The Periodic Table
Trends and bonding basics including ionic bonding.
Dr. Melinda Oliver
CHM 101
https://youtu.be/nWsVfNRwBm0
The first part of the video below goes through all of the examples on pgs 11 and 12 in this week's notes. The second half of the video will be used next week.
https://youtu.be/eWQH3I47W6Y
Groups on the Periodic Table-Pay attention to metals vs nonmetals
Opposites Attract: Metals lose and nonmetals gain electrons when brought together!
Metals will lose valence electrons to form positive ions-cations and nonmetals will gain electrons to make 8 valence electrons forming negative ions or anions. We will begin with metals that tend to lose ALL of their valence electrons (not true for all metals). The nonmetals are quite predictable-they will always gain to make 8 valence electrons.
Daniel Radcliffe sings "The Elements" - The Graham Norton Show - Series 8 Episode 4 - BBC One
Periodic Trends
The figure to the right shows all of the relevant periodic trends we will be discussing in the course. The most important trend is that for atomic size. The other trends, ionization energy and electronegativity are the exact opposite so if you know the trend for size, you will know the other two trends as well. When we talk about trends on the periodic table, it is understood that we refer to left-->right for horizontal trends and top-->bottom for vertical trends. First, why do atoms vary in size even when they have the same number of shells (horizontal trend)? A video on the development of the periodic table (#7): http://www.learner.org/resources/series61.html?pop=yes&pid=798#
Periodic Trends in Atomic Size
This 18.5 min video starts with explaining the periodic law and then explains in detail how the most important trend of atomic size varies across and down the table.
A pile of hydrogen atoms is much smaller than a pile of lead atoms!
If the same number of atoms are in the pile, the figure above shows why hydrogen is lighter than air and lead is very heavy. The thing to remember as we leave the structure of the atom and move through trends (depends on how many of those + and - charges you have!) and then to bonding is that those + and - charges even in neutral atoms affect neighboring atoms. The size of an atom will affect how strongly the valence electrons (those in the outermost shell) can feel the the + charges in the nucleus and how a neighboring atom's electrons might be affected by the nucleus as well.
Ionization energy is the energy required to remove an electron from an atom
The figure above shows the trend observed for ionization energy and it is OPPOSITE of size. What you observe for ionization energy is that the smaller atoms on the right of the PT have higher ionization energies than the larger atoms at the left of the PT. The same reasoning applies to groups. Small atoms at the top have higher IE than those at the bottom of the same group.
Electronegativity is the attraction between the nucleus of one atom and the electrons on a neighboring atom
Again, as with ionization energy, electronegativity trends are opposite to those for size. We will be using the electronegativity to account for differences in types of bonding. You can think of electronegativity as "electron hogginess"!
Examples from the notes on trends
This 10 min video covers the examples on page 4 of this week's notes.
Dogs Teaching Chemistry - Chemical Bonds
Dogs as atoms and electrons as bones!
The video above uses dogs as an analogy for atoms fighting over a bone (it would be electrons the atoms would be fighting over in the atomic world!). The video below is a more detailed version.
Ionic Bonding
Big dog-larger electronegative atoms (which means the atom is small) like chlorine win the tug of war over sodium's one valence electron (sodium is a larger atom with very little hold on it's valence electron) and each element now consists of IONS and the compound (two different elements) will always be a solid, will conduct electricity if melted or is dissolved in water (charges must be moving to conduct electricity), be brittle, and this can all be predicted from the PT: METAL + NONMETAL is usually ionic.
Covalent Bonding
A more even tug of war between atoms with similar electronegativities (usually two nonmetals). If the tug of war is completely even: nonpolar covalent. If the tug of war is uneven but not strong enough to form ions: polar covalent. The properties of covalently bonded compounds vary much more than those of ionic compounds. Whether the compound is polar or non polar (degree of ionic character) will help us predict things like melting points, conductivity, state of matter (solid, liquid, or gas). We will cover those dots in our next s'more!
Atomic Hook-Ups - Types of Chemical Bonds: Crash Course Chemistry #22
One more video to further explain bonding. (#8 on the list)
Formulas and Names for Ionic Compounds
- Metal comes first in formula and in name. Do not change name.
- Nonmetal comes second in formula and in name. Drop the ending on the nonmetal and add the suffix -ide. So oxygen becomes oxide, fluorine becomes fluoride and so on.
- Use the CHARGES on each element to predict the formula. We have already been practicing this when we predicted the ion an element would form.
- Charges must add up to zero. Be careful of the criss-cross method for determining formulas, it does not always work.
- Example: sodium + nitrogen. Sodium forms a +1 cation and nitrogen forms a -3 anion. In order to be neutral, you need 3 sodiums and one nitrogen in the formula.
- The name for our example is sodium nitride. PREFIXES ARE NOT USED FOR IONIC COMPOUNDS-ONLY FOR COVALENT COMPOUNDS (di-, tri-, etc).
Polyatomic Ions
These "package" ions are really covalent molecules with a charge. Almost all of them are negative with the exception of a couple. You put them together with other ions exactly as you do the binary types explained above with a few minor variations:
- do not change any endings! Just state the name of the polyatomic ion.
- If you need more than one ion, put a parentheses around the formula for the ion.
- Focus on charge and not the formula for the ion! For example, nitrate has a negative 1 charge. Use this and the charge for sodium (+1) to come up with the formula for sodium nitrate: NaNO3 (3 is subscript). The 3 in the formula for nitrate did not affect how it was combined with sodium.
- Table of ions: http://www.chemistry.wustl.edu/~edudev/LabTutorials/PeriodicProperties/Ions/ions.html
Even more polyatomic ions
What Else Can You Do to Help Your Understanding?
Chemistry is a subject that must be practiced everyday if possible. Work through the lecture examples stopping the video clips and then restarting to check yourself. Take advantage of the online homework in Mastering Chemistry to give you the practice you need to be successful. DO NOT PROCRASTINATE! Check announcements on Sakai everyday and write down due dates from Mastering Chemistry. Keep a printed copy of the most recent course calendar (included in syllabus) next to your work area. Email me with questions!!
Have Questions Later?
If you have trouble opening any links, or just need some clarification...
Dr. Melinda Oliver
Dr. Melinda Oliver
University of South Alabama
Email: oliver@southalabama.edu
Location: University of South Alabama, Mobile, AL, United States
Phone: (251)405-4504