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
16:32 = 1:2
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:
Disproved Rutherford’s Model.
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
o
o
•Democritus
400 B.C. •Proposes the idea of the atom
1770s
•Lavoisier
•Law of Conservation of Matter
1790s
•Proust
•Law of Definite Proportions
1807
•John Dalton
•Founder of modern atomic theory (his model survived for almost 100 years)
1885
•J.J. Thomson
•Discovery of the "proton"
1896
•Henri Becquerel
•Discovery of Radioactivity
1897
•J.J. Thomson
•Discovery of the "electron"
1903
•J.J. Thomson
•"Plum-pudding model"
1909
•Ernest Rutherford
•The Gold Foil Experiment
1911
•Ernest Rutherford
•Rutherford's model of the atom
1911
•Robert Millikan
•Oil drop experiment
1913
•Niels Bohr
•Electrons exist in discrete energy levels
1923
•Robert Millikan
•The Oil Drop Experiment- discovered the charge of an electron, discovered the mass p + = 1836 the mass of e-
1923
•Erwin Schrodinger
•Wave Particle Duality
1925
•Erwin Schrodinger
•Quantum Mechanical Model of the Atom
1932
•James Chadwick
•Discovery of the "neutron"
1945
•Wolfgang Pauli
•Pauli Exclusion Principle
Atoms and Elements: Atomic Structure
Atom


The smallest unit of matter
Has no charge, is electrically neutral
Subatomic Particles
Electron
Symbol
Relative Charge
Relative Mass (amu)
Actual Mass (g)
Proton
e-1
0
(1/1837)
9.11 x 10-28
Neutron
p+
+1
1
n
0
1
1.673 x 10-24
1.675 x 10-24
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
o
The number of protons in the nucleus
**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
PROTIUM
DEUTERIUM
2
H
3
H
1
Another way to represent isotopes: [element name – hyphen]

+
+
+
Z1
-
-
-
A1
TRITIUM
Hyphen notation: Ex: Carbon – 13
H
1
 the isotopes
of Hydrogen
Atoms, Ions and Isotope
Ions: If you don’t have an equal number of protons and electrons, you have an ion








Particles with an electric charge
Atoms of the same element with different number of electrons
Different net charge
Number of protons ≠ number of electrons
Cation: a positively charged ion
Anion: a negatively charged ion
Loss of electrons yields a positive charge
Gain of electrons yields a negative charge
Ex:
C+(1)
- 6 protons, 5 electrons
B-(1)
- 5 protons, 6 electrons

When you have an imbalance between protons and electrons, it is no longer an atom, it is an ion
A neutral atom would look like
C


- 6 protons, 6 electrons
A cation is a positive ion. (+)
An anion is a negative ion. (-)
Average Atomic Mass
Element: Phosphorus
Ratio of Isotopes:
28
14
P
92
Equation
29
14
:
P
30
14
5
:
P
3
= (28 x 92) + (29 x 5) + (30 x 3) =
100
= (isotope abundance x isotope mass #)
isotope abundance
28.11
 Relative Abundance (of each
isotope)