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6.7 Student Handout

6.7 Explaining the Periodic Table
Homework: Bohr-Rutherford worksheet & Pg. 240 #3, 6, 8
You learned that elements are the building blocks of substances.
You also learned that pure substances differ because they consists of different elements
What makes atoms unique is the number of protons they contains
Atomic Number
The number of protons in the nucleus is called the atomic number
It is the number in the top left corner on your periodic table
Each element has its own UNIQUE atomic number
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
Atomic mass = total # of protons + total # of neutrons
Symbol is A in standard notation
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.
The atomic mass states on the Periodic Table is the most abundant
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
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
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.
As you do down the group of alkali metals, the reaction of alkali metals reacts with water become more
vigorous. Why?
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.
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