Download Atoms

Atoms
Atoms are everywhere and they determine all
the properties of the matter they compose.
The Atomic Theory:
Today we know that matter is made up of
atoms, but it has taken a long time to realize
that.
The beginning of the Atomic Theory:
Democritus: (400 BC)
-Greek philosopher who suggested the universe
is made of indivisible units.
•He called the units atoms.
•”atom” comes from the Greek word atomos
meaning unable to be cut or divided.
-He thought movements of atoms caused the
changes in matter that could be observed.
Although Democritus’ theory explained some
observations, he didn’t have the evidence
needed to convince people that atoms existed.
John Dalton:
-English schoolteacher who revised the Atomic
Theory in 1808.
-Dalton agreed with Democritus that atoms
couldn’t be divided.
According to Dalton, all atoms of a given
element were exactly alike, the atoms of
different elements could join to form
compounds.
- Unlike Democritus, Dalton based his theory
on experimental evidence by using the Law
of Definite Proportions.
Law of Definite Proportions: states a chemical
compound always contains the same elements
in exactly the same proportions by weight or
mass.
- Dalton’s theory is considered to be the
foundations for the modern atomic theory.
• Some parts were correct
• His theory could not explain all
experimental evidence.
- Over time and experiments, Dalton’s theory
changed slightly.
Thomson’s Model of the Atom:
1897 – J.J. Thomson (British scientist) suggested
that atoms were NOT indivisible.
- He wasn’t experimenting on atoms, he was
experimenting on electricity. (Studying
cathode rays – “mysterious” rays in vacuum
tubes.)
Thomson’s cathode-ray tube experiment
suggested that cathode rays were made of
negatively charged particles that came from
inside atoms.
- This revealed atoms could be divided into
smaller parts.
- Thomson discovered electrons.
- Thomson proposed a new model of the
atom based on his discovery.
• in his model, electrons were spread
throughout the atom. (like blueberries in a
muffin)
Rutherford’s Model of the Atom:
Shortly after Thomson proposed his model,
Ernest Rutherford (British Scientist) designed an
experiment to test Thomson’s model.
- Rutherford found that Thomson’s model
needed to be revised.
- Rutherford proposed that most of the mass of the
atom was concentrated in the atom’s center!
•
This conclusion was based on the results
of his experiment.
- Rutherford discovered the nucleus.
o His experiment suggested that an
atom’s positive charge is concentrated in
the center of the atom.
o This positively charged, dense core is
called the nucleus!
In Rutherford’s model, negative electrons orbit
the positively charged nucleus in the same way
the planets orbit the sun.
**See Figure 7
Structure of Atoms:
What is an atom?
Atoms are made up of various subatomic
particles.
The three main subatomic particles are
distinguished by mass, charge, and location in
the atom.
** See Figure 1
- At the center of the atom is a small, dense
nucleus.
Inside the nucleus:
Protons-
Positive Charge (p+) = 1 amu
Neutrons- No Charge
(n0) = 1 amu
• p+ and n0 are almost identical in size
and mass. Mass is measured in
Atomic Mass Units (amu)
- Moving around outside the nucleus is a
cloud of very tiny, negatively charged
electrons (e-)
•
The mass of an e- is much smaller than
that of a p+ or n0
Characteristics of Atoms:
1. Each element has a unique number of protons.
- An element is defined by the number of p+
is in an atom.
- As you move numerically through the
Periodic Table, the protons increase by 1 for
each element.
2. Unreacted atoms have no overall charge.
(also called…Neutral Atoms)
- Even though protons and electrons in atoms
have electric charges, most atoms do not
have an overall charge.
o This is because most atoms have the
same number of p+ and e- ….therefore
the charges cancel out.
- If an atom gains or loses e- it becomes
charged.
o A charged particle is called an ion.
3. Positive and Negative charges attract each other
inside the atom.
-This force is called Electric Force.
-p+ and e- attract each other by electric force.
- Electric force holds the atom together.
•this is the force that holds solids and
liquids together.
Atomic Number and Mass Number
Atoms of different elements have their own
unique structures.
Because atoms have different structures they
have different properties.
Atoms of each element have the same number of
protons, but they can have different numbers of
neutrons.
The atomic number of an element tells you how
many protons are in an atom of an element.
- In neutral atoms, the atomic number equals
the number of electrons.
- Each element has a unique number of
protons, therefore each has a unique atomic
number.
Mass Number equals the total number of
subatomic particles in the nucleus.
-Mass number equals the number of protons
plus the number of neutrons in an atom of
the element.
Ex. Fluorine
Atomic Number = 9
Mass Number = 19
** Number of Neutrons = MN – AN
Protons = 9
Neutrons = 10
- the mass number represents the number of
protons and the neutrons…NOT
ELECTRONS! Because protons and
neutrons provide most of the mass of the
atom.
Isotopes
Atoms of an element always have the same
Atomic number, but not always the same mass
number
How can that be?
If protons change…you change the element.
However, the number of electrons can change
and the number of neutrons can change.
If neutrons change…then an isotope is formed.
Isotope: elements that vary in mass
numbers because their number of neutrons
differ.
-isotopes have the same number of protons,
but a different number of neutrons relative
to other atoms of that element.
Some isotopes are more common than others.
Ex. Isotopes of Hydrogen
1. protiummost common
(no neutrons)
2. deuterium- (one neutron)
3. tritiumleast common
(two neutrons)
-to represent different isotopeswrite the mass number and atomic number
of the isotope before the symbol of the
element.
Ex:
Mass Number
35
17
17 protons
17 Electrons
35-17 = 18 neutrons
Cl
Atomic Number
• if you know the mass number and the
atomic number you can calculate the
neutrons.
Mass Number – Atomic Number = Neutrons
***An element’s average atomic mass refers to the
weighted average of all the masses of all the naturally
occurring isotopes of that element! ***
Atomic Masses
The mass of a single atom is very small.
-because working with such tiny numbers is so
difficult, atomic masses are usually expressed in
atomic mass units. (AMU)
-since there are so many isotopes of each
element, the mass number listed in the periodic
table is an average of all the isotopes in nature.
How do we measure/ count atoms since they are so
small?
WE USE MOLES!
Mole is the unit used to count particles.
- a mole is a collection of a very large number
of particles.
1 mole = 602 213 670 000 000 000 000 000
particles
(6.02 x 1023)
• this is called Avogadro’s Number
• named for the Italian scientist
Amedeo Avogadro
- moles and grams can be related.
• 1 mole of a substance is equal to the
Atomic mass from the periodic table.
Converting Moles to Grams to Particles:
1 mole
6.02 x 1023
Atomic Mass
particles
(g)
(Avogadro’s Number)
Compounds also have molar mass.
- to calculate the molar mass of a compound
add up the mass number of all the atoms in
the compound.
Modern Atomic Theory
Dalton’s theory that atoms are indivisible
had to be revised after the discovery that atoms
are make of protons, neutrons, and electrons.
Modern Models of the Atom:
The modern model of the atom is very different
from Rutherford’s model.
- in modern atomic models, electrons can be
found only in certain energy levels, not
between levels.
- Also, the location of electrons cannot be
precisely predicted.
1913- Niels Bohr
-Danish physicist
- suggested that the energy of each electron
was related to the path it follows around the
nucleus.
•
electrons can only be found in certain
levels.
•
electrons must gain energy to move to a
higher level
•
electrons must lose energy to move to a
lower level.
Ex. If the atom is an apartment building
with no stairwells, if the nucleus is in the
basement then electrons would gain energy
by riding up the elevator and lose energy by
riding down the elevator.
-Electrons can be on any floor but not in
between.
By 1925, Bohr’s model of the atom no longer
explained all aspects of electron behavior.
A new model was devised- which no longer
assumed that electrons orbited the nucleus like
planets orbiting the Sun.
- according to the new model, electrons
behave more like waves.
The exact location of an electron can’t be
determined.
- the best scientists can do is to calculate the
chance of finding an electron in a certain
place within an atom.
- best way to show them is in “orbitals”
(levels)
Energy Levels:
Within the atom, electrons have various
amounts of energy and exist on different energy
levels.
- there are many possible energy levels that
electrons can occupy.
- Each energy level holds a certain number of
electrons.
- The number of energy levels that are filled
in an atom depends on the number of
electrons.
o See figure 3
- the electrons in the outer energy level are
called valence electrons
valence electrons determine the chemical
properties of an atom.
Electron Transitions:
As we know, the modern model of the atom
limits the locations of electrons to specific
energy levels. They are never found between
levels.
- instead, it “jumps” from one level to the
next.
Why do they move?
- electrons jump between levels when an
atom gains or loses energy.
- The lowest state of energy of an electron is
called the ground state.
- At normal temperatures, most electrons are
in the ground state.
- If an electron gains energy, it moves to an
“excited state”
o It gains energy by absorbing a particle
of light called a photon.