periodic trends essential questions: what are the period and group trends of different properties?...

Periodic TrendsEssential Questions:

What are the period and group trends of different properties?

How are period and group trends in atomic radii related to electron configuration?

What is a periodic trend?

• The predictable change the elements show when you move across a period or down a group/family

Atomic Radius

• Is defined by how closely an atom lies to a neighboring atom

• The size of the atom can vary depending on what other element its bonded to

• Atomic radius decreases across a period and up a group/family

Atomic Radius within a Period• As you move across a period the atomic radius

decreases

• As you move across the period you are adding an extra proton and an extra electron. The nucleus has an increasing nuclear charge and the extra electron arranges itself into the same energy level.

• As the positive charge increases in the nucleus the electrons are pulled closer to the nucleus

Atomic Radius within a group/family

• Increases down a group/family

• Increasing positive charge in the nucleus but the extra electrons are arranged into the next energy level.

Ionic Radius

• Cations: lose electrons and form positively charged ions, they always become smaller because – 1) physically losing electrons – 2) the increased positive charge pulls the

other electrons closer to itself.

• Anions: gain electrons so they become larger because

Adding an extra electron causes electrostatic repulsion to increase making them spread apart

Ionization Energy

• Is the energy required to remove an electron from a gaseous atom

• Removing electrons– 1st ionization energy = 1 electron– 2nd ionization energy = 2 electrons– 3rd ionization energy = 3 electrons

*2nd & 3rd … ionization energies require significantly more energy

Electronegativity• Is the ability of an atom to attract an

electron to it’s chemical bond

Octet Rule

• Atoms in the s and p blocks want to be like the noble gases and have a full set of valence electrons

• They will gain/lose or share electrons to have 8 electrons in the outer shell.

• Exceptions to the octet rule are in d and f block

Octet Rule

• S block elements will give up their electrons

• P block elements will take up electrons

• Octet rule helps us determine the kind of bond formed.

Summary

• Atomic Size decreases left to right on a period and up a family

• Ionic size, ionization energy, and electronegativity increase left to right a period and up a family.

• The octet rule states that atoms gain, lose or share electrons to acquire a full set of eight valence electrons

Review Questions:

• How are the period and group trends in atomic radii related to electron configuration?

• Atomic radii increase down a group b/c of increasing energy levels. And decrease across a period because of the stronger positive pull from the positively charged nucleus pulling the electrons closer

Review Questions:

• A tale of 2 elements:

Flourine or Bromine

– Which has a larger electronegativity:

– Which has a larger atomic radius:

– Which has a smaller Ionic radius:

– Which has a larger ionization energy:

• F, Br, F, F

Practice Problems:

• Which has the largest atomic radius: Mg, Si, S or Na? the smallest?

• Na (largest)

• S (smallest)

Practice Problems cont’d

• Draw spheres to show the respective sizes of Helium, Krypton, and Radon

Practice Problems

• Can you determine which of 2 unknown elements has the larger radius if the only known information is that the atomic number is of one of the elements is 20 greater than the other? Explain.

Review Questions:

• Why would it be harder to remove the second electron from lithium than it is to remove a fourth electron from carbon?

• Because Lithium’s second removed electron is an inner core electron, not a valence electron. Carbon’s fourth removed electron is still a valence electron