Download ch. 4 atomic structure

CH. 4 ATOMIC STRUCTURE
4.1 The Structure of an Atom
GREEK MODELS OF ATOMS
Philosopher Democritus believed that all matter
consisted of extremely small particles that cannot
be divided.
He called these particles “atoms” from the Greek
word “atomos” which means uncut or indivisible
He believed there were different types of atoms
ex: liquids had round and smooth atoms and
solids had rough, prickly atoms
Aristotle did not think there was a limit to the
number of times matter could be divided.
DALTON’S ATOMIC THEORY
EVIDENCE FOR ATOMS
Dalton
determined that all compounds have
a fixed composition.
For example, a 100 gram sample of
magnesium combines with 65.8 grams of
oxygen.
A 10 gram sample of mangnesium
combines with 6.58 grams of oxygen.
The ratio remains constant.
DALTON’S THEORY
All matter is made up of individual
particles called atoms, which cannot be
divided.
4 MAIN POINTS OF
DALTON’S THEORY
All
elements are composed of atoms.
All atoms of the same element have the
same mass, and atoms of different elements
have different masses.
Compounds contain atoms of more than one
element.
In a particular compound, atoms of different
elements always combine in the same way.
THOMSON’S MODEL OF
THE ATOM
Some
materials gain the ability to attract or
repel other materials, this is referred to as a
positive or negative electric charge.
Object with opposite charges attract, or pull
together.
J.J. Thomson used an electric current to
learn more about atoms.
Thomson’s Experiments
DESCRIPTION OF
THOMSONS EXPERIMENTS
He
used a glass tube, with 2 metal disks on
each end with opposite charges. Each disk
had wire that connected it to an electric
current. When connected, a glowing beam
appeared in the space between the disks.
He hypothesized that the beam was made up
of charged particles that interact with the air
in the tube causing it to glow.
DESCRIPTION (CONT’D)
Thomson also concluded that the particles in the
beam had a negative charge because they were
attracted to the positive plate. (Remember
opposites attract)
He hypothesized that these particles came from
inside atoms. The following supported this:
– No matter what metal he used the particles produced
were identical
– The mass of the particles was about 1/200 the mass of a
hydrogen atom, the lightest atom of all.
THOMSON’S MODEL
An
atom does not have a – or + charge.
How can an atom contain – particles and
still be neutral?
In Thomson’s model, the atom contains
negatively charged particles evenly
scattered throughout a positively charged
mass of matter.
THOMSON’S MODEL
(CHOCOLATE CHIP ICE CREAM)
Chocolate Chips=negative
particles
Vanilla Ice
Cream=positively charged
mass of matter
By spreading out the
chocolate chips, their –
charges balance out the +
charges.
RUTHERFORD’S ATOMIC THEORY
Discovered that Uranium emits fast moving
particles that have a + charge.
He named these particles alpha particles.
Through the gold foil experiment, he concluded
that the + charge of an atom is not spread out like
Thomson had proposed.
Rutherford determined this + charge is
concentrated in a small, central area he called the
nucleus
The nucleus is a dense, positively charged mass
located in the center of the atom.
4.2 The Structure of an Atom
Properties of Subatomic
Particles
The 3 types of subatomic particles are:
1. Proton
2. Electron
3. Neutron
Protons
A
positively charged subatomic particle that
is found in the nucleus of an atom.
Each proton is assigned a charge of +1.
Some nuclei contain more than 100 protons.
Electrons
Greek
word means “Amber”
A negatively charged subatomic particle
that is found in the space outside the
nucleus.
Each electron has a charge of -1
Neutrons
A
neutral subatomic particle that is found in
the nucleus of an atom.
It has a mass almost exactly equal to that of
a proton.
Comparing Subatomic
Particles
Protons, electrons, and neutrons can be
distinguished by mass, charge, and location in an
atom.
Protons and Neutrons are similar in mass.
Electrons have a charge that is equal in size, but
opposite of the charge of a proton.
Neutrons have no charge.
Protons and Neutrons are located in the nucleus,
electrons are found outside the nucleus.
Atomic Number
Atomic Number of an element is equal to the
number of protons of that element.
For example, Hydrogen has 1 proton therefore the
atomic number is 1.
Atoms of different elements have different
numbers of protons.
The numbers are used to identify elements since
no two are alike.
Each + charge is balanced by a – charge, so the
atomic number is also equal to the # of electrons
in an atom.
Each atom of Hydrogen, also has 1 electron.
Mass Number
The
Mass Number is the sum of the protons
and neutrons in the nucleus of an atom.
Example
Aluminum has 13 protons
14 neutrons
27 Mass Number
o Number of neutrons= Mass #- Atomic #
Isotopes
All atoms do have the same number of protons
and electrons.
However, they do not have the same number of
neutrons.
Isotopes are atoms of the same element that have
different numbers of neutrons.
Isotopes have the same atomic # but different
mass #’s because they have different numbers of
neutrons.
Examples of an Isotope
Oxygen has 8 protons.
Some atoms of oxygen have 8 neutrons and a
mass # of 16.
Some oxygen atoms have 9 neutrons and a mass #
of 17.
These isotopes are referred to as Oxygen –16 and
–17.
Both can react to from H2O or combine with iron
to form rust.
4.3 Modern Atomic Theory
Bohr’s Model of the Atom
Bohr’s
model of an atom is similar to a
solar system with planets revolving around
a sun.
He agreed with Rutherford’s model of a
nucleus surrounded by a large volume of
space.
Bohr’s model is focused on the electrons.
Energy Levels
Bohr’s model suggests that electrons move with
constant speed in fixed orbits around the nucleus.
Each electron has a specific amount of energy.
If it gains or loses energy, the electron energy can
change.
These possible energies that electrons can have are
referred to as energy levels.
No two elements have the same set of energy
levels.
Energy Levels (cont’d)
An
electron can move from one energy
level to another when it loses or gains
energy.
It is possible for it to move more than 1
energy level.
The size of the jump determines the amount
of energy gained or lost.
Electron Cloud Model
An Electron Cloud is a visual model of the most
likely locations for the electrons in an atom.
Scientists use this to describe how electrons move
around the nucleus.
Think of an airplane propeller, when the plane
isn’t moving you can count the blades present,
however, when it starts to move the blades are still
there, they are just hard to locate at any specific
time.
Atomic Orbitals
The
electron cloud represents all the orbitals
in an atom.
An orbital is a region of space around the
nucleus where an electron is likely to be
found.
Atomic Orbitals (cont’d)
The lowest energy level has only one orbital
Higher energy levels have more than one orbital.
Energy
Level
1
# of Orbitals Maximum #
of Electrons
1
2
2
4
8
3
9
18
4
16
32
Electron Configurations
Electron Configurations are arrangement of
electrons in the orbitals of the atom.
The most stable configuration is the one in which
the electrons are in orbitals with the lowest
possible energies.
When this happens, the atom is said to be in its
“ground state”
If the electrons move to higher energy levels this
is called the “excited state”
The excited state is less stable, and eventually the
electron will lose energy and return to ground state