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Lecture Notes
Chapter 4-The Structure of the
Early Ideas of the Atom:
Democritus: atomos: small cells
Aristotle: Rejected the concept of
the atom
John Dalton: English teacher.
Dalton’s Atomic Theory:
B. Dalton’s Atomic Theory:
1. All matter is made-up of tiny indivisible particles
called atoms.
2. The atom is the smallest form of matter.
3. Atoms are represented by symbols called
4. Atoms are the foundation of the universe.
5. All atoms of the same elements will be the same.
6. All atoms of different elements will be different.
7. Atoms of different element can combine in definite
proportions to form more compounds
Dalton was right about most of his
theory. What part of Dalton’s Theory
is not correct with today’s
Answer: Atoms are divisible into small
subatomic particles.
A Model of the Atom
1. Sir William Crookes: Developed the
use of the cathode ray tube in the
study of the atom.
a. cathode ray tube: vacuum tube
with a (+) end called the anode and a
(-) end called the cathode.
b. Crookes study show movement
from the cathode (-) to the anode (+).
From -  +
2. JJ Thomson: Used the cathode ray tube to
show the charge to mass ratio of the stream
of light in the tube.
a. Compare the charge to mass ratio to other
known values
b. Concluded that the charged particles are
less than the mass of the lightest atom
c. Identified the first subatomic particle called
the electron.
3. Robert Millikan: Use
Thomson’s charge to mass
ratio to define the mass of an
a. Mass of electron = 1/1840 the
mass of hydrogen.
4. Ernest Rutherford: Gold Foil
a. The atom has a dense center
called the nucleus.
b. The nucleus has a positive
c. The positive charge is due to the
d. The atom is mostly empty space.
(diagram: Link)
5. James Chadwick: Discovered the
a. Chadwick was a student of Rutherford.
b. Discovered that the nucleus is heavier
than could be account for with just
c. The extra mass called the neutron and
had NO charge.
1. (no charge=neutral=neutrons)
Structure of the Atom
From the above models, the atom is known
to have the following basic points:
**Every atom of the same atom has the same
number of protons. THE NUMBER OF
**The number of neutrons can change from
atom to atom of the same element. This is
called an isotope, (same atom but different
number of neutrons).
Conclusion: The Atom
Within the nucleus:
*Protons: (+) charge; half the mass
of the atom; number is fixed
*Neutrons: NO charge; the other
half of the mass of the atom
Outside the nucleus:
* Electrons: (-) charge; in the
electron cloud about the nucleus;
NO mass
Conclusion: The Atom
• The number of protons NEVER changes for the
same atom. Found in the nucleus
• Elements are identified by the number of
• In a neutral atom (net charge = 0), the # of
protons = # of electrons
• All the mass of the atom is in the nucleus.
• The # of neutrons + the # of protons = the mass
• The # of protons = the atomic number
• Periodic Table
Formed with an unblanced between p+
and eCharged form of an atom brought about
by the gain of e- (-) or loss of e- (+)
Ions are formed:
1. response to another atom
2. metals form (+) ions
3. non-metal form (-) ions
4. unlike charges attract; like
charges repel.
Same atoms with different number of
neutron in the nucleus:
**Accounts for different masses for the
same atom.
**amu: atomic mass units, is the
average of the mass in one mole.
**Accounts for some radioactivity
Relative Abundance
How much is present based on the
amount of the sample given.
-It is the decimal form of the percentage
-Used to find the amu.
In a given sample of sulfur, 81.3% has a
mass of 32.45 g and 18.7% has a mass
of 30.39 g. What is the amu of S?
Textbook: #78 and #104
The spontaneous emission of
radioactive (high energy) particles.
Radioactivity is caused by a proton to
neutron ratio. The greater the
difference between the p+ and no, the
more unstable.
An unstable atom will emit energy until
a more stable form is reached. This
is called radioactive decay.
Types of Radiation, page 122
• Alpha radiation: (α), contains two
protons and two neutrons giving a 2+
charge. Note that the mass and the
atomic number changes.
• Beta radiation: (β), contains a single
electron and carries a 1- charge. Note
the mass does not change.
• Gamma radiation: (γ), Very high
energy particle with no mass and no
Radioactive Questions
1. Why are some atoms radioactive/
2. Write the symbols used to denote alpha,
beta, and gamma radiation. Write the
mass and charge of each.
3. What is the primary factor that
determines if a nucleus is stable or
4. Boron-10 emits alpha particles and
Cesum-137 emits beta particles. Write a
balance equation.