Download Report

CYCLE 6
Developing Ideas
ACTIVITY 4: Atoms and the Periodic Table--
KEY
Purpose
Mendeleev developed his periodic table based on a limited number of
macroscopic observations and measurements—atomic weights (now called
atomic mass) and a few known physical properties and chemical reactivity.
Mendeleev’s table was revised as new elements were discovered and as
experimental evidence about atom structure emerged. The modern periodic
table now organizes the more than 110 elements according to those same
trends or periodicity in physical and chemical properties described by
Mendeleev, but also accounts for atom structure.
In this activity you will investigate how the organization of Mendeleev’s
periodic table relates to the structure of the atom. This activity will help you
answer the question:
How does the structure of the atom relate to the
organization of the periodic table?
Collecting and Interpreting Evidence
Exploration #1: How does atom structure relate to the organization of the
periodic table?
Your instructor will provide you with an element card mat, similar to the one
you used in Activity 3. This element card mat gives the relationship between
the organization of elements and atom structures, rather than the relationship
between organization of elements and their observable properties. In the
element card mat, the atom is depicted using a modified Bohr model: the
number of protons (p) and neutrons (n) in the nucleus of the element’s most
abundant isotope is shown in the center of the atom; the orbits (energy levels) are
shown as concentric circles around the nucleus; the electrons that reside within each
energy level are shown as dots. Even though scientists believe the Bohr model to
be inaccurate, it is often used in science books to help students study atom
structure and the relationship to the periodic table.
© 2007 PSET
6-57
Cycle 6
A small version of this element card mat is shown below. Examine
the element card mat provided by your instructor, or the image
below, and answer the questions.
1A
2A
3A
4A
5A
6A
7A
1
2
3
4
What general trend do you observe in the atomic number as you move
from left to right across the table? From top to bottom?
Increases left to right and increases top to bottom.
How is the atomic number of an element different from the element to
its left? To its right?
Greater by 1 than element on left, less by 1 than element on right.
6-58
8A
Activity 4: Atoms and the Periodic Table
What is the relationship between atomic number and number of protons
in the nucleus?
Atomic number = # of protons
What is the relationship between atomic number and number of
electrons in the atom?
Atomic number = # of electrons
For elements with atomic number 1-20, is there a definite relationship
between the number of protons and the number of neutrons in the
nucleus of the most abundant isotope? For elements with atomic number
greater than 20?
For elements 1-20, some have 1:1 ratio, but no definite relationship between #
protons and # of neutrons for any.
What is the relationship between the number of occupied energy levels
(concentric circle(s) containing dots) and the period (horizontal)
number?
# occupied energy levels = period #
What is the maximum number of electrons that can occupy the first
energy level? The second energy level? The third energy level1?
1st = 2, 2nd = 8, 3rd = 18
What is the relationship between the number of electrons in the
outermost energy level and the group (vertical) number?
# electrons in outermost energy level = group #
The following points summarize additional scientists’ ideas that relate atom
structure to the organization of the periodic table:
1
You may be wondering why gallium (and Period 4 elements to its right) have has an
additional 10 electrons in the third energy level as compared to potassium and calcium.
Recall that 10 transition elements separate calcium and gallium. For each of those elements,
one electron is added to the third energy level instead of to the fourth energy level. To
understand why, more advanced models of atom structure and electron arrangement are
needed.
6-59
Cycle 6
a)
Moving across the periodic table, elements increase by one atomic
number. An atom of an element with an atomic number of ‘Z’ will
have ‘Z’ protons in the nucleus and ‘Z’ electrons moving around the
nucleus. The atomic number is the whole number listed on the
periodic table.
b)
Isotopes of an element differ in their number of neutrons in the
nucleus. The mass number ‘A’ of a particular isotope is equal to the
sum of the protons and the neutrons in the isotope’s nucleus. The
mass number is not given on the periodic table. For many elements,
the mass number for the most abundant isotope can be predicted from
the atomic mass of the element. See if you can figure out the
relationship!
c)
The atomic mass is a weighted average of the mass numbers of the
various isotopes of an element. The atomic mass is the decimal
number listed on the periodic table and has units of amu (atomic
mass units). An amu is equivalent to the mass of one proton.
Use the information provided (ideas a, b, and c) and a periodic table
to complete the table below. One element has been completed for
you.
Table 1: Atomic Structure for Elements (Most Abundant Isotope)
Element
Carbon
Sodium
Nitrogen
Chlorine
Calcium
Iron
Chromium
Iodine
Platinum
Gold
6-60
Chemical Atomic
# of
# of
# of
Mass
Element
Symbol Number Protons Electrons Neutrons Number Atomic
Mass
C
Na
N
Cl
Ca
Fe
Cr
I
Pt
Au
6
11
7
17
20
26
6
11
7
17
20
26
6
11
7
17
20
26
6
12
7
18
20
30
53
78
79
53
78
79
53
78
79
72
117
118
12
23
14
35
40
56
52
127
195
198
12.01
22.98
14.00
35.45
40.08
55.84
126.90
195.08
197.97
Activity 4: Atoms and the Periodic Table
In the table above you were provided and completed information for the most
abundant isotope of each element listed. Scientists differente between
different isotopes of the same element by writing the mass number after the
name or as a superscript before the symbol. For example, the most abundant
isotope of carbon has 6 neutrons and a mass number of 12. Therefore,
scientists call it carbon-12 or 12C.
Carbon-13, or 13C, is an isotope of carbon that is used in a variety of
applications, including breath tests for the helicobacter pylori bacteria which
causes stomach ulcers, as well as air pollution and climate change studies.
How many neutrons does carbon-13 have?
7 neutrons
Carbon-14, or
materials.
14C,
is radioactive isotope of carbon that is used for dating
How many neutrons does carbon-14 have?
8 neutrons
Only non-radioactive isotopes are used when determining the atomic mass of
an element. For carbon, 99% of the non-radioactive isotopes are carbon-12
and 1% are carbon-13. Thus, 99 of 100 carbon nuclei will have a mass number
of 12 and 1 of 100 carbon nuclei will have a mass number of 13. The
calculation of the atomic mass accounts for the abundance of each isotope
99 nuclei are 12C
(99 x 12)
1 nucleus is 13C
+
(1 x 13)
100
= 12.01
Calculate the atomic mass of boron if 80% of the non-radioactive
isotopes are boron-11 and 20% are boron-10. Show your work.
80 nuclei are 11B
(80 x 11)
+
100
20 nuclei are 10B
(20 x 10)
=
10.8
6-61
Cycle 6
Exploration #2: How does electron arrangement relate to trends
in chemical behavior?
Using the orbit model we are reminded of the relationship between an
electron’s distance from the nucleus and its energy. The further away from
the nucleus an electron is, the higher the energy level the electron occupies,
and therefore the greater the potential energy possessed by an electron. The
electrons that are furthest from the nucleus, or that reside in the highest
energy level, are called the valence electrons. All other electrons of an atom
are called the core electrons.
Your instructor will provide you with a different element card mat. This
element card mat depicts a particular chemical behavior of atoms, the
formation of ions. Ions are charged atoms that form when atoms gain or lose
electrons. We will explore how ions interact with each other in the
homework.
A small version of this element card mat is shown below. Examine the
element card mat provided by your instructor, or the image below, and
answer the questions.
1A
1
2
3
4
6-62
2A
3A
4A
5A
6A
7A
8A
Activity 4: Atoms and the Periodic Table
Complete the table based on patterns you observe in the element card mat. One
column has been completed for you as an example.
Table 2: Valence Electrons and Ion Formation2
Group
1A
How many valence
electrons initially?
Atom gains or
loses valence
electrons?
3A
4A
5A
6A
7A
8A
2
3
4
5
6
7
8
Loses
Loses
Loses
Gains
Gains
Gains
--
2 Lost
3 Lost
4 Lost
3 Gained
2Gained
1Gained
--
2+
3+
4+
3-
2-
1-
--
1
Loses
How many valence
electrons are either
lost or gained?
Charge on ion
formed?
2A
1 Lost
+1
Which electrons are lost or gained in the formation of ions—core
electrons or valence electrons?
Valence electrons
Why are ions positive when they lose electrons and negative when they
gain electrons? (Hint: compare the number of positive protons versus the
number of negative electrons after losing and gaining electrons)
# (+) protons > # (-) electrons when they lose electrons = net positive charge;
# (+) protons < # (-) electrons when they gain electrons = net negative charge
2
Elements in Group IV and other metalloids can form positive or negative ions depending on
the other elements they are bound to. We have elected to only display the ion that gives the
clearest relationship between position and ion formation for later activities.
6-63
Cycle 6
What is the relationship between the number of electrons lost and the
number value of the charge on a positive ion? Explain why this is.
They are the same. If 1 is lost, then 1+ charge, if 2 are lost, then 2+ charge, etc. Same
reason as previous question: take away an electron and there is one more (+) proton
than (-) electrons, so 1+.
What is the relationship between the number of electrons gained and the
number value of the charge on a negative ion? Explain why this is.
They are the same. If 1 is gained, then 1- charge, if 2 are gained, then 2- charge, etc.
Same reason as previous question: add an electron and there is one more (-) electron
than (+) protons so 1-.
What kind of charge—positive or negative—are you more likely to find
on metal ions?
Positive—they lose electrons
What kind of charge—positive or negative—are you more likely to find
on nonmetal ions?
Negative—they gain electrons
After an atom loses or gains valence electrons, the arrangement of
electrons in its ion resembles the arrangement of electrons in which
group (1A-8A) of elements?
The noble gases, group 8A.
6-64
Activity 4: Atoms and the Periodic Table
Summarizing Questions
Discuss these questions with your group and note your ideas. Leave
space to add any different ideas that may emerge when the whole class
discusses their thinking.
S1. Describe the organization of the elements in the periodic table based on
atom structure (i.e. protons, neutrons, electrons)?
Atomic number (and # protons, # electrons) increases by 1 going from left to right. The #
neutrons generally increases from left to right, but with no set pattern. Energy levels
closest to the nucleus fill until they reach their “maximum” capacity, then the next energy
level up begins to fill. First energy holds a maximum of 2, 2nd holds up to 8, and 3rd
holds up to 18.
S2. Describe the organization of the elements in the periodic table based on
basic arrangement of electrons.
In same group, each element has same number of valence electrons. In same period,
each element increases by one valence electron from left to right and valence electrons
occupy the energy level that is equal to the period #.
S3. Which electrons—core or valence—might have more power in
predicting similar physical properties and chemical reactivity of
elements in the same group? Explain your reasoning.
Valence electrons. Elements with the same number of valence electrons form
compounds in the same ratios, combining power, have similar chemical reactivity (i.e.
toward water, oxygen, metals, etc.), and form ions with the same magnitude of positive
or negative charge, etc.
6-65
Cycle 6
S4. Positron Emission Spectroscopy (PET) is a medical procedure used to
study the chemical processes involved in the working of healthy or
diseased human brains. When using PET, molecules that our bodies
normally use (like oxygen, water, and glucose) are labeled with isotopes
such as 15O and 18F, and then injected into the patient. The PET Scanner
then records the signals that those isotopes emit as they journey through
the brain.
In the space below, draw modified Bohr models (like those used in
Exploration #1) for 15O and 18F. Label them clearly.
S5. Using only the modern periodic table to help you, complete the table for
55Fe2+ ion:
Table 3. 55Fe2+ ion
6-66
Atomic number
26
Number of protons
26
Ion charge
2+
Number of electrons
24
Mass number
55
Number of neutrons
29