Happy Martin Luther King Jr. Day - Be the Diamond

Our bodies are constantly replacing the cells and molecules they are composed of.

 A Carbon atom that has passed through:

Jesus, Mohammad, Moses, the succession of Dali Lamas, George Washington, Martin Luther 

King Jr., Hitler, Osama bin Laden, Kim Jong-il, and Kim Jong-un

Will pass through you at some point in your life.

I hope it changes you for the better. Happy Martin Luther King Jr. Day

Back to Basics: Atomic Structure

1

The identity of an element is determined by the number of protons in its nucleus. 

The nucleus contains positively charged atomic mass units called protons, and neutral atomic mass units called neutrons. 

So Hydrogen, for example, has one proton. All elements with one proton are Hydrogen atoms.

The mass number of an atom is the number of protons plus the number of neutrons.

Two atoms with the same number of protons, (thus the same identity) but have a different number of neutrons are called isotopes.

2

An example of an isotope would be Tritium, which is a Hydrogen atom (1 proton) with 2 neutrons.The superscript 3 in the image above is the sum of the protons and neutrons in Tritium. The subscript is the number of protons.

Protium is the most common isotope of Hydrogen, it has one proton and one electron only, no neutrons.

Another common example would be carbon- 12 and carbon-14. Carbon-12 has 6 protons and 6 neutrons, while carbon-14 contains 6 protons and 8 neutrons. There are many other isotopes of Carbon, and their molar masses average out to 12.01g per mole which is the value on the periodic table.

The molar mass we see on a periodic table is the average mass of all the isotopes of an element.

Sources:
1) "Blausen 0342 ElectronEnergyLevels" by BruceBlaus - Own work. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Blausen_0342_ElectronEnergyLevels.png#mediaviewer/File:Blausen_0342_ElectronEnergyLevels.png

2) "Protium deuterium tritium" by Lamiot for french version, from Dirk Hünniger - self, translation from Dirk Hünniger (german wikipedia). Licensed under CC BY-SA 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Protium_deuterium_tritium.jpg#mediaviewer/File:Protium_deuterium_tritium.jpg


3) Tro, Nivaldo J., Robert S. Boikess, Joseph H. Bularzik, and William M. Cleaver. "Liquids, Solids, and Intermolecular Forces." Chemistry A Molecular Approach. Upper Saddle River: Prentice Hall, 2008. 76. Print.

Online Chemistry Notes: Electronegativity and Polarity

Electronegativity is a word we use to describe how much an atom is pulling electrons.

The Textbook Definition is:
"Electronegativity: The ability of an atom to attract electrons to itself in a covalent bond."

Fluorine is the most electronegative element.

On a periodic table, Electronegativity increases from left to right across a row, and it also increases up a group from bottom to top.

This is an important concept in chemistry. Electronegativity reappears throughout both semesters of General Chemistry and follows you into Organic and Biological Chemistry if you take those courses. (and you should, or at least sit in on a few classes if you don't want to risk your GPA) 

It's important to understand Electronegativity because it allows us to determine if a molecule is polar. 

What does polar mean? think of it as a molecule with a positive side (pole) and a negative side (pole). Just like a magnet.

Molecules are composed of different atoms, and the more electronegative atom in a molecule will be the negative pole.

 It's pulling those electrons in the covalent bond toward itself. And electrons are negative, so the atom becomes partially negative.

 Similarly, the less electronegative atom will be the positive pole since the electrons are moving away from it.

There are exceptions to this rule. If the pulling force from the electrons, known as the dipole moment, cancels each other out due to symmetric molecular geomoetry, then there will be no net dipole moment, and the molecule will be non-polar. 

Think of CO2

"Carbon dioxide 3D ball" by Jynto (talk) - Own workThis chemical image was created with Discovery Studio Visualizer.. Licensed under CC0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Carbon_dioxide_3D_ball.png#mediaviewer/File:Carbon_dioxide_3D_ball.png

We have a carbon in the middle double 

bonded to 2 Oxygen molecules. 

These oxygen molecules are more electronegative than carbon. So in the bonds between oxygen and carbon, the electrons are being pulled toward the oxygen. 

But since the bond angle is 180 degrees, their pulling force (dipole moment) cancels each other. For that reason CO2 is nonpolar.

Let's look at another molecule H2O:

"Water-3D-balls". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Water-3D-balls.png#mediaviewer/File:Water-3D-balls.png

Now the more electronegative atom, Oxygen, is in the center, and the two Hydrogen atoms are angled away due to 2 lone pairs of electrons on the Oxygen atom (not shown in picture).

The electrons in the bonds between Oxygen and Hydrogen will be pulled toward the oxygen making oxygen partially negative. 

Since the electrons are moving away from the Hydrogen atoms, the Hydrogen atoms become partially positive. 

Most importantly, since the molecular geometry is bent, the dipole moments do not cancel, like they do in Carbon Dioxide, and they cause a net dipole moment toward the oxygen. 

This is why Water is polar and carbon Dioxide is not.

Source:
Tro, Nivaldo J., Robert S. Boikess, Joseph H. Bularzik, and William M. Cleaver. "Liquids, Solids, and Intermolecular Forces." Chemistry A Molecular Approach. Upper Saddle River: Prentice Hall, 2008. 466. Print.

Online Chemistry Notes: Chloroform vs. Bromoform

Below are the two molecules Chloroform ( CHCl3) and Bromoform( CHBr3). They are both polar molecules, but Bromoform will have a higher boiling point because Bromine is a larger molecule than Chlorine.
                               o                                     o                         B.P = 61.7  C          B.P. = 147-151 C

 Even though Chlorine is more electronegative, and it's bonds are a little more polar, the electronegativity of Bromine is close enough to Chlorine to negate this factor. 

So since the polarity of the two molecules cancel each other out, The London-dispersion forces determine which will have a higher boiling point.For this same reason, HBr has a higher boiling point than HCl.

Here is a link from Wayne Breslyn on how to draw the Lewis Structure of Bromoform:

Source:
Tro, Nivaldo J., Robert S. Boikess, Joseph H. Bularzik, and William M. Cleaver. "Liquids, Solids, and Intermolecular Forces." Chemistry A Molecular Approach. Upper Saddle River: Prentice Hall, 2008. 466. Print.

Updated by Z. on Jan. 17, 2015

Online Chemistry Notes: Dichloromethane and polarity

The image below is a molecule called Dichloromethane.

Its'  formula is CH2Cl2. Notice the arrows? Those are dipole moments. 

Since Chlorine is more electronegative than Carbon, there are  dipole moments away from the carbon atom toward each chlorine atom. 

These two small arrows combine to make a NET DIPOLE. This is the large arrow. 

This net dipole causes the end of the molecule with the chlorine atoms to have a partial negative charge. At the same time, the Hydrogen atoms will have a partial positive charge.

 These partial (or delta) positive and negative charges cause this molecule to be polar.

Here is a Youtube video by Wayne Breslyn showing you how to draw the resonance structure: 

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Online Chemistry Notes - Intermolecular Forces

(Zoom In to 200% for a better view)