Name
Date
Period
Chapter 9: Lesson 4- Models of Atoms
How Did Atomic Theory Develop?
1. COMPARE AND CONTRAST: How is the cloud model of the atom different from Bohr’s model?
2. EXPLAIN: How did Dalton’s atomic theory lead to the different model scientists have today?
3. Describe the modern model of the atom.
Understanding Main Ideas
Answer the following questions in the spaces provided.
4. What three particles are found in an atom?
5. Which two particles are found in an atom’s nucleus?
6. Explain why scientists use models to study atoms.
7. Which two particles in an atom are equal in number?
8. How are elements identified in terms of their atoms?
9. What two particles account for almost all of the mass of an atom?
Building Vocabulary
Fill in the blank to complete each statement.
10. Atoms of the same element that have the same number of protons but different numbers of neutrons
are called _________________.
11. The sum of protons and neutrons in the nucleus of an atom is the _____________________________.
12. An electron’s movement is related to its _______________________________________.
Read the passage and study the figure below it. Then to answer the questions that follow the figure.
Exactly How Small Is It?
Measuring the size of an atom is not easy. For one thing, an atom is very, very tiny. Scientists, however,
have developed several ways to estimate the relative sizes of atoms. For elements that exist as two
identical molecules bonded together, such as oxygen and hydrogen, scientists can use a technique called
X-ray diffraction to estimate the distance between the nuclei. Once scientists do that, they can calculate the
atomic radius, which is one-half the distance between the nuclei.
It’s important to remember that the atomic radius is not a measurement of a single atom’s size but only its
size relative to other atoms. In other words, scientists know that oxygen atoms are larger than hydrogen
atoms, but they don’t know the exact size of any single atom of oxygen. When comparing sizes of atoms,
one must be careful to compare sizes based on similar measuring techniques.
The figure below shows the atomic radii of several elements. The atomic radius is given in units of
picometers (pm). One picometer is equal to 0.000000000001 meter.
13. What determines the size of an atom?
14. Why is it difficult to measure the size of an atom?
15. What is the distance between nuclei in a hydrogen molecule (H2)?
16. What would be the distance between nuclei of a bromine molecule (Br2)?
17. What is the atomic radius of oxygen in meters?
Write the letter of the correct answer on the line at the left.
18. ____ The positively charged particle in an
atom’s nucleus is the
19. ____ An element’s identity can be determined
from its
A electron
a
atomic number
B neutron
b
number of neutrons
C proton
C number of isotopes
D isotope
d
20. ____ The smallest particle an element can be
divided into and still be that element is the
a
electron
b
neutron
C isotope
d
atom
energy levels
21. ____ The model of the atom that described
electrons scattered throughout a ball of positive
charge was proposed by
a
Niles Bohr
b
Ernest Rutherford
C J. J. Thomson
d
John Dalton
If the statement is true, write true. If the statement is false, change the underlined word
or words to make the statement true.
22. ____________________ An element’s mass number tells the number of protons in its
nucleus.
23. ____________________ Negatively charged particles in an atom are called electrons.
24. ____________________ The cloud model of the atom describes the location of electrons as
specific orbits around the nucleus.
25. ____________________ Atoms with the same number of protons but different numbers of
neutrons are called isomers.
26. ____________________ The sum of the protons and neutrons in an atom is called the atomic
number.
27. ____________________ An object that helps explain ideas about the natural world is called a
model.