ATOMIC STRUCTURE
In this unit we will be describing elements and compounds. Both are made from
atoms. Therefore by studying the atom (the basic building block of the universe) we
study elements and compounds.
MODELS
A model is anything that represents the properties of another object or system. Some models are
miniature copies of the things they represent, while other models represent things that can-not be
seen. The model has been changed many times by many different scientists. As each new
discovery occurred it described the atom with better precision and understanding.
DEVELOPMENT OF ATOMIC MODELS
We know that matter is anything that has mass and volume but what is matter
made of? Atoms cannot actually be seen. We rely on _________________________
and create _________________ to explain our observations.
EARLY GREEK MODEL: Read pages 111-118 in PS textbook to fill in blank spaces.
The ancient Greeks believed that all matter was made up of 4 elements,
____________, ____________, ____________, and_______________________.
Democritus is credited with providing the term atom. He believed that matter
could only be broken down so far and eventually the smallest piece of matter would be
obtained. He described this piece as _____________, which means, “not to be cut” or
“indivisible.” We now call it the atom. (HINT: good essay question on a test)
IT TOOK OVER 2000 YEARS FOR THIS TO BECOME ACCEPTED.
THOMPSON’S MODEL
He described his model as a pudding where positive and negatively charged
particles were floating around like raisins in plum pudding. He is credited for discovering
the __________________. Thompson knew that electrons are ____________charged.
He also knew that all atoms must be _________. So he concluded that there must be
something ___________________ also in an atom. He was never able to find it though.
RUTHERFORD’S MODEL
He is credited for finding the positively charged particle that Thompson could not
find. He performed a very famous experiment called____________________________.
He fired a stream of negatively charged particles at a very thin sheet of gold foil.
Surrounding the gold foil was a screen to detect these particles.
Diagram:
If Thompson was correct in his theory the negatively charged particles would
pass right through “the pudding” like “bullets” through tissue paper” with only a slight
angle of deflection.
What actually occurred was some particles passed through with ____________
deflection at all. Even more amazing was that some particles were deflected almost
_______________________as if it hit something solid. Therefore Rutherford concluded
that the positively charged particles were clustered together in a __________ packed
center and called it the ______________________. The __________________ are
scattered outside near the____________________ of the atom. He also stated that
atoms were mostly__________________________________. (HINT: KNOW THAT)
BOHR MODEL
Bohr focused on finding where in the atom ____________________ are located.
He stated that electrons orbit the nucleus in specific paths like planets orbit the sun.
Energy determines the amount of distance an orbit can be from the nucleus.
________________ energy is required the farther away the orbital or shell is from the
nucleus. Therefore an electron that is farther away from the nucleus the more energy it
must have.
EARLY MODERN MODEL
The modern model is based primarily on Rutherford and Bohr’s model. The main
difference is the location of electrons. Electrons actually travel in random paths at
certain distances away from the nucleus. The exact location of the electron is
impossible to know (right now). However using very complicated formulas we can
predict the probability of where to find them. There is also densely packed center called
the nucleus where protons and neutrons are found.
Summary of the modern model
All elements are composed of_______________. All atoms of the same element
are alike in those________________________________ that determine the chemical
properties of the element. However they can (and do) have different_______________.
Atoms of different elements are ______________________. In particular they have a
different number of ______________________. Atoms have detailed structure that
temporarily changes during a _____________. Compounds are formed when 2 or more
atoms are joined together. They are joined in a ______________.
Atoms can be broken down physically (not chemically). The particles no longer
have the same chemical composition as the original atom. These particles are
called____________________________________. They are the _______________,
_____________________, & ______________________.
Nucleons are particles that make up the ____________________ of an atom.
They are the Proton and the Neutron. Protons have a ____________________ charge.
Neutrons have no charge. They are said to be ________________. Electrons are
outside the nucleus and have a ________________________ charge.
The mass of subatomic particles is measured in units called A.M.U. This stands
for Atomic Mass Units. An A.M.U. is a standard measurement used in order to measure
the mass of such small items. It is based on the mass of the carbon atom. A carbon
atom has a mass of 12. So an A.M.U is defined as 1/12th the mass of a carbon atom.
Protons have a mass of 1 AMU. Neutrons also have a mass of 1AMU. Electrons
have a mass of 1/1836th of a proton. That is to say that an atom would need to have
1836 electrons to equal the mass of 1 proton. When calculating the mass of atom
electrons are considered not to add any additional mass.
PARTICLE
MASS
LOCATION
CHARGE
PERIODIC TABLE
Atomic number is defined as the number of_______________________ only. So
to find the number of protons find the atomic number.
THE NUMBER OF PROTONS NEVER CHANGE IN
A STABLE (NORMAL) ATOM
It just so happens that the number of protons must equal the number of
_________________. This is to ensure that all atoms remain neutral. This is also why
the neutron must remain neutral.
The mass number (atomic mass) is the mass of the atom. The mass is due to the
mass of all the protons plus all the neutrons.
Atomic Mass = # of Protons + # of Neutrons
Therefore to calculate the Atomic mass add the # of protons plus the # of
neutrons.
To calculate the number of Neutrons:
# of Neutrons = Atomic mass – Atomic Number
Remember the atomic number is the # of protons.
Isotopes
Isotopes are atoms of the same element having different masses due to a
different number of____________________.
Example:
Atomic mass is really the average atomic mass. It is the_________ of all the known
______________ based on ____________________
Example
ELECTRONS
Remember the number of electrons must equal the number of protons in an
atom. Electrons cannot be located exactly. We use models to represent the area of
highest probability of finding an electron (remember the cubes with dots). These areas
are called _______________________________, __________________________,
_______________________or________________________________________.
For simplicity reasons we will use the Bohr model to show the location of the
electrons. However it should be noted that this model is not specific and the current
models can locate electrons much more exactly (ORBITAL MODEL). Both models are
based on probability.
Each Principal Energy Level (shell,orbit etc) shows where the electrons are.
Remember the location is based on energy. Electrons can circle the nucleus anywhere
inside these areas. They may also change shells. With these changes energy is
released or absorbed by the electron. Electrons are said to be in the________________
when they absorb energy. Electrons cannot circle in the areas between the shells.
Each Shell can accommodate only a very specific number of electrons.
ACCORDING TO THE BOHR MODEL
1st Principal Energy Level or K Shell = 2 electrons
2nd Principal Energy Level or L Shell = 8 electrons
3rd Principal Energy Level or M Shell = 8 electrons
4th Principal Energy Level or N Shell = 8 electrons
Drawing Atoms Using the Bohr Model
Ions- If an atom loses or gains electrons the atom now has a _______________ and is
called an ion.
A positive charge is caused by ___________ electrons
A negative charge is caused by ___________ electrons
Examples:
RADIOACTIVITY:
The atom no longer remains stable. The Nucleus breaks down and releases
particles and energy. The atom can change and become a different element. This is
called___________________________.
Alpha Decay:
Beta Decay:
Gamma Radiation:
FORCES WITHIN THE ATOM
Electromagnetic Force:
Oppositely charged particles attract each other. Same charge particle repel each
other.
Strong Force:
This is the strongest force. Over powers the electromagnetic force and allows
protons to stay together. Only works when protons are close together and have limited
range.
Weak Force:
This is responsible for radioactive decay (the breakdown of an atom). Then sun
is powered by the weak force. (not actually the weakest force)
Gravity
This is the weakest force. All objects in nature attract each other. The role of
gravity in the atom is uncertain.
LAW OF CONSERVATION OF MATTER
Matter exists and cannot be made from nothing. Also once matter exists it must
always exist. This is called the Law of Conservation of Matter. It states:
______________________________________________________________________
______________________________________________________________________
The law of conservation of matter demands that all matter must be accounted for
in a chemical reaction. Therefore all chemical equations must be _________________.
Balancing equations is done so that the law of conservation of energy is obeyed.
HINT: you are responsible for knowing why equations are balanced and
knowing whether an equation is balanced or not.
Example:
CHEMICAL EQUATIONS
Chemical equations show chemical reactions. During a chemical reaction
substances are ______________________________________________________.
Examples: