6.7 Explaining the Periodic Table
Homework: Bohr-Rutherford worksheet & Pg. 240 #3, 6, 8
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You learned that elements are the building blocks of substances.
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You also learned that pure substances differ because they consists of different elements
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What makes atoms unique is the number of protons they contains
Atomic Number
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The number of protons in the nucleus is called the atomic number
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It is the number in the top left corner on your periodic table
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Each element has its own UNIQUE atomic number
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Symbol is Z in standard notation
Atomic Mass (Mass Number)
Atom is described as mostly empty space in Rutherford’s atomic model. An electron has a mass of about
9.109x10-31 kg; its mass is relatively insignificant. Therefore, the mass of an atom consists mainly of the
contents of its nucleus – protons & neutrons
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Atomic mass = total # of protons + total # of neutrons
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Symbol is A in standard notation
Isotopes
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Isotopes – are atoms that have the same number of protons but
different amounts of neutrons (so different atomic masses). For
example, carbon has three different atomic masses, 12, 13 and 14.
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The atomic mass states on the Periodic Table is the most abundant
one.
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Atomic Mass unit is amu.
Bohr-Rutherford Diagram
 Steps to draw Bohr-Rutherford Diagrams
1. Determine the number of neutrons, protons & electrons.
* Remember:
# of protons = # of electrons = atomic number
# of neutrons = atomic mass – atomic number
2. Draw the nucleus (with # of protons & neutrons)
E.g. P+ = 2, no = 2
3. Draw correct number of electrons on each energy shell
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For the first 20 elements, the maximum number of electrons in each energy shell is:
 2 electrons on the first ring
 8 electrons on the second ring
 8 electrons on the third ring
 32 electrons on the fourth ring
Draw a Bohr-Rutherford (B-R) diagram for the following elements.
Example # 1: Lithium
Carbon
Example #2: Beryllium
Example #3: Boron
Example #4:
Patterns in the Periodic Table
* As you go down each family (group), the number of electron shells increases, a new electron shell is added with
each new period (row)
* With each group, all atoms have the same number of valence electrons in their outermost orbits. For example,
group 1 has 1 valence electron. Group 2 has 2 valence electrons. Group 13 has 3 valence electrons.
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As you do down the group of alkali metals, the reaction of alkali metals reacts with water become more
vigorous. Why?
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Evidence suggests that electrons that are farthest from the nucleus have the weakest attraction to the
nucleus, So, they react faster and more vigorous with water.
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E.g. outermost electrons of sodium atom is farther from the nucleus than lithium does. So, the
reactivity increases as you go down Group 1