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Atoms and Elements
Atoms and Elements
Dr.SamuelHunt,United Arab Emirates,Teacher
• Atoms are incredibly small
• All of our senses, sense atoms
• There are about 91 elements found in
Over 20 have been made in laboratories.
• Each kind of atom is unique
Carbon is not Hydrogen
They have different properties
Structure, magnetic meaning they can attract and
repel other atoms, melting, boiling, electrical,
stability, reactivity (attract and repel), etc…
www.ThesisScientist.comThe Divisibility of Matter
• Ultimate particle
Upon division, eventually a
particle is reached which can no
longer be divided.
Atoms are 10 m
We detect particles at 10 m
In theory particles 10 m exist,
we don’t have instruments that
“Nothing exists except atoms and empty space; everything
else is opinion.” - Democritus 460–370 B.C.
www.ThesisScientist.comModern Evidence for Atoms
IBM's Almaden Research Center in San Jose,
California, April 1990
www.ThesisScientist.comSizes of Atoms
• Using compositions of compounds and
assumed formulas, Dalton was able to
determine the relative masses of the atoms.
Dalton based his scale on H = 1 amu.
We now base it on C-12 = 12 amu exactly.
Unit = atomic mass unit.
Amu or dalton.
• Absolute sizes of atoms:
Mass of H atom= 1.67 x 10 g.
Volume of H atom = 2.1 x 10 cm .
www.ThesisScientist.comThe Atom Is Divisible
• Work done by J. J. Thomson and others proved
that the atom had pieces called electrons.
• Thomson found that electrons are much smaller
than atoms and carry a negative charge.
The mass of the electron is 1/1836 the mass of a
The charge on the electron is the fundamental unit
of charge that we call –1 charge unit.
www.ThesisScientist.comPlum Pudding Atom
• How can you prove something is empty?
• Put something through it.
Use large target atoms.
Use very thin sheets of target so they do not absorb ―bullet‖.
Use very small particles as ―bullet‖ with very high
But not so small that electrons will effect it.
• Bullet = alpha particles; target atoms = gold foil
a particles have a mass of 4 amu & charge of +2 c.u.
Gold has a mass of 197 amu and is very malleable.
If atom was like
• a plum pudding,
• all the a particles
• should go
Very few of the
a particles Nuclear Atom
do not go through.
Most a particles
go straight through.
Some a particles
go through, but are deflected.
The Nuclear Model
1. The atom contains a tiny dense center called the
The amount of space taken by the nucleus is only about
1/10 trillionth the volume of the atom.
2. The nucleus has essentially the entire mass of the atom.
The electrons weigh so little they contribute practically no
mass to the atom.
3. The nucleus is positively charged.
The amount of positive charge balances the negative charge of
4. The electrons are dispersed in the empty space of the
atom surrounding the nucleus.
Like water droplets in a cloud.
• How could beryllium have 4 protons stuck
together in the nucleus?
Shouldn’t they repel each other?
• If a beryllium atom has 4 protons, then it should
weigh 4 amu, but it actually weighs 9.01 amu
Where is the extra mass coming from?
Each proton weighs 1 amu.
Remember: The electron’s mass is only about
0.00055 amu and Be has only 4 electrons—it can’t
account for the extra 5 amu of mass.
www.ThesisScientist.comThere Must Be Something Else There
• To answer these questions, Rutherford
proposed that there was another particle in
the nucleus—it is called a neutron.
• Neutrons have no charge and a mass of 1
The masses of the proton and neutron are both
approximately 1 amu.
www.ThesisScientist.comSubatomic Mass Mass Location Charge Symbol
particle g amu in atom
Proton 1.67262 1.0073 nucleus 1+ p, p , H
Electron 0.00091 0.00055 empty space e, e
Neutron 1.67493 1.0087 nucleus 0 n, n
www.ThesisScientist.comThe Modern Atom
• We know atoms are composed of
three main pieces—protons,
neutrons, and electrons.
• The nucleus contains protons and
• The nucleus is only about 10 cm
• The electrons move outside the
nucleus with an average distance
of about 10 cm.
Therefore, the radius of the atom is
about 10 times larger than the radius
of the nucleus.
www.ThesisScientist.comSome Notes on Charges
• There are two kinds of
charges, called positive and
• Opposite charges attract.
+ attracted to –.
• Like charges repel.
+ repels +.
– repels –.
• To be neutral, something
must have no charge or equal
amounts of opposite charges.
www.ThesisScientist.comThe Nature of Electrical Charge
• Electrical charge is a fundamental property
of protons and electrons.
• Positively and negatively charged objects
attract each other.
• Like charged objects repel each other.
+ to +, or to .
• When a proton and electron are paired, the
result is a neutral charge.
Because they have equal amounts of charge.
www.ThesisScientist.comPractice—An Atom Has 20 Protons.
Determine if Each of the Following
Statements Is True or False?
• If it is a neutral atom, it will have
• If it also has 20 neutrons, its mass
will be approximately 40 amu.
• If it has 18 electrons, it will have
a net 2 charge.
• Each element has a unique number of protons in its
All carbon atoms have 6 protons in their nuclei.
• The number of protons in the nucleus of an atom is called
the atomic number.
Z is the short-hand designation for the atomic number.
Because each element’s atoms have a unique number of
protons, each element can be identified by its atomic
The elements are arranged on the Periodic Table in order of
their atomic numbers.
• Each element has a unique name and symbol.
The symbol is either one or two letters
One capital letter or one capital letter + one lower case letter.