Chapter 14- The Atom
The Big Idea
Atoms are the incredibly small building blocks of matter
14.1 Discovering the
Invisible Atom
How small are they?
So small that the number of them in a
baseball is roughly equal to the number
of ping pong balls that would fit in a
hollow sphere the size of the earth.
We can’t see them with the best light
microscopes made because they are
smaller than wavelengths of visible
light.
Where did they come
from?
The origin of atoms goes back to the the
birth of the universe.
Hydrogen (H) was the first.
The lightest atom – hydrogen (H) makes up
over 90% of the known universe.
Heavier atoms are produced in stars.
Extremely high heat and pressure caused
hydrogen atoms to fuse together making
the heavier atoms
A Breath of Air
1022 atoms of air make up a liter of air
There are about 1022 liters of air in the
atmosphere
You have roughly a liter of air in your lungsevery time you exhale, you exhale roughly
the same number of atoms as there are liters
of air in the universe
14.2 Evidence for Atoms
First direct evidence
1827- Robert Brown (Scottish botanist)
Brown was examining pollen grains in a
drop of water under a microscope
He noticed that they were moving
Brownian Motion
At first he thought they were a life form but
later found that dust and soot moved the
same way
Movement was called Brownian motion
Einstein was later able to explain that
Brownian motion results from collisions
between invisible atoms and visible particles
The grains were moving because they were
being jostled by atoms in the water
Can we see atoms?
We still can’t see atoms directly but can generate
images of them indirectly using a scanning probe
microscope.
Opened up the field of
Nanotechnology
Atomic Structure
Early 1900’s scientists cane to realize that atoms are
made of smaller particles called subatomic particles
Protons, neutrons and electrons.
Protons and neutrons are bound together at the
atom’s center to form the atomic nucleus
The nucleus makes up most of the atom’s mass
Orbiting the nucleus are tiny electrons.
Atoms are mostly
empty space
Atoms are mostly empty space- a core nucleus with
tiny electrons buzzing around it.
This is like our solar system.
Read page 263 to find out why
we don’t just pass through each
other if we are mostly empty
space.
Elements
Everything in the universe is made up of slightly
more than 100 types of distinct atoms called
elements.
The variety in matter is caused by combinations of
these atoms.
Of the 100 or so elements, only 90 are naturally found
in nature. The rest are man-made.
The periodic table
Element- def.- a material made up of only one type of
atom
All of the elements are listed in a chart called the
periodic table.
Each element is designated by a one, two or three
letter symbol that comes from letters of the
element’s name. Some of the names came from Latin
Ex- gold’s symbol Au comes from its Latin nameaurum; lead’s symbol Pb comes from plumbum
Elements & Table
Only the first letter in an element’s name is capitalized.
Cobalt is Co; CO means a compound made with carbon (C) and
oxygen (O).
A vertical column is called a GROUP. Atoms in the same group
have similar properties.
A horizontal row is called a PERIOD. As you go across a row, the
properties of matter gradually change. This change is called a
Periodic Trend.
Each period corresponds to one full cycle of a trend. For
example, one trend is that the size of atoms gradually decreases
in moving from left to right across any period. Atomic size is a
periodic (repeating) property.
14.3 Protons and
Neutrons
Proton- a particle in the atomic nucleus that
carries a positive electric charge.
Heavy- about 2000 times heavier than an electron.
In a neutral atom, the number of positively charged
protons equals the number of negatively charged
electrons orbiting around the atom.
The quantities are the same but the charges are
opposite
Example- oxygen has 8 protons and 8 electrons
Atomic Number
Each element is identified by its atomic
number
This is the number of protons in its
nucleus.
The number of protons makes the atom
what it is. If somehow the proton number
changes, so does the atom ( this is difficult
to do- nuclear fission or fusion)
Neutrons
Neutrons are also located in the nucleus
Neutrons are particles that have about the same
mass as protons but have no electric charge. They are
electrically neutral. (that’s how it got its name)
Electrons
Extremely small nuclear particles that have about
1/2000 the mass of a proton or neutron
They have an electrically negative charge (opposite
charge of proton)
They are the only particle that participate in chemical
reactions
They are arranged in electron shells
Nanotechnology
Please read the top of page 268 about
nanotechnology.
There will be worksheet questions and test questions
on this material
14.4 Isotopes and
Atomic Mass
An atom has a definite number of protons but its number
of neutrons can vary.
For example, there are three forms of hydrogenOne with 1 proton and 0 neutrons
One with 1 proton and 1 neutron
One with 1 proton and 2 neutrons
They all have the properties of hydrogen because they
have the same number of protons
These different forms where the neutron number varies
are called isotopes
Isotopes
Isotopes are identified by their mass number
Mass Number- the total number of protons and
neutrons they contain. So mass number is the total
number of nucleons (particles in the nucleus)
The average mass number for each atom is listed on
the periodic table. It is a fractional number because it
is a weighted average of all the isotopes and their
abundance in nature.
To use this number, round it to the nearest whole
number.
To write symbols and
isotopes
Using the atomic and
mass numbers
Atomic number = # of protons; # of electrons in
neutral atom
Mass number (rounded to whole number) =
# proton + # neutrons
Mass number – atomic number (protons) = # neutrons
Example: uranium
Atomic number 92 (92 protons, 92 electrons)
Mass number 238 238 – 92 = 146 neutrons
14.5 Electron shells
Electrons orbit the nucleus in distinct shells
Each shell can only hold a certain number of
electrons
There are at least seven shells and an atoms location
on the periodic table reflects how many shells it has.
Shell 1 – can hold only 2 electrons
Shell 2- 8
shell 5 – 18
Shell 3 – 8
shell 6 - 32
Shell 4 -18
shell 7 - 32
Valence electrons
The electrons in the outermost shell of an atom are called
the valence electrons
These are the electrons that are exposed to the
environment and are the first to interact other atoms.
They are the ones that participate in chemical bonding.
ATOMS COMBINE TO FORM MOLECULES, WHICH ARE
TIGHTLY HELD GROUPS OF ATOMS, BY WAY OF VALENCE
ELECTRONS. VALENCE ELECTRONS ARE THE ONLY ONES
THAT PARTICIPATE IN A REACTION.