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The Atom
Objectives:
1) Describe the characteristics of protons, neutrons and
electrons.
2) Describe and draw the patterns of electrons for the
first 20 elements in the periodic table using BohrRutherford diagrams.
• All matter on Earth is composed of
one or more of the 118 elements on
the periodic table
• Elements are made up of atoms
RECALL
Rutherford proposes that the atom contained a central
nucleus with a small, dense, positive charge in the center.
James Chadwick proposed that the nucleus of the atom contains
protons and neutral particles called neutrons.
Mass of a neutron is about the same as the mass of a proton and a
neutral atom has the same number of protons and electrons.
Niels Bohr
1913
Studied under Rutherford.
Electrons are in orbits. Rather like
planets orbiting the sun. With each
orbit only able to contain a set
number of electrons.
Bohr’s Atom
electrons in orbits
nucleus
We Now Know that ….
• An atom is the smallest particle of an
element that still has the same properties
of that element
• Atoms are incredibly small – about a
million times smaller than the thickness of
a human hair
• They are made up of even smaller
subatomic particles: protons, neutrons,
and electrons
HELIUM ATOM
Shell
proton
+
-
N
N
+
electron
What do these particles consist of?
-
neutron
Atomic Structure
Particle Charge
Name
Location
Mass
Symbol
proton
+
nucleus
Heavy
p+
neutron
neutral
nucleus
Heavy
n0
electron
-
energy levels
outside the
nucleus
Light
e-
In Summary
•Protons have a positive (+) electrical charge, and
are found in the nucleus (centre) of an atom
•Neutrons are electrically neutral and are also
found in the nucleus of an atom
•Electrons have a negative (-) electrical charge and
found in energy levels around the nucleus
What makes atoms different from
each other?
• Each element or atom has its very own number
of protons
• For example, calcium has 20 protons, while
magnesium has 12
• The number of protons in an atom is called the
atomic number
• The periodic table is arranged based on
the atomic number of each element
• In a neutral atom, the number of protons
equals the number of electrons
• # protons = # electrons
• Therefore calcium has 20 electrons, while
magnesium has 12
How we determine the number of
neutrons?
• The mass number is the total number of
protons and neutrons in an atom
• mass number = # protons (or atomic
number) + # neutrons
• Since we always know the number of
protons (atomic number on the periodic
table), we can subtract that value from
the mass number to get the number of
neutrons
• # neutrons = mass number – atomic
number (or # protons)
Example of ATOMIC STRUCTURE
2
He
4
Atomic number
the number of protons in an atom
Atomic mass
the number of protons and
neutrons in an atom
number of electrons = number of protons
Practice
Element
Symbol
Helium
He
Atomic
Number
2
magnesium
4
Protons Electrons Neutrons
2
2
2
24
26
zinc
Mass
Number
56
35
Bohr-Rutherford Diagrams of
the Atom
p+
no
Bohr-Rutherford Diagrams of
the Atom
• A Bohr-Rutherford Diagram is a drawing
that shows the number of protons,
neutrons and electrons for the first 20
elements.
• In Bohr-Rutherford diagrams, each energy
level is shown as a ring around the nucleus
• Evidence indicates that only a limited
number of electrons can exist in each
energy level
ATOMIC STRUCTURE
Electrons are arranged in Energy Levels or
Shells around the nucleus of an atom.
•
first shell
a maximum of 2 electrons
•
second shell
a maximum of 8 electrons
•
third shell
a maximum of 8 electrons
•
fourth shell

a maximum of 2 electrons
How to draw a Bohr-Rutherford
diagram of an atom:
•
Determine the number of protons,
electrons, and neutrons.
•
Draw the nucleus and indicate the number
of protons and neutrons.
•
Add the electrons one at a time, starting
with the first energy level (the ring closest
to the nucleus) and working your way out.
•
•
•
•
Remember!
# protons = the atomic number
# electrons = # protons
# neutrons = mass number (round off
the atomic mass) - # protons
Example #1
Hydrogen
Example #2
Sodium
SUMMARY
1. The Atomic Number of an atom = number of
protons in the nucleus.
2. The Atomic Mass of an atom = number of
Protons + Neutrons in the nucleus.
3.
The number of Protons = Number of Electrons.
4.
Electrons orbit the nucleus in shells.
5.
Each shell can only carry a set number of electrons.