an atom.
# neutrons = Atomic Mass – Atomic Number
Example Uranium 238. The atomic number is 92 for Uranium.
# neutrons = 238 – 92 = 146
This isotope of Uranium has 146 neutrons.
A) Molecular and Formula Mass
1) The mass of the elements is compared to the mass of carbon.
2) The atom is too small to use grams. Therefore, the unit that measures the mass of an element is
an “atomic mass unit”, the symbol is amu or U.
3) The mass of carbon is 12 amu, the mass of hydrogen is 1 amu. Carbon is 12 times heavier than hydrogen.
4) On the periodic table the atomic mass is always the larger of the two numbers.
5) Molecular mass or formula mass is the sum of the atomic masses of the atoms in a compound.
Example #1 Find the mass of H2O ( Round the mass to the closest whole number.)
(Subscript) x (Mass)
H2O H = 2 x 1 amu = 2 amu
O = 1 x 16 amu = 16 amu
The mass of water =18 amu
Example #2 KH2PO4
K = 1 x 39 amu = 39 amu
H = 2 x 1 amu = 2 amu
P = 1 x 31 amu = 31 amu
O = 4 x 16 amu = 64 amu