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Chapter 4
The Structure of the Atom
4.1 Early Ideas About Matter
• Greek philosophers – formed explanations
based on life experiences
Democritus – 400 BC
• Belived matter was made of tiny particles
called atomos
– Cannot be created, destroyed, or divided
• Aristotle disagreed
John Dalton - 1803
•
•
Revived & revised Democritus’ ideas
Came up with 4 part atomic theory
1. All elements are composed of tiny
particles called atoms which cannot be
broken into smaller parts
2. All atoms of the same element have
identical properties. Atoms of different
elements have different properties
3. Atoms combine in whole-number ratios to
form compounds
4. Chemical reactions take place when
atoms rearrange. Atoms of one element
are NOT changed into atoms of a different
element
Conservation of mass
• Result of separation, combination, or
rearrangement of atoms
• Atoms are not created or destroyed
4.2 Defining the Atom
• The smallest particle of an element that
retains the properties of the element
• Atoms are very small
– One Cu atom = 1.28 x 10-10 m
– Solid copper penny = 2.9 x 1022 atoms
– If 1 atom = size of an orange, an orange
would be as big as the earth!
• Scanning tunneling microscope (STM)
allows individual atoms to be seen
– Scans across the surface of atoms
– silicon
J.J. Thomson – 1890s
• Worked with a cathode ray tube
• Discovered cathode ray was attracted to +
charged plate
• What does this mean?
• By measuring the effects of both magnetic
and electric fields on a cathode ray,
Thomson determined the mass of the
charged particle is less than the hydrogen
atom
• What does this mean?
• Thomson’s model
– Matter is neutral so some positive charge
needs to balance out the negative electrons
– Plum pudding model
Rutherford - 1911
– Shot alpha particles (+ charged) at gold foil.
– Based on Thomson’s model he expected to
pass through and hit screen on other side but
not all did!!!
• What does this
mean?
• Rutherford concluded:
– Atom consists of mostly empty space
• If an atom had a diameter of 2 football fields the
nucleus would be the size of a nickel
– Almost all + charge & mass are found in
nucleus
• Rutherford model
James Chadwick - 1932
• What we knew
1. Electrons & Protons have opposite
charges
2. Neutral atom has same number of
electrons as protons
3. Mass of atom was too much to just be
made of protons & electrons
• Showed nucleus
contained a particle
with the same mass
as a proton but no
charge – neutron
Completing the model of the atom
• Spherically shaped
• Small, dense, + charged nucleus
– Contains 99.97% of atoms mass
• Fast moving electrons traveling through
empty space
– Held to atom by attraction to nucleus
• Atoms are neutral so # of protons = # of
electrons
Electron Cloud Model
Particle
Electron
Proton
Neutron
Symbol
Location Charge
Mass
4.3 How Atoms Differ
• The number of protons in an atom
identifies it as a particular element
• Number of protons = atomic number
• Periodic table is organized in order of
increasing atomic number
• All atoms are electrically neutral therefore
• # of protons =
• Determine the number of protons and
electrons in an atom of Molybdenum
Mass Number
• Mass number = protons + neutrons
• Neutrons =
Atomic Shorthand
• Beryllium- - -atomic number 4
mass number 9
• The atomic number is written as a
subscript.
• The mass number is written as a
superscript.
Be
Element Atomic Protons
Number
35
10
Electrons
Neutrons
19
20
Mass
number
80
20
Isotopes
• Atoms with the same number of protons
but different number of neutrons
– Oxygen – 16, Oxygen – 17, Oxygen – 18
– One isotope is more common than the others
Ions
• Atoms with an electrical charge
• Ions form by gaining or losing
ELECTRONS
• Cation
• Anion
Element
Atomic
#
Protons Electrons Neutrons
O2Al3+
19
14
6C
Potassium - 41
18
Mass
number
What can change in an atom?
• Protons – NEVER!!!
• Neutrons – if changed an isotope is
formed
• Electrons – if changed an ion is formed
Proton Changed
• Whole new atom!!!
• Oxygen loses a proton it becomes
Nitrogen
• Oxygen gains a proton it becomes
Fluorine
Neutron Changed
• Different version of the same atom is
formed (isotope)
• Oxygen – 16 has 8 Neutrons
• Oxygen – 17 has 9 Neutrons
• Oxygen – 18 has 10 Neutrons
Electrons Changed
• Charged version of the same atom is
formed (ion)
• Oxygen gains 2 electrons – 10 electrons &
8 protons
– Cation
• Oxygen loses 2 electrons – 6 electrons &
8 protons
– Anion
Mass of Atoms
• Mass of protons & neutrons = 1.67 x 10-24
g
• This is too small to be practical so
scientists use atomic mass units (amu)
• Carbon – 12 = 12 amu
– 1 amu = 1/12 mass of carbon – 12 atom
• Proton = 1.007276 amu
• Neutron = 1.008665 amu
• Electron = 0.000549 amu
• Atomic mass = weighted average mass of
the isotopes of an element
• Boron has two naturally occurring
isotopes: Boron – 10 (19.8% abundance,
10.013 amu) and Boron – 11 (80.2 %
abundance, 11.009 amu). What is the
atomic mass of Boron?
• Unknown element X has two isotopes. 6X
has an atomic mass of 6.015 amu and a
7.59 % abundance. 7X has an atomic
mass of 7.016 amu and a 92.41%
abundance. What is the atomic mass of
the unknown element? What is the identity
of the unknown element?
4.4 Unstable Nuclei and
Radioactive Decay
• Unstable atoms emit radiation to gain
stability
– Stability is gained by losing energy
• Nuclear reactions can change elements
into new elements
Radioactivity
• The process in which some substance
spontaneous emit radiation
• Radiation – rays and particles emitted by
the radioactivity
• Nuclear reaction – reaction that involves a
change in an atom’s nucleus
• Radioactive decay – spontaneous process
in which atoms lose energy by emitting
radiation
– Unstable elements do this until they are stable
(often a different element)
Types of Radiation
• Three types of radiation based on their
electric charge
– Alpha (α)
– Beta (β)
– Gamma (γ)
Alpha Radiation
• Radiation deflected toward negative
charge
– So its charge is
• Made up of alpha particles
– 2 protons + 2 neutrons
Beta radiation
• Radiation deflected toward positive charge
– So its charge is
• Made up of beta particles
– electron
Gamma radiation
•
•
•
•
High energy radiation with no mass
No charge
Usually accompany alpha & beta radiation
Account for most of the energy lost during
radioactive decays
Nuclear Stability
• Primary factor in determining stability is
ratio of protons to neutrons
• Atoms with too many or too few neutrons
are unstable
– Will radioactively decay until they become
stable