Atomic Structure

Democritus (460 BC – 360 BC)
 Ancient Greek philosopher
▪ No experiments performed!
 Major Contribution: The Atom
▪ He proposed that everything was made of these atoms
and they were all indivisible
 Was his theory correct?
▪ No! There are subatomic particles!

John Dalton (1766-1844)
 Major Contribution: Atomic Theory (1808)
▪ This began the modern era of chemistry
 Four Principles:
▪ Elements are composed of indivisible particles
called atoms.
▪ All atoms of a given element are identical.

John Dalton
▪ Compounds are composed of atoms of one or more
elements, and will form only in whole-number ratios.
▪ This is called the Law of Multiple Proportions
▪ i.e. H2O exists, while H2.35O does not
▪ A chemical reaction involves the combination,
separation, or rearrangement of atoms, not their
creation or destruction
▪ This is called the Law of Conservation of Mass
 Was his theory correct?
▪ Mostly! Parts 1 & 2 have problems!
John Dalton
“When we attempt to conceive
the number of particles in an
atmosphere [gas], it is
somewhat like attempting to
conceive the number of stars in
the universe; we are
confounded with the thought.
But if we limit the subject, by
taking a given volume of any
gas, we seem persuaded that…
the number of particles must
be finite”
- John Dalton on his approach
to the theory of atoms, 1808

J.J. Thomson (1856-1940)
 Major Contribution: The Electron
 Cathode Ray Tube Experiment (1897)
▪ Nobel prize (1906)

J.J. Thomson

Thomson’s Atomic Model
 Also known as the Plum Pudding Model
 Was his theory correct?
▪ No! Missing parts of atom!

Ernest Rutherford
(1871-1937)
 Two Major
Contributions:
▪ The nucleus
▪ The atom is mostly
empty space
 Gold Foil Experiment
(1910)
▪ Nobel prize in Chemistry
(1908)

Ernest Rutherford’s Gold Foil Experiment

Rutherford’s Atomic Model
 Was his theory correct?
▪ Mostly! Missing neutrons and location of electrons!

Niels Bohr (1885-1962)
 Major Contribution: Planetary Model
of the Atom
▪ Nobel Prize in Physics (1922) for spectrum
of hydrogen
 Atomic Line Spectra
▪ Bohr observed that when light was given off
from an atom, there were only single lines
visible
▪ Bohr proposed that each line represented
an electron in a different orbit

Atomic Line Spectra

Bohr’s Atomic Model
Nucleus
Electrons

Current Theory of the Atom
 Many scientists contributed to developing
quantum mechanics, which is the current model
of the atom
 Known as the electron cloud model
▪ The cloud is an area of probability where the electron is
found
▪ These electrons, moving at extremely high speeds,
effectively occupy the entire area of the cloud, the same
way that moving fan blades effectively occupy the entire
area through which they pass.

Current Model of the Atom:
Probability
cloud where
electrons found
Nucleus

Parts of the Atom
 Proton
▪ Positive
▪ Nucleus
 Neutron
▪ Neutral
▪ Nucleus
 Electron
▪ Negative
▪ Orbitting Nucleus
 Atomic Number
 Atoms are identified by their number
of protons
▪ This is referred to as their atomic number
▪ Think of atomic # like a social security
card for each element

In atoms that have a neutral charge, the
numbers of electrons equals the number of
protons
 When an atom gains a charge, it is called an ion
Sodium Atom (Na)
11 Protons
11 Electrons
Sodium Ion (Na+)
11 Protons
10 Electrons
Chlorine Atom (Cl)
17 Protons
17 Electron
Chlorine Ion (Cl-)
17 Protons 18 Electrons
 Mass Number
 The mass of an atom is the number
of protons plus the number of
neutrons
▪ This is referred to as mass number
 The mass of protons and neutrons
are equal

Why is the electron not
part of the mass?
 It takes roughly 1800
electrons to equal the
mass of 1 proton, so it is
left out.
 If we wanted the mass of
an elephant, would we
weigh the flies buzzing
around it? Doesn’t
count!


The periodic table does not give the mass
number, but always the atomic number
For simplicity, we round the number on the
periodic table to get the mass number
Round this number to whole number

Isotopes are elements that have the same
number of protons, but contains a different
number of neutrons
 Compare: carbon-12 vs. carbon-14
▪ The number indicates the mass number
▪ Both contain the same number of protons (6), so
carbon-14 must have two extra neutrons

The masses given on the periodic table
are an average of all the isotopes on the
planet
 We refer to the masses on the periodic table
as the average atomic mass of an element

This explains why the atomic masses are
not whole numbers – it is an average!