AP Chemistry Chapter 6 Notes

Periodic Law – properties of the elements are a periodic function of their atomic number The moon’s phases and magazine subscriptions are also periodic…

The earliest and most successful arrangement of the elements was by Dmetri Mendeleev in the 1870s. He arranged the elements known at that time by their physical and chemical properties into groups.

His arrangements of the elements left some gaps, which he claimed were elements not yet discovered. With his table he predicted the characteristics of these missing elements, and was correct.

During the 1910s, Henry Moseley used x-ray spectra to determine the atomic number (number of protons) for elements and proved that each element had a different amount of protons.

Moseley rearranged the elements based on atomic number, as it is arranged today, and found gaps which he also claimed were undiscovered elements. Some of these elements were found quickly, while others were not found until after his death due to their highly radioactive (unstable) nature.

There are three key items in understanding and explaining all the trends (and exceptions) on the periodic table.

1. Effective Nuclear Charge

There are three key items in understanding and explaining all the trends (and exceptions) on the periodic table. 1.Effective Nuclear Charge 2.Energy Levels

There are three key items in understanding and explaining all the trends (and exceptions) on the periodic table. 1.Effective Nuclear Charge 2.Energy Levels 3.Coulombs Law

1. Effective Nuclear Charge – pull of the protons in the nucleus on the valence (outer) energy level electrons

The greater the atomic number, the greater the number of protons, and the greater the effective nuclear charge.

Effective Nuclear Charge has the greatest effect moving from left to right across a period (it increases).

Which element has more effective nuclear charge, P or S?

Effective Nuclear Charge has the greatest effect moving from left to right across a period (it increases). Which element has more effective nuclear charge, P or S? Which element has more effective nuclear charge, S or Se? (Why won’t this work?)

2. Energy levels - the principal quantum level of the electrons, sometimes called shells

As elements increase in atomic number they also increase in the number of electrons. These electrons occupy higher and higher energy levels.

Higher numbered energy levels are farther away from the nucleus. 1s 2s 1s 2s 3s

Energy Levels have the greatest effect moving from top to bottom within a group (it increases).

Which element has more energy levels, S or Se?

Energy Levels have the greatest effect moving from top to bottom within a group (it increases). Which element has more energy levels, S or Se? Which element has more energy levels, P or S? (Why won’t this work?)

3. Coulombs Law states that the force of attraction between things is directly proportional to the size of the charge and inversely proportional to the square of the distance between them.

What two things are attracted to each other in an atom?

Which one can move?

Which situation would show the stronger force of attraction? Explain. 2p + 12p + 1e -

Which situation would show the stronger force of attraction? Explain. 2p + 1e -

For the following trends, you must know the general trends (memorize), but using the three key items you must be able to explain the trend as well (understand).

1.Atomic Radius size of the atom a. increases from top to bottom within a group Why does it increase from top to bottom?

1.Atomic Radius size of the atom b. decreases from left to right within a period Why does it decrease from left to right?

2.Ionization Energy energy required to remove an electron from a gaseous atom a.increases bottom to top Why is it easier to remove electrons from atoms at the bottom of the P.T.?

2.Ionization Energy energy required to remove an electron from a gaseous atom b. increases left to right Why is it easier to remove electrons from atoms at the left of the P.T.?

2.Ionization Energy energy required to remove an electron from a gaseous atom c. This is sometimes called metallic character, as metals tend to lose electrons easily. Which element would exhibit the most metallic character?

3.Electron Affinity energy released when an electron is added to a gaseous atom a.increases bottom to top Why is more energy released when electrons are added to atoms at the top of the P.T.?

3.Electron Affinity energy released when an electron is added to a gaseous atom b. increases left to right Why is more energy released when electrons are added to atoms at the right of the P.T.?

3.Electron Affinity energy released when an electron is added to a gaseous atom c. This is sometimes called nonmetallic character, as nonmetals tend to gain electrons easily. Which element would exhibit the most nonmetallic character?

4. Electronegativity measure of an atom’s pull on another atom’s electrons a.increases bottom to top Why are atoms at the top of the P.T. able to pull stronger on a different atom’s electrons?

4. Electronegativity measure of an atom’s pull on another atom’s electrons b. increases left to right Why are atoms at the right of the P.T. able to pull stronger on a different atom’s electrons?

5. ions charged atoms a. cations – positive ion formed from an atom losing an electron The ionic radius is always smaller than the original atom. Why?

5. ions charged atoms b. anions – negative ion formed from an atom gaining an electron The ionic radius is always larger than the original atom. Why?

Let’s summarize the trends. Remember you must know the trend (memorize) but you also need to be able to explain why the trend exists.

Atomic Radius

Ionization Energy, Electron Affinity, Electronegativity

Effective Nuclear Charge Energy Levels

Metallic Character

Nonmetallic Character