Monday, February 19, 2018

VSEPR - Valence Shell Ectron Pair Repulsion Theory

Valence Shell Electron Repulsion Theory

Electrons do not like each other and when looking at molecular structures - electrons and unshared electrons (the two dots paired together) will space out evenly so they are as far apart as possible.

Most of the names of the shapes of hints like tri, tetra, planar, etc. Students need to memorize these shapes and be able to visualize them for given formulas.

For help with VSEPR - read this.

Friday, February 16, 2018

Polar Vs NonPolar Covalent Bonds

Anyone who has ever had to share something with someone else knows that sometimes isn't exactly even. Covalent molecules or bonds are no different. 

If a molecules is nonpolar covalent, it is sharing its electrons equally. The best example of this is in diatomic molecules. Diatomic molecules are two of the same atom bonded together - so they would have exactly the same pull. Symmetrical molecules are also nonpolar.
Polar covalent bonds occur when electrons are not equally shared. One atom, usually more electronegative, has a stronger pull on the electrons and shares them unequally. The other atom that is less electronegative has a smaller hold on the electrons and is thus can be slightly positive. 

One way to remember this is... "Polar Bears do not share... equally."

Thursday, February 15, 2018

Covalent Bonding

If it is a - and -, the bond is covalent. The electrons are shared in the bond. To get the formula, you have to draw the Lewis Dot structures for the elements and connect the dots that don't have friends. You write the formula based on your drawing. To name it, use prefixes to indicate the number of atoms in the formula and the second one ends in -ide. For these it doesn't matter which element comes first.

There are 4 atoms that commonly form diatomic molecules with a covalent bond... and hydrogen is one of them! That weird atom!

Wednesday, February 14, 2018

Chemistry Cat Valentine

Ionic Bonding

Students learned about ionic bonding. Ionic bonding happens between metals and nonmetals (positives and negatives). The electrons are given and taken in this ionic bond. 

To get the formula, you criss cross the charges. To name it, you say the name of the metal, then the name of the nonmetal with an -ide ending. If it is a metal from DForP block, then you use a roman numeral to indicate the charge of the metal. Basically everyone gets a Roman numeral except S-block, Boron, and Aluminum
After learning the basics, students practice. 

Ionic Bonds for Dummies

Here is a cool interactive where you can build models to simulate ionic bonding.

Tuesday, February 13, 2018

Formula Writing

Today we discussed how to write chemical formulas, and what the numbers associated with a chemical formula mean.

Coefficients are the big numbers in front and are distributed to the whole molecule (which means you may have to multiply). Coefficients tell you how many molecules are present. 
     3He = He He He :)

Subscripts are the little lower numbers and they indicate the number of atoms and only apply to the atom it is to the right of. Subscripts tell you how many of each atom are present. Students wrote their name as a chemical compound and thought it looked pretty interesting. Some students have long formulas, other short.

We then led into counting atoms for real chemical formulas using subscripts and coefficients.
Be careful....

Monday, February 12, 2018

Roman Numerals for metals

Time to learn about roman numerals.... Here is a handy clock if you are unfamiliar with them. Pretty much you need to know 1-7. 1 is represented with I, five with V and 10 with X.  4 and 6 and 7 is where it gets tricky. 4 is 1 before 5 - so its Roman numeral is IV. 6 is one after five so its roman numeral is VI.

Students also learned how to identify the charges of metals with more than one oxidation state using Roman numerals. Metals in the D, F, and lower P get Roman numerals - basically all metals but the S block, Aluminum and Boron get roman numerals. The roman numeral tells you the charge. We have to use this system because those odd metals can actually be found in more than one form - some with 2 possible charges - some with more than four!

Friday, February 9, 2018

Valence Electrons and Ions

Students learned about valence electrons. Valence electrons are the outermost electrons and are the electrons that are used for bonding and participate in reactions. Valence electrons are only found in the S and P blocks. The max number of valence electrons is 8. Students practiced counting valence electrons and drawing Lewis Dot Structures.

Students also practiced identifying which noble gas an element wanted to be like. All elements want to be like two noble gases - it is just a matter of figuring out which is closer. Elements want to be like noble gases because they have full outer electron shells, or full valences. This makes them stable and non reactive which is why noble gases are sometimes called the inert gases.

Today students learned how to use valence electrons and dot structures to determine the charge of an atom. Atoms either want to gain electrons or lose electrons to become like those noble gases they envy.
  • Ions are atoms or molecules that have a net charge, either positive or negative. There are two kinds of ions:
  • Anions are negatively charged ions because they have negative net charges. This means that there is a greater number of electrons (-) than protons (+). For example, the anion, fluoride (F 1-), has a one negative charge because it has a total of nine protons and ten electrons. Thus, the net charge for fluoride is 1 negative.
  • Cations are positively charged ions because they have  positive net charges. This is due to these ions having more protons (positive charges) than electrons (negative charges). For example, calcium (Ca 2+) is a cation ion with 20 protons and 18 electrons. The net charge for Calcium is 2 positive. (from here)

Thursday, February 8, 2018

Organic Chemistry Primer

Organic Chemistry is the study of carbon chemistry! Carbon has four valence electrons and thus always likes to make four bonds. 

The basics for naming organic molecules include: counting the number of carbons, looking at the number of bonds between carbons, and checking for functional groups. 

Prefixes are based on the number of carbons. 1=meth, 2=eth, 3=prop, 4=but, and after that the prefixes mimic the covalent prefixes. The suffix identifies the bond type. A single is an alkane and thus the suffix is -ane. Doubles - alkene; triples - alkyne. 

  Click on the image to make it larger.

Compound Interest's original post can be found here.

There are many organic functional groups, but at this level of chemistry, students just need to be familiar with alcohols, aldehydes, and carboxylic acids. 

Monday, February 5, 2018

Periodic Trends

It is all about the electrons! Always!

Electro- negativity is how badly atoms want electrons. The most electronegative atoms are Fluorine, Chlorine, and Oxygen. Everyone wants to be a Noble Gas... and halogens are the closest so they are the most electronegative.

Ionization energy is how difficult it is to remove electrons. It is difficult to remove electrons from atoms that are electronegative.

Atomic radius increases as you move down the periodic table because atoms have more mass, but actually decreases from left to right because atoms are holding on to their electrons tighter (because they are more electronegative).

Shielding has to do with how protons are blocked by the electron shells - the more shells there are, the more blocking there is. So something in period 5 (with 5 shells) has more shielding than an element in period 2 with two electron shells. Sheilding is constant across the periodic table because the number of shells is constant.

Compound Chemistry has an infographic on the Periodic Trends here