Download Chapter 7 Models of Atomic Structure

Chapter 7 Models of Atomic Structure
Grade 9 Science
Class Notes For Chapter 7
Atom
- Is the smallest particle of an element that is still that
element.
- contains subatomic particles called protons and
electrons.
Nucleus
- a tiny core that is very small in volume dense compared
to the rest of the atom and intensely positive.
An Electron Cloud
- “envelope” that is very large in volume. Light compared
to the nucleus and negatively charged. The electron
cloud id the region of the atom where the electrons move
rapidly around the nucleus.
An Atom is the smallest particle of an element that is still that
element.
Rutherford’s model of an atom:
Rutherford’s model describes a nucleus which is a tiny core that
is very small in volume, dense compared to the rest of the atom,
and positively charged. Also that there is an “envelope” that is very
large in volume, light compared to the nucleus, and negatively
charged.
Bohr model of the atom:
Bohr’s model compared the nucleus to the Sun, and the electrons
to the planets . The Sun exerts an enormous gravitational pull on
the planets, but they do not spiral inward and crash. Why? Because
they revolve at just the right speed to remain in their orbits.
Similarly, the atom’s positive nucleus exerts a strong force of
attraction on the negative electrons. The electrons do not spiral
inward and crash, however, because they are moving rapidly in
fixed regions around the nucleus. These regions are threedimensional and sphere-like. For this reason, they are called
electron shells. (page 242-textbook).
Bohr felt the nuclear model was accurate as far as it went, but was
incomplete.
Therefore, scientists today agree upon the Bohr-Rutherford
Model as a representative model for the structure of an atom. The
Bohr-Rutherford model of an atom uses aspects of both models.
The Bohr-Rutherford Model of an atom can be called the first
“modern” view of the atom.
The central part of an atom is called the Nucleus.
The nucleus is made up of protons and neutrons.
Protons
- have a positive charge
Neutrons
- have no charge
Electrons
- negatively charged particles
The Atomic Number of an atom is the number of protons it has.
The Atomic Mass or Weight of the atom is total number of
protons and neutrons in atoms nucleus.
The number of neutrons of an atom can be determined by
subtracting the atomic number from the atomic mass (which is
the total number of proton and neutrons in the nucleus) of an
element.
What is radioactivity?
The spontaneous emission of radiation by an atom.
Three types of radiation were discovered by Ernest
Rutherford:
a particles
b particles
g rays
Protons and electrons are the only particles that have a
charge.
Protons and neutrons have essentially the same mass.
The mass of an electron is so small we ignore it.
What are isotopes?
Atoms of the same element with different masses due to the fact
Isotopes have different numbers of neutrons.
For example:
1
H
1
2H
1
3H
1
This Hydrogen atom has one proton, 1 electron and 0
Neutrons.
This Hydrogen atom has one proton, 1 electron and 1
Neutrons.
This Hydrogen atom has one proton, 1 electron and 2
Neutrons.
Charges on Atoms:
An ion is an atom, or group of atoms, that has a net positive
or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
For Example:
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
Cl
17 protons
17 electrons
Cl17 protons
18 electrons
Al
13 protons
13 electrons
Al 3+
13 protons
10 electrons
Remember:
The number of electrons is the same as the number of
protons. When there is a charge present that indicates either
more electrons (a negative charge) or less electrons (a
positive charge).
Be sure to Read through the Power Point Presentation
available on the school home site, teachers page: Tlomond,
for Bonus question material.
Bohr Model or Electron Shell diagram:
Be able to draw an electron shell diagram (Bohr Model) for
any given element on the periodic table up to Ar with an
atomic number of 18.
Be able to explain the 2-8-8 Rule for drawing an electron
shell diagram (Bohr Model).
The 2-8-8 Rule states that there can only be a maximum of
two electrons in the first shell of an atom, then there can
only be a maximum of eight electrons in the second shell of
an atom and there only be a maximum of eight electrons in
the third shell of an atom.
To help further your understanding of the material covered
in chapter 7. Refer to your class notes, the notes available
here as well as the pages in the text, SciencePower 9, 229,
234, 235, 237, 242-243, 246-248.