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Chapter 3 – Atoms and Moles
Answer Key
1. What were Empedocles’s, Democritus’s, and Aristotle’s atomic theories of matter? Be sure
to discuss pros and cons of each.
a. Empedocles started the 4-element theory of earth, air, fire, and water. It was simple, popular, and easy
to understand, but incorrect
b. Democritus originated the atomic theory. His atomic theory was simple, unpopular, and also
incorrect.
c. Aristotle supported the 4-element theory and added the characteristics of moist, dry, hot, and cold to
the elements. He also originated the concept of transmutation of the elements. His theory was simple,
popular, and grossly incorrect.
2. What was Dalton’s 5 part atomic theory of matter and his model of the atom?
a. All matter is composed of extremely small particles called atoms, which cannot be
subdivided, created, or destroyed.
b. Atoms of a given element are identical in their physical and chemical properties.
c. Atoms of different elements differ in their physical and chemical properties.
d. Atoms of different elements combine in simple, whole number ratios to form compounds
e. In chemical reactions, atoms are combined, separated, or rearranged but never created,
destroyed, or changed.
3. Describe how Thomson’s CRT experiment led to the discovery of the electron. Describe a
CRT and Thomson’s model of the atom.
a. The electron was discovered using cathode rays. English physicist, J.J. Thomson pumped
most of the air out of a glass tube and applied a voltage to two metal plates, called electrodes,
which were placed at opposite ends of the tubes. The anode was attached to the positive
terminal, so it has a positive charge. The other electrode, called a cathode had a negative
charge because it is attached to the negative terminal of the voltage source. The glowing
beam that originated at the cathode and struck the anode was made of negatively-charged
particles, called electrons.
b.
4. How is the atomic mass on the periodic table determined?
a. Atomic mass = #p+ + #n0, or it is the weighted average atomic mass of all the isotopes of an element
5. Explain Rutherford’s Gold Foil Experiment, what he discovered, and his model of the atom.
a.
b. The nucleus is the dense, central portion of the atom. The nucleus is made up of protons and
neutrons. The nucleus has all of the positive charge, nearly all of the mass, but only a very small
fraction of the volume of the atom
c.
6. What led to the discovery of the neutron? Who discovered the neutron?
a. The mass of an atom was known to be greater than the mass of the protons, except for hydrogen.
James Chadwick discovered the neutron.
7. What is atomic number? Mass number? An isotope?
a. Atomic Number (Z) is the number of protons in the nucleus of an atom and is found on the periodic
table
b. Mass number is the sum of the protons and neutrons of an isotope of an element; it is also the
weighted average atomic mass of all of the isotopes of an element; it is found on the periodic table
c. An isotope is an atom with the same number of protons, but differing numbers of neutrons. For
example, there are three isotopes of carbon, carbon-12, carbon-13, carbon-14. They all have 6
protons, but differ in their numbers of neutrons.
8. Differentiate between electrons, protons, and neutrons with relation to their charge, size
and location in the atom.
Electron
Negative
Small
Orbitals
Proton
Positive
1 amu
Nucleus
Neutron
No charge
1 amu
Nucleus
9. What is the law of definite proportions?
a. The law of definite proportions states that two samples of a given compound are made of the same
elements in exactly the same proportions by mass regardless of the sizes or sources of the samples.
10. What is the law of conservation of mass?
a. The law of conservation of mass states that the mass of the reactants in a reaction equals the mass of
the products. This is further evidence that matter is composed of atoms, since atoms would not be
destroyed in a chemical reaction.
11. What is the law of multiple proportions?
a. Chemists discovered that if two or more different compounds are composed of the same elements, the
ratio of the masses of the second element is always a ratio of small whole numbers. This example
illustrates the law of multiple proportions
12. How did Bohr change Rutherford’s model of the atom?
a. According to Bohr’s model, electrons can be only certain distances from the nucleus. Each distance
corresponds to a certain quantity of energy that an electron can have. The difference in energy
between two energy levels is known as a quantum of energy.
b.
13. What kind of relationship does frequency and wavelength have?
a. c = λν
14. What kind of relationship does energy and frequency have?
a. E = hν
15. What do the quantum numbers tell you?
a. A quantum number is a number that specifies the properties of electrons
16. How does an electron get excited?
a. An electron gets excited by absorbing energy.
17. What is a mole?
a. A mole is a number of things, it is equal to 6.02 x 1023 things
18. List and describe the 4 Quantum numbers. Be sure to include the name, its symbol, and
what it describes about the orbital.
•
The principal quantum number, symbolized by n, indicates the main energy level occupied by the
electron.
•
•
Values of n are positive integers, such as 1, 2, 3, and 4.
•
As n increases, the electron’s distance from the nucleus and the electron’s energy increases.
The main energy levels can be divided into sublevels. These sublevels are represented by the
angular momentum quantum number, l.
•
This quantum number indicates the shape or type of orbital that corresponds to a particular
sublevel.
•
A letter code is used for this quantum number.
•
•
l = 0 corresponds to an s
•
l = 2 to a d orbital
orbital
•
l = 3 to an f orbital
l = 1 to a p orbital
•
•
The magnetic quantum number, symbolized by m, is a subset of the l quantum number.
•
It also indicates the numbers and orientations of orbitals around the nucleus.
•
The number of orbitals includes
•
one s orbital
•
three p orbitals
•
five d orbitals
•
seven f orbital
The spin quantum number, indicates the orientation of an electron’s magnetic field
relative to an outside magnetic field. A single orbital can hold a maximum of 2 electrons,
which must have opposite spins.
19. What is the electron configuration of the following elements: K, Ba, Kr, P, Ag.
a. K = [Ar] 4s1
b. Ba = [Xe] 6s2
c. Kr = [Ar] 4s2 3d10 4p6
d. P = [Ne] 3s2 3p3
e. Ag = [Kr] 5s2 4d9 or [Kr] 5s1 4d10
20. Define the 3 basic forces of nature. Which is the strongest force and which is the
weakest force?
a. Nuclear force – the strongest force that binds the nucleus of an atom together and is what
is harnessed in nuclear reactions
b. Electrostatic force – the medium force of charged particles. Coulomb’s law governs this
force
c. Gravity – this is the weakest force, caused by an object’s mass. It is always attractive
21. Calculate the energy and frequency of a wave with the wavelength of
5.9 x 10-19m.
c = λν, 3.0 x 108 m/s = (5.9 x 10-19m) x ν
a.
ν = 5.08 x 1026 1/s
b. E = hν
E = (6.626 x 10-34 J s) (5.08 x 1026 1/s) = 3.37 x 10-7 J
22. Describe the mechanics and lessons of the glass discharge tubes we looked at
with the diffraction glasses.
An electron in a hydrogen atom can
move between only certain energy
states, shown as n = 1 to n = 7.
In dropping from a higher energy
state to a lower energy state, an
electron emits a characteristic
wavelength of light.
The same was true for the other
elements observed. Each had a
unique spectrum, resulting from a
unique electron configuration.
23. How many moles is 10 grams of Nitrogen?
a. 10 g N2 / 28.02 g/mol = 0.357 mol N2
24. How many grams is 0.5 moles of calcium?
a. 0.5 mol Ca x 40.08 g/mol = 20.0 g Ca
25. How many atoms is 6 moles of neon?
a. 6 mol x 6.02 x 1023 atoms/mol = 3.61 x 1024 atoms
26. How many moles is 5.2 x 1021 atoms of lithium?
a. 5.2 x 1021 atoms Li / 6.02 x 1023 atoms/mol = 0.00864 mol Li
27. Calculate the average atomic mass for magnesium. It has 3 isotopes: Mg-24 has
% abundance of 78.99, Mg-25 has a % abundance of 10, and Mg-26 has %
abundance of 11.01.
(24 x 0.7899) + (25 x 0.10) + (26 x 0.1101) = 18.96 + 2.5 + 2.86 = 24.32 g/mol or 24.32 amu