Arnoldi
Chemistry
Atomic Theory One
Ch 4 Reading (sections 1 – 3)
Ch 5 Reading (sections 1 and 3)
4.1 Defining the Atom
1. Important Definitions
atom
smallest piece of an element that keeps the element’s identity
2. Why is it so hard to study atoms? (Look at the Question on page 92)
They are too small to be seen.
3. How did Democritus describe the atom?
indivisible and indestructible
4. What is one of the weaknesses of Democritus’ theory?
there was no evidence or experimentation to back it up
5. Dalton had several main ideas in his atomic theory:
All matter is (elements are) made of
All atoms of the same element are
completely different.
atoms which are indivisible.
identical. All atoms of different elements are
To make compounds, atoms of different elements combine
number ratios.
chemically in simple, whole
6. What made Dalton’s theory superior to Democritus’ theory?
He had experimentation to back up his ideas.
7. What tool helps us to see and manipulate atoms?
electron microscope
4.2
Structure of the Nuclear Atom
1. Atoms are not indivisible. They are broken down into several smaller particles that are collectively
called
subatomic particles.
2. Electrons have a negative charge.
They were discovered by JJ
Thomson using a cathode ray
tube. A cathode ray is a stream of electrons produced at the negative electrode ( cathode) of a
tube containing gas at low pressure.
3. How did Thomson figure out that the cathode ray had a negative charge?
The ray was attracted to positive charges (opposites attract) and repelled
from negative charges (likes repel).
4. Read the Build Connections box on page 96. do you have a CRT TV or computer monitor in your
house?
5. Positively charged subatomic particles are called
protons and were discovered by Goldstein.
6. James Chadwick discovered the neutral subatomic particle, which is called a
neutron.
7. Describe or diagram Rutherford’s experiment. What were the conclusions from this experiment?
Rutherford bombarded gold foil with alpha particles (positively charged Helium atoms).
Most of the alpha particles went right through the gold foil. A few were deflected.
Because most of the particles went through, Rutherford concluded that the atom was
mostly empty space.
Because some of the positive particles were deflected, he believed there was a small,
dense, positively charged center in the atom.
8. Today, how would you describe the structure of the atom? (look at the Key Question on page 99).
protons and neutrons are in the nucleus, electrons are distributed outside of the nucleus
4.3
Distinguishing Among Atoms
1. Definition
a. Atomic Number
the number of protons in an atom
b. Mass Number
the number of protons and neutrons in the atom
c. Isotopes
atoms of the same element that have the same atomic number
but different mass numbers
d. Atomic Mass
a weighted average of the atoms in a naturally occurring
sample of the element
2. What one factor distinguishes one element from another?
the number of protons
3. What is true for all neutral atoms? the number of protons
= the number of electrons
4. How can you determine the number of neutrons in an atom?
take the mass number minus the number of protons
5. Answer practice problem # 21 (a and b) on page 103 here:
a.
protons are 47, so electrons are also 47
neutrons = 108 – 47 = 61
b.
protons are 82, so electrons are also 82
neutrons = 207 – 82 = 125
6. How many protons, electrons and neutrons in Carbon – 13?
carbon has 6 protons on the periodic table, so there are also 6 electrons
neutrons = 13 – 6 = 7
7. Which subatomic particle stays the same in two different isotopes of the same element?
protons
8. Why do isotopes have different mass numbers?
they have different numbers of neutrons
9. Why is it called an ISOtope? (see the Build Vocabulary box on page 104)
iso is Greek for same, they have the same number of protons
10. What is the unit and its abbreviation that we use to measure the mass of an atom?
atomic mass units, amu
5.1
Models of the Atom
Pages 118 to 119 only
1. Where did Bohr place electrons in his atomic model?
in energy levels, orbiting around the nucleus
2. Look at the picture of the two ladders on page 119. What is this picture trying to illustrate
about the difference in energy between different energy levels?
the energy difference between levels 1 and 2 is not the same as the energy
difference between levels 2 and 3, etc.