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Chapter 4 Early Atomic Theories
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Periodic Table
The Periodic Table is more than a list of elements. The Periodic Law states that Chemical and Physical
properties repeat in a regular pattern when the elements are arranged according to increasing atomic
number, the Periodic Table results. Elements end up in columns because of similar properties. Each
column is called a group or Family. A period occurs between members of the Family.
The table is set up according to the number of protons in the nucleus; called the atomic number (Z).
When arranged this way the chemist found that the properties of the elements started to repeat on a
regular bases. The chemist then started to list the elements with similar properties under each other,
Groups or Families. Grouping the elements creates rows, which we call Periods.
Some common Family or groups:
Group 1 (1A) - Alkali Metals
Group 17 (7A) – Halogens
Other classifications shown below are:
Representative elements
Lanthanide series
Metals, Nonmetals and Metalloids
Group 2 (2A) - Alkaline earth metals
Group 18 (8a or 0) - Noble Gases
Transition metals
Actinide series
Each block in the table contains important information:
Atomic number – Z – the number of protons, which never changes.
Atomic mass – the weighted average of the atomic masses of the isotopes (by relative abundance).
Mass number – A the number of protons + neutrons – changes because the number of neutrons can
vary within the element. The mass number is the average atomic mass rounded.
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Representative elements show all the possible variation in chemical and physical properties.
Metals exhibit the following properties:Luster – shiny.
heat and electricity
Malleable – hammered into sheets.
Ductile – drawn into wires
Conductors of
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Non-metals are Dull, Brittle and Nonconductors- insulators
Metalloids or Semimetals have Properties of both metals and non metals are Semiconductors
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To understand why the periodic table has this set up we must understand the structure of the atom.
The original idea of the atom came from Ancient Greece (400 BC). The Greek philosophers (lovers of
knowledge) tried to make sense of their world by observing their surroundings. Greek society was slave
based and so it was beneath the famous to work with their hands. The Greeks settled disagreements by
argument (debates). They used logic and reason to make sense of what they saw.
The Greeks Philosophers did not perform experiments in the study of nature (experimentation - an
activity carried out under controlled conditions was suggested by Galileo).
Democritus, a Greek philosopher, argued that there are two possibilities for matter: (i) matter is
continuous or (ii) matter is discontinuous. If matter is continuous a piece of matter can be subdivided
forever and never reach the end of these divisions. If matter is discontinuous then matter can be
subdivided until you reach a point that any further division will destroy the matter (you no longer have
the same matter you have something different).
An analogy would be to start with a box of marble divide in half eventually you get down to one marble
which if you divide again you no longer have a marble.
In Greek, the prefix "a" means "not" and the word "tomos" means cut. Our word atom therefore comes
from atomos, a Greek word meaning not cuttable.
Aristotle - another famous Greek philosopher argued that matter is made of 4 elements:
Fire - Hot
Air - light
Earth - cool, heavy
Water – wet
Matter was seen as a blend of these in different proportions to get all the variations in matter.
Aristotle was more famous so his ideas won out over those of Democritus. Aristotle’s ideas carried
through to the Middle Ages. It was during the Middle Ages that the alchemists were trying change lead
to gold. During the passage of time the ideas proposed by Democritus were not forgotten.
Dalton’s Atomic Theory (The re-emergence of the Atomic Theory)
During the late 1700’s, John Dalton (England) a teacher who summarized results of his experiments and
those of other’s. In Dalton’s Atomic Theory he combined ideas of elements with that of atoms (a major
contribution to today’s Atomic Theory).
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Dalton’s Atomic Theory
1. All matter is made of tiny indivisible particles called atoms.
2. Atoms of the same element are identical in size, shape and mass; atoms of different elements are
different.
3. Atoms of different elements combine in whole number mass ratios to form compounds. This is
known as Law of Definite Proportions. Each compound has a specific mass ratio of elements.
Water is always 8 grams of oxygen for each gram of hydrogen. Atoms of the same element can
sometimes unite in more than one ratio with another element to form more than one compound.
Oxygen and hydrogen can react to form two different compounds: water and hydrogen peroxide.
Water is 8 grams of oxygen per gram of hydrogen. Hydrogen Peroxide is 16 grams of oxygen per
gram of hydrogen. The oxygen-to-oxygen ratio is 16 to 8 which is a 2 to 1 ratio. This is true
because you have to add a whole atom, you can’t add a piece of an atom.
4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed
– Law of conservation of mass.
Law of Definite Proportions
Each compound has a specific whole-number ratio of elements; ratio is by mass [Definite Proportions]
Water
H2O H:O = 2:16 = 1:8 by mass
Carbon dioxide CO2 C:O = 12:32 = 3:8 by mass
Methane
CH4 C:H = 12:4 = 3:1 by mass
8.0 g oxygen reacts with 1.0 g hydrogen to form H2O. Water has a mass ratio = 8:1 (oxygen to
hydrogen).
Law of Multiple Proportions
If two elements form more that one compound, the ratio of the second element that combines with 1
gram of the first element in each is a simple whole number.
• In hydrogen peroxide 32.0 g oxygen reacts with 2.0 g hydrogen (H2O2) O:H = 16:1
• Ratio of the masses of oxygen in hydrogen peroxide and water is 16:8 = 2:1
• Therefore H2O2 contains twice as many oxygen atoms as H2O [Multiple Proportions]
Water is 8 grams of oxygen per gram of hydrogen. (H2O)
Hydrogen Peroxide is 16 grams of oxygen per gram of hydrogen. (H2O2)
16 to 8 is a 2 to 1 ratio
This is true because you have to add a whole atom, you can’t add a piece of an atom.
Is this true for carbon dioxide (CO2(g)) and carbon monoxide (CO(g))? (C = 12g and O =16g)
The Parts of Atoms
As scientists began to develop methods for more detailed probing of the nature of matter, the atom
(supposedly indivisible) began to show signs of a more complex structure.
J. J. Thomson - English physicist. 1897 was studying the electrical conductivity of gasses and made a
piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out.
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When voltage is applied an electric current passes and makes a beam appear to move from the negative
(cathode) to the positive (anode) end. These were called Cathode Rays because he thought he was
dealing with light.
By adding an electric field around the vacuum tube, the cathode ray changed direction.
Thompson concluded that Cathode rays consisted of beams of particles that have a negative charge
because they were attracted to the anode (positive electrode).
J.J. Thomson is remembered as the discoverer of the electron, the first subatomic particle. Thomson also
was the first to attempt to incorporate the electron into a structure for the atom. Matter is electrically
neutral so Thomson proposed the existence of positive particles.
Thomson’s model of the atom is often referred to as Thomson's "plum pudding model," where the
pudding represents the sphere of positive electricity and the bits of plum scattered in the pudding are the
electrons. Today we may wish to think of it the "blueberry muffin" model. All those round little
blueberries (electrons) surrounded by the bread (positive) of the muffin.
Rutherford’s experiment
Ernest Rutherford, an English physicist. (1910), believed in the plum pudding model of the atom. He
wanted to see how big the atoms were. Rutherford used radioactivity to produce Alpha particles -
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positively charged pieces given off by uranium. He shot the alpha particles at gold foil that was a few
atoms thick.
He expected the alpha particles to pass through without changing direction very much because the
positive charges in the atom were spread out evenly over a large area which would result in a weak +
force field. The force fields in the gold atoms were not enough to stop the alpha particles.
What he observed
How did Rutherford explain his observations? The atom is mostly empty space with a small dense,
positive piece at centre. This centre is called the nucleus. Rutherford arrived at this model by
interpreting his empirical data. Mass – mass interaction (collisions between objects like one billiard ball
and another ball) would lead to small deflections. Large deflections meant stronger forces were at play.
He knew that alpha particles carry a positive charge, so he suggested that there was a charge – charge
interaction. The atom must have an area of large positive charge that will repel the positive charge of the
alpha particle. This positive area must be small and massive. Small – not many alpha particles are
strongly deflected. Massive – a few alpha particles are deflected back to the source. Rutherford saw the
atom as shown in the diagram below:
Since most of the alpha particles went through, the atom is mostly empty. Alpha particles that were
deflected were interacting with the positive force field of the nucleus. The alpha particles would undergo
a very large deflection if they got close to its strong positive force field. Because the alpha particles were
deflected straight back the nucleus had to be heavy or massive. Since very few alpha particles were
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reflected back the nucleus had to be very small. This small dense positive area of the atom is the
nucleus. Rutherford did not place the electrons around the nucleus (no structure given), he said that they
occupied the space around the nucleus.
Modern View
The atom is mostly empty space. An analogy would be to picture a marble (nucleus) at the centerline of
a football stadium (seats where you find the electrons). The two regions are:
1. the nucleus, which contains protons and neutrons ( the nucleons)and
2. an electron cloud, which is the region outside the nucleus where you might find an electron
(extra nuclear).
Name
Symbol Charge Relative Mass (amu) Actual Mass (g)
Electron e11/1840
9.11 x 10-28
+
Proton
p
1+
1
1.67 x 10-24
Neutron n0
0
1
1.67 x 10-24
The above table shows us that the mass of the atom is found in the nucleus while the electron cloud
determines the volume (size) of the atom. The radius of the nucleus is approximately 10 -15 m. The
radius of the atom is measured in picometres, 10-12 meters.
The modern picture of the atom includes many more subatomic particles. The chemist however need
only be concerned with the three mentioned above, as that is all they need to explain chemical and
physical properties.
How does the atomic structure relate to the set up of the periodic table?
The table is set up according to the number of protons in the nucleus; this is called the atomic number
(Z). When arranged this way the chemist found that the properties of the elements started to repeat on a
regular bases. The chemist then started to list the elements with similar properties under each other. This
creates rows, which we call Periods and columns that are called families or groups.
Counting the Pieces
Atomic Number = number of protons (p+)
The number of protons determines kind of atom – 2 protons in the nucleus means that this is a Helium
atom.
Chemists use Z as a symbol for atomic number.
In a neutral atom there is the same number of electrons (e-) and protons (atomic number).
Mass Number = number of protons + neutrons [Sum of p+ and nº (p+ + nº)]. The symbol used for mass
number is A. The neucleons (p+ and nº) make up the mass of the atom.
A
The chemist uses the Nuclear Symbol to describe the nucleus of the atom ZE
E – the symbol of the element
A – mass number (the number of protons plus the number of neutrons)
Z – atomic number (the number of protons) that equals the number of electrons in a neutral atom.
Here is an example of a nuclear symbol:
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The element symbol, Li, is that for lithium.
The three, subscripted left, is the atomic number and the seven, superscripted left, is the mass number.
The number of neutrons is 7 – 3 = 4. The number of electrons = the number of protons, therefore Li has
3 electrons.
Here's another example:
The 22 is the atomic number for titanium (22 protons and 22 electrons) and 48 is its mass number.
Number of neutrons is 48 - 22 = 26.
Now, write the nuclear symbol for the chlorine isotope with 18 neutrons.
Here is the answer:
This question did not give the mass number, so you to add the atomic number and number of neutrons
(17+18) to get the mass number of 35.
What is the nuclear symbol for the krypton isotope with 48 neutrons?
Answer:
Dalton was wrong when he said that every atom of an element was identical to every other atom of that
element. Atoms of the same element can have different numbers of neutrons and therefore different
mass numbers. These atoms of an element that differ in the number of neutrons are called isotopes. Two
isotopes of carbon are
12
6C
14
6C
To name these isotopes, you put the mass number after the name of the element
12
6C - carbon- 12
14
6C - carbon -14
The mass shown in the Periodic Table is the weighted average of the isotopes for the element. The
weighted average is calculated by using the mass of the isotope and the percentage abundance.
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Average Atomic Mass
How heavy is an atom of oxygen? There are different kinds of oxygen atoms, so the best way to
represent the oxygen would be by using the average atomic mass. The average atomic mass is based on
abundance of each isotope in nature. We do not use grams for mass because the numbers would be too
small. To measure Atomic Mass we use the unit Atomic Mass Unit (amu). The atomic mass unit is
defined as one-twelfth the mass of a carbon-12 atom.
Each isotope has its own atomic mass. The mass on the periodic table is the weighted average, which is
calculated from percent abundance of each isotope. This calculation is shown below:
Calculating weighted averages
You have five rocks, four with a mass of 50g, and one with a mass of 60g. 80% (.80) have a mass of 50g
and 20% (.20) have a mass of 60g. What is the weighted average mass of a rock?
Weighted average mass = .80 x 50g + .20 x 60g = 40g +12g = 52g. This value shows that most of the
rocks had a mass of fifty.
Weighted average mass = % as decimal x mass + % as decimal x mass + % as decimal x massCalculate
the weighted average atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu
and the rest has a mass of 64.93 amu.
Weighted average mass = .691 x 62.93amu + .309 x 64.93amu
= 43.48amu + 20.06amu
= 63.54amu
Magnesium has three isotopes. 78.99% magnesium 24 with a mass of 23.9850 amu, 10.00% magnesium
25 with a mass of 24.9858 amu, and the rest magnesium 26 with a mass of 25.9826 amu. What is the
weighted average atomic mass of magnesium?
Weighted average mass = .7899 x 23.9850amu + .1000 x 24.9858amu + .1101 x 25.9826amu
= 18.95amu + 2.499amu + 2.861amu
= 24.31amu
Here are two tips:
1) The element name (or symbol) uniquely determines the atomic number. In the
example above Ti is the only element with an atomic number of 22. So, if you need
the atomic number, and all you know is the specific element, go to a periodic table
and find its atomic number.
2) Suppose you are asked to write a nuclear symbol for silver, do this:
a) Select the element from the table; this will determine its atomic number.
b) Take the element's weighted average atomic mass and round it off to
the nearest whole number. More often than not, this will be the mass
number of the most abundant stable isotope
The nuclear symbol for silver – the answer:
Notice the mass number is rounded off from the atomic weight on the periodic table
(107.868)
Isotopes - atoms of the same element can have different numbers of neutrons and therefore have
different mass numbers
When naming, write the mass number after the name of the element
Chapter 4 Early Atomic Theories
Hydrogen – 1
Hydrogen – 2
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Hydrogen – 3
Review Questions
1. Identify the following elements as metals, metalloids or nonmetals.
a) sodium
f) iron
b) boron
g) aluminum
c) calcium
h) silver
d) antimony
i) silicon
e) chlorine
j) zinc
k) neon
l) iodine
m) selenium
n) manganese
o) phosphorus
2. Which of the above elements in question 1 are representative elements?
3. In which family and/or group are the elements in question 1 found?
4.
a)
b)
c)
Use the lettered locations to identify each class of elements:
a) transition metals
d) d) inner transition metals
b) metals
e) e) metalloids
c) nonmetals
5. List the properties that allow us to classify elements as metals, nonmetals or metalloids.
6. What is an atom?
7. State the main ideas for Dalton’s atomic theory.
8. What are the charges and relative masses of the three subatomic particles that are of interest to
the chemist?
9. Describe the makeup of the nucleus of the atom.
10. What information does the atomic number of an atom give?
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11. What information does the mass number of an atom give?
12. What is an atomic mass unit?
13. What are isotopes?
14. Name three ways that isotopes of an element differ.
15. What is the atomic mass of an element? Explain why the atomic masses for most elements are
not whole numbers.
16. Element carbon is atomic number 6. How many protons and electrons are in a carbon atom?
17. The atomic number of an element is 11. What is the element?
18. How many protons are in the nuclei of the following atoms?
a. sulfur
b. phosphorus
c. calcium
d. cadmium
19. Two isotopes of oxygen are oxygen – 16 and oxygen – 18. Write the nuclear symbol for each.
20. Use the periodic table to determine the number of neutrons in these atoms.
a. 12C
b. 15N
c. 226Ra
21. What parts of Dalton’s atomic theory no longer apply to our view of the atom?
22. The element copper is found to contain the naturally occurring isotopes 6329Cu and 6529Cu. The
relative abundances are 69.1% and 30.9% respectively. Calculate the average atomic mass of
copper.
23. Uranium has three isotopes with the following percent abundances:
234
235
238
92U (.0.0058%);
92U (0.71%);
92U (99.23%). What is the atomic mass of uranium?
24. Complete the following table:
Atomic Mass
Number
Number of
number number of protons neutrons
7
7
9
Nuclear
symbol
19
27
45
16
28
Symbol of
element
10
39
59
Number of
electrons
Se
8
14
2
2
1H
83 Sr
38
92
146
14 C
6
79 Br
35
17 O
8
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25. The table below shows some of the data collected during Rutherford’s gold foil experiment:
Angle of deflection (degrees) Number of deflections
How did Rutherford explain these results?
5
8 000 000
10
5000 000
15
100 000
30
8 000
45
1 500
60
500
75
220
>100
190
26. Explain how empirical data helped Thomson and Rutherford modify their view of the atom.
Briefly explain how each viewed the atom after their experimentation.
27. How are the three isotopes of hydrogen alike? How do they differ?
28. The 4 isotopes of lead are shown below:
1.37%
26.26%
Use the above data to calculate the atomic mass for the element lead.
20.82%
51.55%