Name_________________________________
Periodic Trends
Five main trends are produced by three main factors.
List the five trends:
1. _________________________________
2. _________________________________
3. _________________________________
4. ________________________________
5. _________________________________
List the three main factors:
1. ___________________________________
2. ___________________________________
3. ___________________________________
To understand why the trends exist we must learn about the factors.
1. Effective Nuclear Charge – the attraction of the positive protons in the nucleus (Z) less the repulsion of other
electrons to an electron. Remember that protons are located in the nucleus and have a positive charge. The
negative electrons are attracted to the positive nucleus – opposite charges attract. However, there can be many
electrons surrounding the nucleus and they will repel each other – like charges repel.
a. Draw the example for Sodium (Na) below:
b. What is the nuclear charge in this atom?
c. How many “inner shell electrons” are present in this atom?
The Effective Nuclear Charge can be determined by using the formula ______________________________
Where:
Effective Nuclear Charge = ________
Number of Protons = ____________
Shielding Constant = ____________
The Shielding Constant (S) remains constant across a ___________, however the number of protons (Z)
______________ across a period as you go left to right. How does this affect the Zeff (Effective Nuclear Charge)?
d. Calculate the Zeff for Na
e. Calculate the Zeff for Mg
f. Calculate the Zeff for Al
g. Calculate the Zeff for Si
h. Calculate the Zeff for P
Conclusion: As you move from left to right across a periodic table the effective nuclear charge (Zeff) ________________.
This is because the number of ________________ increases increasing the nuclear charge, but the shielding
_______________________________.
As the effective nuclear charge increases the force of attraction between the nucleus and the valence electrons
increases.
Practice Problems:
1) What is the shielding constant (S) for Boron?
2) What is the shielding constant (S) for Aluminum?
3) What is the shielding constant (S) for Nitrogen?
4) What is the effective Nuclear Charge (Zeff) on electrons in the outermost shell of Boron?
5) What is the effective Nuclear Charge (Zeff) on electrons in the outermost shell of Potassium?
6) What is the effective Nuclear Charge (Zeff) on electrons in the outermost shell of Nitrogen?
7) Inner Electrons vs. Outer Electrons: Which electrons have greater shielding from the nucleus? Why?
2. Coulomb’s Law – a law that says the energy of an atom is directly related to the product of the charges and
indirectly related to the distance between the charges.
3. Shielding - or screening is the fore of repulsion between the inner electrons and the electrons in the valence shell.
Shielding reduces the effective nuclear charge, and reduces the attraction between the nucleus and the valence
shell electrons.
Problems:
1) Across a period from left to right the Zeff
a. Increases
b. Decreases
c. Remains the same
2) Down a group from top to bottom Zeff
a. Increases
b. Decreases
c. Remains the same
3) The effective nuclear charge acting on an electron is larger than the actual nuclear charge? True of False
4) A selenium atom has 34 electrons. Electrons in the ___________ subshell will experience the lowest effective
nuclear charge
a. 1s
b. 3p
c. 3d
d. 4p
e. 5p
5) In which subshell does an electron of an arsenic (As) atom experience the greatest shielding?
a. 2p
b. 4p
c. 3p
d. 3s
e. 1s
6) In which subshell does an electron of a calcium atom experience the greatest effective nuclear charge?
a. 1s
b. 2s
c. 2p
d. 3s
e. 3p
Conclusions:
For any given atom, the electrons closest to the nucleus experience the ________________ effective nuclear charge.
(They have the least shielding)
The electrons farthest from the nucleus experience the _________________ effective nuclear charge. (They are
shielded the most)
As you move across the periodic table from left to right the effective nuclear charge felt by the outer electrons
____________________.