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History of the Atom
IMPORTANT PEOPLE
1) Democritus:
o When you keep on dividing something in half, you get something really small. (Atomos)
o First person to come up with the idea of atoms.
o The world was made of tiny particles or “Atomos” which means indivisible—
discontinuous theory
o Smallest particles of matter cannot be destroyed
2) John Dalton:
o Came up with modern atomic theory.
o Dalton’s Model: (1807)
Atom was indivisible, uniformly dense, solid sphere that participated in , but was
unchanged by chemical rex
Think of a marble hard and solid
o Dalton’s Law of Multiple Proportions:
The mass of one element forms a simple whole number ratio when combining
with a fixed mass of another element
Example:
water H2O O = 16 g
hydrogen peroxide H2O2 O = 32 g
3) JJ Thomson:
o Discovered the proton (1885) and electron (1897)
o Used a cathode ray. (proof of the existence of the electron)
In a cathode ray tube, electrons travel from a negatively charged cathode to a
positively charged anode.
When he added the deflecting coils (magnets), he was able to deflect the beam of
light
Since the cathode ray was deflected toward the positive plate, it must be a
composed of negatively charged particles
Can see where hits because fluorescent screen glows
Used in TV and computer screens
o Plum pudding model (1903)
electrons imbedded in a solid sphere of positive charge
4) Ernest Rutherford:
o Gold-foil experiment.
Atom is mostly empty space
Has a small densely packed positively charged core
Disperses the plum pudding model
o Discovered the nucleus.
o Rutherford Model
5) Niels Bohr:
o Disproved Rutherford’s Model.
o Bohr model (1913)
Electrons in the atom can exist in stationary states which emit no radiation
known as the planetary model
electrons act like particles
6) Schrodinger:
o Quantum mechanical model
o also known as the electron cloud or wave model
o “probability regions" of finding an electron
o Electrons are not particles, but waves.
7) Chadwick:
o Discovers the neutron.
o In contrast with the helium nuclei (alpha particles) which are positively charged, and
therefore repelled by the considerable electrical forces present in the nuclei of heavy
atoms, this new tool in atomic disintegration need not overcome any electric barrier and
is capable of penetrating and splitting the nuclei of even the heaviest elements
8) Werner Heisenberg:
o Both Bohr and Schrodinger are correct.
o Correct description of an atom.
o Quantum theory
o Electrons act like both particles and waves
o Uncertainty principle:
The position and the velocity of an object cannot both be measured exactly, at the
same time, even in theory.
9) Mendeleev:
o Came up with modern day periodic table.
o Periodic Law:
Placed elements in horizontal rows by atomic mass and in columns by chemical
properties.
Problem: atomic mass doesn’t increase regularly
o Modern Periodic Law: (Mosley)
The properties of elements repeat periodically when the elements are arranged in
increasing orders by their atomic number.
10) Lavoisier (1770s)
o The Law of Conservation of Matter
The mass of the reactants before a chemical reaction is equal to the mass of the
products after the reaction
Matter (mass) can’t be created or destroyed, just rearrange
Atoms and Elements: Atomic Structure
Atom
The smallest unit of matter
Has no charge, is electrically neutral
Subatomic Particles
Electron Proton Neutron
Symbol e- p+ n
Relative Charge -1 +1 0
Relative Mass (amu) 0
Actual Mass (g) 9.11 x 10
1.673 x 10
1.675 x 10
Atomic # (Z) = The # of protons in a nucleus.
The # of electrons. ( ONLY in a neutral atom)
Nucleon (protons/neutrons):
Particles that occupy the nucleus including the proton and neutron
Atomic Symbols
Atomic number (Z)
o The number of protons in the nucleus
o **atoms of the same element must ALWAYS have the same number of protons
Atomic Mass (A)
The number of protons plus neutrons in the nucleus
Also known as the mass number
Expressed as atomic mass units or amu
1 amu = 1.66 x 10 -24 grams
1 amu = 1 proton = 1 neutron
An electron has 1/1837th the mass of a proto
Isotopes: Atoms of a single element (same atomic #) that differ in the # of neutrons in their nucleus (diff
masses)
amu = atomic mass units
a very, very small unit of mass used to express the mass of atoms and molecules
atomic mass unit is equal to one-twelfth of the mass of the nucleus of a carbon-12 atom.
If you change the # of protons, you are changing the element
Mass # (A) = The combined # of protons and neutrons in a nucleus
A = Z + N
the isotopes of
Hydrogen
A
H H H
Z 1 1 1
Another way to represent isotopes: [element name – hyphen]
Hyphen notation: Ex: Carbon – 13
PROTIUM DEUTERIUM TRITIUM
Average Atomic Mass
Element: Phosphorus
Ratio of Isotopes:
Relative Abundance (of each
isotope)
Equation = (28 x 92) + (29 x 5) + (30 x 3) = 28.
= (isotope abundance x isotope mass #)
isotope abundance
P
