Download Lesson 4: Atomic Structure

Lesson 4: Atomic Structure
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Define the word atom
Describe the four points of Dalton's atomic theory of matter
Identify and describe the two kinds of electrical charges
Describe how particles with the same charge affect each other
Describe how particles of different charges affect each other
Discuss how atoms are related to electricity
Explain how cathode rays and radioactivity are related to atomic structure
Explain how Rutherford's experiment showed the existence of the nucleus
Describe alpha, beta, and gamma radiation
Name and describe the three subatomic particles
Determine the number of protons, neutrons, and electrons in an atom or ion
Explain how an ion differs from an atom
Determine number of subatomic particles from the symbol of a nuclide
Identify isotopes
Calculate average atomic mass from relative abundance
Describe the changes that accompany nuclear reactions
Define radioactivity
Perform calculations with half-lives
Atomic Structure, Atomic Number, Mass Number
The atom is the basic unit of matter. It is the smallest particle of an element that
still has the characteristics of that element. Every atom has a positively charged
central nucleus, composed of positively charged protons (1+) and uncharged
neutrons. The nucleus is surrounded by a number of negatively charged electrons (1-).
Like magnets, particles of opposite charge are attracted to one another, and particles with
the same charge are repelled from one another. The structure of the atom that we are
familiar with today has a long history to which many people contributed. The discovery of
these particles and their arrangement, relative masses, and charges required many
experiments.
The word atom comes from the Greek word atomos, meaning "indivisible," and indeed the
idea that all matter is composed of tiny, indivisible particles goes back to ancient Greece.
The first scientist in modern times to revive this idea was a British scientist named John
Dalton. In 1803, after observing that elements in a given compound always exist in the
same proportions, Dalton formulated his atomic theory of matter. The theory had
four main points:
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Each element is composed of extremely small particles called atoms
All atoms of a given element are identical; the atoms of different elements
are different and have different properties (including different masses)
Atoms of an element are not changed into different types of atoms by
chemical reactions; atoms are neither created nor destroyed in chemical
reactions
Compounds are formed when atoms of more than one element combine; a
given compound always has the same relative number and kind of atoms.
Michael Faraday was an English chemist who, in 1832-33, carried out a series of
experiments attempting to use electricity to isolate elements from known
compounds. His work led him to discover that the amount of electricity applied to a
sample compound is related to the amount of an element that is isolated. He
concluded that the structure of the atom must play a role in this.
Building on Faraday's experiments, English physicist J. J. Thompson discovered that
electricity consisted of tiny subatomic particles called electrons. In 1897, Thompson was the
first person to measure the mass-to-charge ratio of a single electron. He found that the
particles he observed were common to all atoms. Thomson also knew that the negative
charge he measured must be balanced by a particle of positive charge, since there was no
overall charge on an atom. Thomson devised his own model of the atom, which is often
described as the blueberry muffin model: he pictured an atom as a uniform ball of positive
charge, with the electrons scattered uniformly throughout, like blueberries in a muffin.
The American physicist R. A. Millikan determined the unit of charge using oil drops and an
electrical field. By calculating the velocity with which a charged drop was deflected from the
electrical field, Millikan was able to calculate the charge on the drop. The mass could also be
calculated. By performing many experiments, Millikan was able to calculate the charge on
an individual electron and the mass of an individual electron.
The true arrangement of particles in the atom was discovered by Ernest Rutherford, another
English physicist. Rutherford performed his famous platinum and gold foil experiments with
a college student who was named Ernest Marsden. They determined that some positively
charged helium nuclei, alpha particles (He2+) were deflected by platinum foil, at angles
greater than 90 degrees. Sometimes the particles were deflected almost 180 degrees, back
the way they came! For an alpha particle to be deflected that much, it must pass very close
to a group of particles that also had a positive charge. The group of particles would have to
have a strong positive charge, concentrated in a relatively small area. That is what the
nucleus of an atom is – a relatively small area with a concentrated positive charge.
The individual protons and neutrons were more difficult to isolate and identify. At this time,
it was known through Rutherford’s and others experiments that while the proton and
electron had the same charge, that the mass of the proton was much greater. In any atom,
the number of protons and electrons are the same. The existence of an uncharged particle
was discovered by comparing the number of positive charges (number of protons) in an
atom with the mass. The number of mass units was almost twice the number of protons.
Where was the extra mass coming from?
James Chadwick’s experiments with beryllium and positively charged alpha particles
revealed the presence of the neutron. By bombarding beryllium with alpha particles,
Chadwick was able to observe that an uncharged particle was given off, which had the same
mass as a proton. Chadwick used a specific type of beryllium, beryllium-9. The number 9 is
the mass number, which indicates the total number of protons and neutrons in an
atom. The number of protons in an atom of a specific element always identifies the
element. The number of neutrons in atoms of the same element may be different. When
neutrons are different in atoms of the same element, they are called isotopes. The
element neon has two common isotopes, neon-20 and neon-21. Both isotopes have 10
protons, since neon is element number 10 in the Periodic Table. By subtracting the number
of protons from the mass number, we see that neon-20 has 10 neutrons, and neon-21 has
11 neutrons. Beryllium-9 has 4 protons and 3 neutrons.
Mass number = number of protons + number of neutrons.
Each element is identified by a unique atomic number, which is shown on the Periodic Table.
The atomic number is equal to the number of protons in each atom of the element.
The number of protons and electrons in a neutral atom of an element is always the
same.
How does this work? Let’s complete the chart for an example, oxygen-16. The mass
number is the number after the dash, 16.
Symbol Atomic
Number
O
8
Number of
Protons
8
Number of
Electrons
8
Mass
Number
16
Number of
Neutrons
8
For an atom of Ca-40, the mass number is the number after the dash, 40. The atomic
number can be obtained by looking for Ca on the Periodic Table. The atomic number is 20,
the number of protons.
Mass number – number of protons = number of neutrons.
Number of neutrons equals 20. If no charge is specified, the number of electrons is the
same as the number of protons. The number of electrons is therefore 20.
Symbol Atomic
Number
Ca
20
Number of
Protons
20
Number of
Electrons
20
Mass
Number
40
Number of
Neutrons
20
We can follow this process for any isotope for which a mass number is designated. If we do
not know the mass number, we can use the Periodic Table to obtain an average atomic
mass, which is usually a decimal, then round it to the nearest whole number. For instance,
the element sulfur, S, has an average atomic mass on the Periodic Table of 32.07. By
rounding this number to the nearest whole number, 32, we now have the mass number for
a common isotope of sulfur. We can obtain the number of protons from the atomic
number on the Periodic Table, and the number of electrons will be the same if no
charge is indicated.
Ions and Ion Formation
The number of electrons can vary if charged particles called ions are formed. How can we
determine the number of protons and electrons in an ion? Remember that the atomic
number always gives the number of protons. We can find the atomic number on the
Periodic Table, by looking in the same box as the symbol for an element.
Each proton has a 1+ charge. An atom of an element has the same number of
protons and electrons, and that means the total charge adds to zero. Atoms of an
element are often said to be neutral for this reason.
Example: (Use the formula: positive charge + negative charge = net charge)
The element sodium, Na, has atomic number 11, meaning 11 protons and 11 electrons:
(11+) + (11-) = 0. There is zero net (overall) charge, the atom is neutral; it is not an ion.
An atom of chlorine has atomic number 17, 17 protons and 17 electrons: (17+) + (17-) =
0. Zero net charge, this is a neutral atom.
Let’s try this for ions. An ion is an atom that carries a charge either through gaining or
losing an electron. A cation is an atom that loses an electron and has a positive charge. An
anion is an atom that gains an electron and has a negative charge. We do not observe
negative ions by themselves, they are balanced by positive ions. Using an everyday
substance as our example, we can determine the number of protons and electrons for a
positive ion, Na1+, and a negative ion, Cl1-. These ions are found in table salt, sodium
chloride, NaCl. The ions are formed from the elements sodium, Na, and chlorine, Cl.
The sodium ion, Na1+, has atomic number 11. This means 11 protons, 11 positive charges.
The total charge is 1+, overall. That means that there is one “extra” or unbalanced charge.
What about the other 10 positive charges? They are still there in the nucleus, and their
charge is balanced by 10 electrons. Use the formula positive charge + negative charge
= net charge.
We already know that 11 protons = 11+, and net charge = 1+ ; so substitute into the
equation:
(11+) + number of negative charges = 1+
The math on this one is easy! There are 10 negative charges (10-), which means 10
electrons. The sodium ion, Na1+ , has 10 electrons; a neutral atom of sodium has 11
electrons. The sodium atom loses one electron to form the positive ion. Where does the
electron go? Let’s take a look at the negative ion in the NaCl compound.
For the chloride ion, Cl1-, the process is the same. The atomic number is 17, indicating 17
protons. The net charge is 1-. How many of those negative charges are balanced with a
positive charge?
Remember: Positive charge + negative charge = net charge
(17+) + number of negative charges = 1-number of negative charges = 18- , and number
of negative charges equals number of electrons, 18!
A neutral chlorine atom has 17 electrons. The chloride ion has 18 electrons. A chlorine atom
has to gain an electron to form the negative ion. Therefore, ions are formed by the
loss or gain of electrons. This process works for any ion, for which you know the
symbol and charge. The symbol allows you to determine the atomic number by looking at a
Periodic Table.
Calculating the Average Atomic Mass
The average atomic mass of an
of its isotopes.
Mass
Element Isotope
Number
12
Carbon
C 12
6
13
C 13
6
element is determined by averaging the natural abundance
Mass (amu)
Fractional
Abundance
12 (exactly)
98.89%
Average Atomic Mass
12.01
13.003
1.11
Chloride
Silicon
35
Cl
17
37
Cl
17
28
Si
14
29
Si
14
30
Si
14
35
34.969
75.53
37
36.966
24.47
28
27.977
92.21
29
28.976
4.70
30
29.974
3.09
35.45
28.09
Example: Use the table above to calculate the average atomic mass of silicon.
Multiply the mass by the percent abundance for each of the three isotopes of silicon.
(Change the percentage abundance to a decimal by moving the decimal point two places.)
(27.977) (0.9221) = 25.80 amu
(28.976) (0.0470) = 1.362 amu
(29.974) (0.0309) = 0.926 amu
Find the sum of the results. 25.80 + 1.36 + 0.926 = 28.08 amu
Round to three significant figures, 28.1 amu.
The average atomic mass of silicon is 28.1 amu.
Nuclear processes and Radioactivity
The French researcher Antoine Henri Becquerel, made the first observations of
natural, spontaneous radioactivity. While studying fluorescent properties of substances,
he noticed that photographic plates placed next to certain uranium compounds became
darkened, as if they had been exposed to light! This is not what he had intended to study,
but this accidental discovery had an enormous impact. One of the students in his lab was
Marie Curie, who went on to win the Nobel Prize in physics in 1903 with her husband Pierre
Curie. She was awarded another Nobel Prize in 1911 for chemistry for her work on radium
and polonium. Marie Curie is one of only three people to win two Nobel Prizes in science.
Her daughter and son-in-law shared a Nobel Prize in chemistry in 1935.
Because of these discoveries, Rutherford and other researchers were aware of the existence
of elements that are naturally radioactive. This means that the elements undergo a specific
type of change by emitting particles of different sizes and charges, with differing amounts of
energy, from the nuclei of atoms. Nuclear decay (radioactive decay) causes the number of
protons in the nucleus to change. Since the identity of an element is defined by the number
of protons in its nuclei, nuclear decay results in the transmutation of one element to
another. This type of change, nuclear change, is very different from ordinary chemical
reactions.
Two types of particles are emitted during natural nuclear decay: alpha particles
and beta particles. Alpha particles consist of two neutrons and two protons bound
together--the equivalent of a positively charged helium ion (He2+). Let’s ignore the charge
for a moment, and look at the symbol in a new way. Alpha particles are often written in the
following form:
4
He
2
The 4 in the upper left is the mass number. The 2 in the lower left is the atomic number.
The helium ion still has its 2+ charge; it is just not shown in many nuclear reactions. The
arrangement of particles in the nucleus and the changes that occur in radioactive decay
processes get special attention, and the charge is typically ignored. Charges are most often
important in ordinary chemical reactions, where they receive much more consideration.
An example of alpha decay is the naturally occurring decay of uranium-238
238
U
92
234
4
Th

+
90
He
2
An alpha particle also has a 2+ charge, even though we do not see/write the charge in
nuclear reactions. Note that the element starting material on the left of the arrow, uranium238, is not the same as the element products helium-4 or thorium-234.
A beta particle is formed when a neutron from the nucleus breaks down into a proton and
an electron. The electron, called a beta particle, is expelled from the nucleus. It is important
to remember that a beta particle (β-) is not one of the original set of electrons that are
outside the nucleus.
An example of naturally occurring beta decay is thorium-234:
234
Th
234
Pa

90
β-
+
91
The beta symbol is used to be sure that we realize that this is not an electron from outside
the nucleus. We can also recognize the correct mass and charge by using another symbol,
e:
234
Th
90
234

Pa
91
0
+
e
-1
Note that the element starting material on the left of the arrow, thorium-234, is not the
same as the element product protactinium-234.
In addition to alpha and beta particles, nuclear decay also produces gamma rays.
Gamma rays are very high energy electromagnetic radiation with essentially no
charge or mass. They have more penetrating power than X-rays.
Half-Lives
Not all radioactive elements decay at the same rate. Some artificially created isotopes are
very unstable and only exist for a few minutes before breaking down. At the other extreme,
the radioactive isotope rubidium-87 decays to strontium-87 extraordinarily slowly; it takes
60 billion years (!) for 50% of a particular sample of 87Rb to decay. The time it takes for one
half of a sample of a radioactive isotope to decay is called the isotope's half-life. The halflife of 87Rb is therefore 60 billion years. Carbon-14 has a half-life of 5,730 years, and the
half-life of the artificial radioisotope iodine-131 is 0.022 years (about eight days).
Part 1
Discovering Atomic Structure: Select the correct answer to each of the following
questions. (Each question is worth one point)
1. The scientist who suggested that the structure of the atom was somehow related to
electricity was
Ben Franklin
Democritus
Michael Faraday
John Dalton
2. The physicist Henri Becquerel discovered radioactivity while working with a sample of
radium
silicon
curium
uranium
3. Which of the following is not a component of the radiation emitted by a radioactive
sample?
alpha radiation
delta radiation
gamma radiation
beta radiation
4. Rutherford's alpha-scattering experiment showed that the charge on the nucleus of
the atom must be
positive
neutral
negative
too small to be detected
Part 2
Select the term from the list that matches each description below. (Each question is worth
one point)
5. __________ spontaneous emission of radiation from an element.
radioactivity
static electricity
electron
6. __________ negatively charged particle found outside the atomic nucleus.
static electricity
electron
coulomb
7. __________ small core at the center of an atom containing a positive charge.
alpha particle
gamma radiation
atomic nucleus
8. __________ particle with a 2+ charge that is emitted by radioactive elements.
electron
cathode ray
alpha particle
Part 3
Answer the following question. (Each question is worth one point)
9. Through a series of experiments, what was Millikan able to determine about
electrons?
By performing many experiments, Millikan was able to calculate the charge on an
individual electron and the mass of an individual electron.
Part 4
Modern Atomic Theory: Use the periodic table to determine how many protons,
neutrons, and electrons are present in each of the following atoms. Type your
answers in the text box provided.
Click here to view the periodic table (Each question is worth nine points)
10.
Atom
Protons
Neutrons Electrons
iodine-125
53
72
53
potassium-39
19
20
19
iron-56
26
30
26
Atom Protons Neutrons Electrons iodine-125 53 72 53 potassium-39 19 20 19 iron-56
26 30 26
11.
Atom
Protons
Neutrons
Electrons
oxygen-18
8
10
8
cobalt-60
27
33
27
hydrogen-3
1
2
1
Atom Protons Neutrons Electrons oxygen-18 8 10 8 cobalt-60 27 33 27 hydrogen-3 1 2
1
Part 5
Directions: Type the chemical symbol for each of the ions described below. Since
we can't use special formatting tools in our textboxes, we'll have to use another
way of writing these symbols. Look at the following example. Type the chemical
symbol for the ion with 33 protons and 36 electrons. The answer is: arsenic with
a charge of 3- ; you would type: As 3- (Each question is worth one point)
12. Cl- has 17 protons and 18 electrons.
Cl13. Li+_ has 3 protons and 2 electrons.
Li+
14.
Mg^2+ has 12 protons and 10 electrons.
Mg^2+
15. O^2- has 8 protons and 10 electrons.
O^2Part 6
Use the periodic table to determine the number of protons and electrons in each
of the following ions. Type your answers in the text box provided.
Click here to view the periodic table
(Each question is worth eight points)
16.
Ion
Protons Electrons
2+
Cu
29
27
F9
10
+
H
1
0
Na+
11
10
Ion Protons Electrons Cu2+ 29 27 F- 9 10 H+ 1 0 Na+ 11 10
Part 7
Atomic Structure: Choose the best answer for the following questions. (Each question is
worth one point)
17. The nucleus of the atom is composed of which two particles?
Proton and electron
Proton and neutron
Electron and neutron
18. The charge of a proton is:
+1
no charge
-1
19. The charge of a neutron is:
+1
no charge
-1
20. Experiments by J.J. Thomson led to the discovery of the:
Proton
Neutron
Electron
21. The nucleus of an atom is a:
Relatively small area with a
concentrated positive charge
Relatively small area with a
concentrated negative charge
Relatively large area with a weak
positive charge
Part 8
Mass Number: Choose the best answer for the following questions. (Each question is
worth one point)
22. The number of ______ in an atom of a specific element is always the same as the
element's atomic number.
Protons
Electrons
Neutrons
23. The ________ indicates the total number of protons and neutrons in an atom.
Element number
Mass number
Isotope number
24. When the number of ________ is different between atoms of the same element, these
atoms are called isotopes.
Protons
Neutrons
Electrons
25. In a neutral atom, the number of ________ and ________ is the same.
Protons, neutrons
Neutrons, electrons
Protons, electrons
26. If you’re not given the __________ in a problem, you can determine it by rounding the
average atomic mass of the element given on the periodic table to the nearest whole
number.
Mass number
Element’s symbol
Atomic number
Part 9
Ions and Ion Formation: Choose the best answer for the following questions. (Each
question is worth one point)
27. How many protons and electrons are present in a nitrogen atom?
8 protons and 8 electrons
7 protons and 7 electrons
6 protons and 7 electrons
28. How many protons and electrons are present in an argon atom?
18 protons and 18 electrons
18 protons and 17 electrons
18 protons and 16 electrons
29. How many protons and electrons are present in a potassium atom?
18 protons and 19 electrons
20 protons and 19 electrons
19 protons and 19 electrons
30. How many protons and electrons are present in a nitrogen atom?
6 protons and 6 electrons
7 protons and 7 electrons
7 protons and 6 electrons
31. What is the name of the element that has atoms that contain 5 protons?
Boron
Chlorine
Potassium
32. What is the name of the element that has atoms that contain 11 protons?
Magnesium
Sodium
Hydrogen
33. What is the name of the element that has atoms that contain 25 protons?
Cobalt
Oxygen
Manganese
34. What is the chemical symbol for the ion with 12 protons and 10 electrons?
Mg
Mg2+
Ne235. What is the chemical symbol for the ion with 95 protons and 89 electrons?
Am6+
Ac6-
Am636. What is the chemical symbol for the ion with 28 protons and 30 electrons?
O
Zn2+
Ni237. What is the chemical symbol for the ion with 10 protons and 14 electrons?
Ne4-
Ne4+
Si
38. What is the chemical symbol for the ion with 35 protons and 33 electrons?
Br2+
As2-
Br2-