Introduction to Atoms
• Development of Atomic Models
– Who were the first people to think about matter?
– Democritus proposed the idea that matter is
formed of small pieces that could be cut into
smaller parts.
– atomos, “uncuttable”
– Atom is the smallest particle of an element.
– John Dalton, an English chemist, inferred that
atoms had certain characteristics.
Dalton’s Atomic Theory
• All elements are composed of atoms that cannot
be divided.
• All atoms of the same element are exactly alike
and have same mass. Atoms of different
elements are different and have different masses.
• An atom of one element cannot be changed into
an atom of a different element. Atoms cannot be
created or destroyed in any chemical change,
only rearranged.
• Every compound is composed of atoms of
different elements, combined in a specific ratio.
Thomson and smaller parts of atoms
• Further experiments revealed that the atom
itself was composed of even smaller, more
fundamental particles.
• 1897-J.J Thomson (Cambridge University), a
British physicist, found negatively charged
particles.
• Electrons=negatively charged particles
Cathode Rays
• Cathode rays are produced when a high electrical
voltage is applied between two electrodes within a
glass tube, from which air has been partially
evacuated, called a cathode ray tube.
Plum Pudding Model
Atom model proposed by JJ Thomson
Rutherford and the Nucleus
• 1911-Ernest Rutherford, Thomson’s student,
found evidence to disprove Thomson.
• Rutherford’s team aimed a beam of positively
charged particles at a thin sheet of gold foil.
• Most of the particles passed through the foil
undisturbed, but a few particles were deflected.
• He inferred that an atom’s positive charge must
be clustered in a tiny region in it’s center-called
the nucleus
Rutherford’s Gold Foil Experiment
Should have been in a straight line, but particles were deflected.
Rutherford
• Electrons had almost no mass.
• Atom’s mass must also be located in the tiny,
positively charged nucleus.
• Rutherford’s model was mostly empty space
with electrons moving around the nucleus in
that space.
• Protons-positively charged particles in the
nucleus
Bohr’s Model
•
•
•
•
Danish scientist-Niels Bohr
Student of Thomson and Rutherford
Revised the atomic model
Move in certain orbits
A cloud of Electrons
• 1920s the atomic model changed again
• Electrons do not orbit the nucleus like planets
• Energy level or the specific amount of energy
it has.
The Modern Atomic Model
• 1932-British scientist James Chadwick
discovered the neutron.
• Neutron: electrically neutral
• The model has not changed since.
Modern Atomic Model
Describes an atom
as consisting of a
nucleus that
contains protons
and neutrons
surrounded by a
cloudlike region of
moving electrons.
Particles in
an Atom
Particle
Symbol
Charge
Relative
Mass
(amu)
Proton
P+
1+
1
Neutron
N
0
1
Electron
E-
1-
1/1,836
AMU=atomic mass unit
Atomic Number
• Every atom of an element has the same number of
protons.
• For example, the nucleus of every carbon atom has 6
protons.
• Atomic number: number of protons in the nucleus and
defines the element.
• Chemical Symbol: a one- or two-letter abbreviation
• For example: H is Hydrogen; He is Helium
• Most are based off the elements names, but some are
based off their Latin names.
Isotopes and Mass Number
• Number of neutrons can vary
• Isotopes: atoms with the same number of
protons, but a different number of neutrons.
• Examples: Carbon, Neon, Aluminum,
Beryllium, and Chlorine
• Mass number: the sum of the number of
neutrons and protons in the nucleus