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Models for Atoms
Chapter 3
•The atom-building blocks of the
elements
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Atoms are the basis of chemistry and the basis of
everything in the universe. Remember all matter is
composed of atoms!
Atom – smallest particle of an element that still retains
the properties of that element
the atom is a microscopic structure. It is similar to the
cell in biology. Like the cell the atom can be broken
down into it simplest parts as well.
Atomic Structure
Atoms can be broken down into electrons, protons,
and neutrons.
Electrons are negatively charged subatomic
particles. Symbol is e- . (J.J. Thompson)
Protons are positively charged subatomic particles.
(Rutherford)
Neutrons are subatomic particles with no charge.
(James Chadwick)
•
An atom refresher
An atom has three parts:
• Proton = positive
• Neutron = no charge
• Electron = negative
•
The proton & neutron are
found in the center of the
atom, a place called the
nucleus.
• The electrons orbit the
nucleus.
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HELIUM ATOM
Shell
proton
+
electron
N
N
+
-
neutron
Protons and Neutrons are found in the nucleus,
electrons are found in shells outside the nucleus
A quick review
What scientists believed about the elements
Empedoles (~450 BC) • All matter was made of four elements - Earth, Air,
Fire and Water
A quick review
What scientists believed about the elements
Democritus (~400 BC) – Matter was made of tiny
particles that cannot be broken down further (called
atoms)
The same element
can make up
different kinds of
substance.
A quick review
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What scientist believed about the elements
Aristotle (~ 350 BC) –
A return to the “4-element” model
•A quick review
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What scientists believed about the elements
Alchemists (500 – 1600 AD) –
Still believed in the “4 –
element” model!
Tried to turn cheap metal
into gold
•A quick review
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What scientists believed about the elements
Boyle (1650 AD) –
• A return to the atom theory…
• Believed in the existence of elements
Dalton (1808 AD)
Developed the atomic model for matter:
- All matter is made up of atoms
- Each element has it’s own atom and
atomic mass
- Compounds can be created when
atoms from different elements come
together to form molecules
- Atoms cannot be created, destroyed,
or subdivided in chemical changes
Faraday (1800’s) –
Discovered that an atom can gain
an electric charge and form an
ion.
• Modified Dalton’s model….
- Matter must contain positive and
negative charges
- Atoms can combine to form
molecules because of electrical
attraction
Thompson (1904) –
Discovered electrons!!!!
• Developed the “raisin bun” model
- Dough → positive charge
- Raisins → negative charge
- Overall, atoms have a neutral charge
-
Dalton’s model is now incorrect because
atoms are divisible (but only through a
nuclear reaction
Rutherford (1911) –
Experimented using gold foil and discovered that
atoms are
1. not solid
2. most of the atom is made up of space
3. have a small center called a nucleus
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Ernest Rutherford, through experimentation,
discovered the nucleus.
The nucleus is the heaviest part of the atom; it’s
comprised of both protons and neutrons. Thus the
nucleus has a positive charge. The electrons
occupy the outside of the nucleus (the majority of
volume of the nucleus)
Different elements have different numbers of
subatomic particles thus giving different elements
different characteristics/properties
Basics of the Atom
Particle Charge
Location in
the Atom
proton
1+
in nucleus
0
in nucleus
neutron
Mass
~1
a.m.u.
~1
a.m.u.
~0 a.m.u.
orbiting
electron
1–
nucleus
a.m.u.: unit used to measure mass of
atoms
Review Protons
Protons are very important because they
help determine the identity of an element
• They are inside the nucleus
• They have a positive charge +
• Atom Identity – the number of protons an
atom has will determine its identity.
Ex:
8 protons = oxygen
19 protons = potassium
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Review Neutrons
• Found
inside the nucleus
• Have a neutral charge
• Have a relative mass of 1 atomic unit
• Important for creating isotopes (coming
soon!)
Review electrons
• Found
outside the nucleus
• Forms an orbital cloud (the location
where an electron might be found)
• Has a negative charge –
• Different
elements have different
numbers of subatomic particles thus
giving different elements different
characteristics/properties
• So
how do we know how many
subatomic particles an element has?
• Grab
out....
your periodic tables and let’s find
Atomic Number:
Number of protons and it is also
the number of electrons in an
atom of an element.
.
Chemical Symbol of
element
Elements Name
Atomic Mass/Weight:
8
O
Oxygen
16
Number of protons +
neutrons.
Created by G.Baker www.thesciencequeen.net
For each element, indicate the:
1. Mass number
2. atomic number
3. # of protons
4. # of electrons
a. sulfur b. Calcium
c. Iron
•Charged Atoms (ions)
Why is it dangerous to work with electrical
appliances around water?
Tap water and rain water (not pure)
contain charged atoms that can move in
a solution and carry electric current.
• These charged atoms are called ions
(atoms that have become charged by
gaining or losing one or more electrons).
• They have a charge because the number
of electrons is NOT equal to the number of
protons
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The first “modern” model of the atom
What do we believe now
Neils Bohr (1913) –
• Electrons move around the nucleus in
paths called orbitals
• Each electron in an orbit has a definite
amount of energy ; further away from
nucleus = greater energy
• Electrons cannot exist between orbits, but
they can move up or down from one orbit
to another
• Electrons are more stable when they are at
lower energy (closer to the nucleus)
Excited state – when electrons jump to a
higher orbit due to being energized by
light, heat or electricity.
Ground state – When electrons fall to
lower orbits and a more stable state
•Bohr Model
• The
Bohr Model shows
all of the particles in
the atom.
• In the center = write
the # of protons and
neutrons in the
nucleus, using “p”
and “n”
• In a circle around the
nucleus are the
electrons. Represent
electrons with dots.
Created by G.Baker www.thesciencequeen.net
•Electrons have special rules….
• You
can’t just shove all of the
electrons into the first orbit of an
electron.
• Electrons live in something called
shells or energy levels.
• Only so many can be in any
certain shell.
• The order of filling of electrons for
the first 4 orbits: 2, 8, 8, 18
Created by G.Baker www.thesciencequeen.net
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
•So let’s try it….
How to draw a Lithium atom
• First, look at the Periodic Table
• Second, determine the number
of protons (Look @ the atomic
number)
• Then determine the number of
neutrons (Atomic mass –
atomic number)
• Then determine the number of
electrons (Look @ the atomic
number)
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3
Li
Lithium
7
Protons = 3
3
Li
Lithium
7
Electrons = 3
2 in the
1st
shell, 1 in the
2nd
shell
Neutrons = 4
(7-3=4)
Created by G.Baker www.thesciencequeen.net
ELECTRONIC CONFIGURATION
With electronic configuration elements are represented
numerically by the number of electrons in their shells
and number of shells. For example;
Nitrogen
2 in 1st shell
5 in
2nd
shell
configuration = 2 , 5
2
+
5 = 7
14
7
N
ELECTRONIC CONFIGURATION
Write the electronic configuration for the following
elements;
a)
40
20
Ca
b)
2,8,8,2
d)
35
17
Cl
2,8,7
23
11
Na
c)
16
2,8,1
e)
28
14
Si
2,8,4
O
8
2,6
f)
11
5
B
2,3
Isotopes and Radioisotopes
• Isotope
– any of two or more forms of
an element, each having the same
number of protons but having different
mass due to a number of neutrons.
• Isotopes of the same element have the
same physical properties and the same
chemical properties
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Hydrogen has three common isotopes:
Chlorine has two common isotopes:
• Cl- 35 (atomic number 17 and mass number 35; 17
protons plus 18 neutrons)
• Cl – 37 (atomic number 17 and mass number 37; 17
protons plus 20 neutrons)
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There are three isotopes for Carbon. Let’s draw the
isotopes for Carbon using Bohr diagrams:
Remember all that has changed is the neutrons and thus the atomic
mass. (The number beside Carbon is the new atomic mass).
Carbon – 12, Carbon- 13 and Carbon -14
Now write the standard atomic structure for these:
Carbon – 12
Carbon- 13
Carbon -14
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Some isotopes are unstable, or radioactive, which
means that the nucleus has a tendency to break
apart and eject very-high-energy particles into its
surroundings.
The huge amount of energy these particles have can
be both dangerous and useful. An atom with an
unstable nucleus is called a radioisotope.
Decay – splitting apart of unstable nuclei to produce
radioactive particles
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Around the year 1900, three different kinds of
radioactivity were identified:
alpha (
) particles were found to be helium nuclei,
containing 2 protons and 2 neutrons
beta ( )
particles are high energy electrons
gamma rays (Ƴ )- radiation is high – energy
electromagnetic radiation with no mass.
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Radioisotopes can damage living tissue; they may
alter the DNA, which affects how cells divide.
This can cause serious diseases including cancer and
birth defects.
However, radioisotopes can be very useful and are
useful and are used in smoke detectors and also to
diagnose medical problems such as thyroid disease.