Bonding Lab Comments

It’s refreshing and disappointing to read through the Bonding Labs you handed in.

Most of you did fairly well, and I graded casually, so most of you did okay, but I suggest you read through these comments to fine tune your thinking, and the way you explain things in your mind (and how you better explain them on the regents exam).

A few of you have WORD for WORD identical answers.  This cannot be, no matter how close you are working, to have exactly the same answers that run on for several sentences at a time.  Also, a few of you clearly copied the WRONG names for some compounds.  Again, when someone makes a wacky mistake, it’s cute, but it needs correcting.  When two or more people make the same, off-the-wall boo-boo, it’s clearly copied.  Watch who you copy from, if they aren’t smart enough to avoid such dopey boo-boos, you get busted.  Azane is a word that appears NO WHERE ELSE in the Universe except a few lab reports I read over the weekend.  DO NOT COPY!  EVER!

  1. Dipole arrows have a positive sign tail, not a T tail.  The tail indicates the atom with the lower electronegativity value atom in the bond, which ends up “more positive most of the time”.  The arrow point points to the atom with the higher EN value, making that atom “more negative most of the time”.
  2. HO2 is wacky, be careful with water.  It’s H2O, and you know that.  This error is not uncommon, but comes from students attempting to FINISH FAST, and to not think a lot.  Be a thinker.
  3. Compounds like HCl are polar.  There is an “H” side, and a “Cl” side.  The wooden models  will feel like little perfectly balanced dumbbells, but the colors are different, the atoms are different, they have different sizes, different EN values – and therefore different mostly positive/mostly negative sides.  They are POLAR.
  4. Every atom in every Lewis Dot diagram must have the right number of electrons.  You should know before you start drawing how many electrons are required, and how many you are LIMITED to drawing.  Halogens start with seven but must get an octet (8) total.  You can’t leave them out.  Oxygen gets six, carbon four, and others are all exactly counted too.  They get octets when bonding too (with a few exceptions)
  5. Some Lewis Dot diagrams you must memorize are water, oxygen, carbon dioxide, methane, all the HONClBrIF twins, and ammonia.  Ammonia is NOT Ammonium.  The first is a molecule, the second an ion.  NOT THE SAME.
  6. CO2 makes two double polar covalent bonds, in a straight line.  Carbon has 4 valence electrons, it needs to make four total bonds to get to an octet, and it almost ALWAYS does.  Each oxygen has six valence electrons.  They must make these 2 double bonds, which are polar because of different EN values.  CS2 is almost identical, oxygen has 6 valence electrons, SO DOES SULFUR.  O and S are in the same group, they bond similarly.  CS2 makes two double polar covalent bonds too.
  7. In Part C, naming the kinds of bonds present, FIRST thing ever is to look to see if the first atom is really a metal cation.  If it’s a metal, it MUST BE AN IONIC BOND.  The only way that a metal bonds to a nonmetal is by forming into ions by transferring electrons.
  8. You must always check EN values, you cannot ASSUME that different atoms always have different EN values making polar bonds.  Don’t be LAZY.  CS2 makes NON POLAR bonds – both C and S have EN values of 2.6    NBr3 has both atoms with a 3.0 EN Value.  It’s easy to see if you are putting your finger in the box, or guessing wrong.
  9. Carbon Monoxide (CO) has a special bond and a normal one as well.  FIRST, a double polar covalent bond, and then the special “coordinate covalent” bond.  This is not a triple bond!
  10. Ozone has the resonating bond.
  11. Carbon dioxide (CO2) has two double polar covalent bonds.  Silicon Si, is DIRECTLY UNDER CARBON on the periodic table – meaning that silicon and carbon should bond similarly.  THEY DO.  SiO2 is exactly like CO2, two double polar covalent bonds, both are in a straight line.
  12. When drawing nitrogen, N2, the unshared pair of electrons are to the outside (not on top).  The electrons repel away from each other in a straight line.  If you put them ON TOP, the molecule becomes polar.
  13. skip this one.
  14. Bond polarity is due to electronegativity value difference.  No difference = nonpolar.  An EN difference means polar bonds.  The greater difference means more polar bonds, when comparing them to other bonds.
  15. Molecular polarity is DIFFERENT.  Molecular polarity is due to either having radial symmetry (nonpolar molecules) or not having radial symmetry (polar molecules).
  16. Bond polarity and Molecular polarity are not the same.
  17. The Intermolecular Attractions (or bonds) are electron dispersion attraction, dipole attraction, or hydrogen bonds.
    All atoms/compounds have electron dispersion attraction, due the temporary positioning (dispersion) of the electrons in the atom/compound.  Good examples of this are the HALOGENS: F2, Cl2, Br2 and I2 – because they all have single nonpolar bonds, all are nonpolar molecules (with radial symmetry) and all are diatomic.  Similar but with different numbers of electrons, causing a general INCREASE in the amount of electron dispersion attraction they generate.  The first two are gases at STP, but Br2 is a liquid (particles more attracted to each other), and I2 is a solid (the most electrons have so much intermolecular attraction, they get stuck together.
    Dipole attraction requires a dipole molecule –  a molecule with a positive side and a negative side.  There are no nonpolar molecules can have dipole attractions, they don’t have dipoles.  Examples include OF2 or SCl2.  These molecules can’t include HYDROGEN atoms, or else they would have HYDROGEN BONDING.
    Molecules like H20 and NH3 have hydrogen bonds (polar molecules with polar bonds with H present), but CH4 does NOT have hydrogen bonding.  CH4 is a nonpolar molecule, the polar bonds are “balanced” out by the radial symmetry.
  18. Ionic Bonding is between METALS and NONMETALS.  They transfer electrons from cation to anion.  If the first element is a metal, it’s going to be IONIC bonding.  We do not use prefixes for ionic componds.  We sometimes use roman numerals. When ionic bonds form, between positive cations and negative anions, it goes without saying that the compounds are “polar”.  They have a positve and negative pole.  We do not need to state that ionic bonds are polar, of course they are.
  19. Covalent bonds form between only NONMETALS.  They share electrons.  These bonds can be single, double, triple (depending upon how many pairs of electrons are being shared – one pair, two pairs, or three pairs) AND they can be either polar or nonpolar (if they have no difference in EN values, they are nonpolar bonds).
  20. Be sure to always read the RUBRIC, the set of boxes that tells you how to score points.  Your lab reports need a cover, and usually a single sentence stating why you did this lab.  Don’t skip that part.  The conclusion goes OUT BACK.  It doesn’t start on the front page.  Carefully put staples where they belong.  Not too close to the corner, pages rip off, and not too inside the corner, I can’t open the lab reports up to see your work.  Your name and class period go on the front page too.
  21. LASTLY, please spread out.  Leave room for comments, and don’t make it so tight I can’t read your little letters.  Certainly don’t save white paper on the other side for nothing, just write bigger or better – PUT MORE SPACE between each question.

 

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