Wednesday, April 27, 2011


So. I know you all have established some sort of relationship with your periodic tables now. I mean, come on, you use it to find out what the symbol for each element is, atomic numbers, ion charges, and atomic masses. Don't tell me you haven't fallen in love with the handy dandy bundle of joy and data. While all that is very appealing to our academic senses, haven't you ever thought of knowing about it a little more deeply? Haven't you ever wondered where all the information came from? And how families got grouped together? (Yes, that was my attempt on a pun). Afterall, like Bruce Lee says "Knowing is not enough; we must apply. Willing is not enough; we must do". How are we supposed to apply all this data if we don't know enough about the Periodic Table? Insightful, no?

In the beginning, scientists knew there were elements that acted out on certain things. But of course the number of those elements were limited. It all started with Aristotle's theory, saying that there were "elements": Fire, Earth, Water, Air. But by 1817, 52 elements were discovered, and scientists were already starting to organize them, or more so, "group" them. In 1857, a good man named William Odling organized the found elements of that time into 13 groups based on their similarities in physical and chemical properties.

The 19th Century was a breakthrough for the Periodic Table. The Law of Octaves was created by John Newlands, stating that chemical elements are arranged according to their atomic weight. He was one of the first scientists who detected a trend or pattern in the elements discovered. This was quite the breakthrough, however Newland's method didn't let scientist predict elements, and he made things ambiguous; he couldn't make his mind up and kept on changing the way he ordered things. Sounds like a confused, smart guy, eh?

Now, Newland must've been so aggravated when Mendeleev discovered something even more greater than he did. It was one of those "I knew that!" moments for him. Dimitri Mendeleev discovered that when you listed the elements in order according to their mass, certain features of elements recurred. Thus, he decided to organize the elements in periods (rows) and groups/families (columns). He even was smart enough to leave gaps in between elements for elements that would be discovered later on. This allowed scientists to predict elements according to their family and their similar properties.

In modern day chemistry society, (aka now), the elements are arranged according to their atomic number as opposed to their atomic mass.

But seriously, you students. Imagine if none of these scientist never made any of these discoveries. We wouldn't be sitting here doing chemistry. Dang. LOL jk.

.....Well that's one way to look at it.

Wednesday, April 20, 2011

Electron Configuration

Electron configuration is a method of describing the orbitals in which electrons occupy, it also states the number of electrons in each orbital. (maximum is 2)

Just to show you what it looks like, here is the electron configuration for silicon:

1s2 2s2 2p6 3s2 3p

You see, electrons only exist in certain energy states. When the electrons absorb or emit a specific amount of energy, it will instantly jump to another orbital. The energy level is basically that amount that an electron of an atom can have. This is called "n". 

The energy difference between 2 energy levels is called the quantum of energy.

Atoms can exist in 2 states:

Ground State: all electrons of the atom are at their lowest possible energy level
Excited State: one or more electron is in an energy level other than the lowest possible one

Here are some other things that you should know:

Orbital: the actual amount of space that is being occupied by an electron
Shell: the set of all orbitals that have the name n-value
Subshell: a set of orbitals of the same type (like 2s and 2p)

s, p, d, and f refer to four types of orbitals.

An atom's electron shells are filled according to the following theoretical constraints:
  • Each s subshell holds at most 2 electrons
  • Each p subshell holds at most 6 electrons
  • Each d subshell holds at most 10 electrons
  • Each f subshell holds at most 14 electrons

There can only be a max of 2 electrons per orbit, so the p subshell would need 3 orbitals, each with 2 electrons. d subshell would need 5 orbitals, each with 2 electrons. f subshell would need 7 orbitals, each with 2 electrons. Sometimes an orbital will only have one electron is there is an odd number of electrons. 

 1s22s2 2p63s3p64s2 3d10 4p65s2 4d10 5p66s2 4f14 5d10 6p67s2 5f14 6d10 7p6

Above is the order in which electrons are filled. For example, a neutral oxygen atom has 8 electrons, so the electron configuration would be:

1s2 2s2 2p

Notice that the p only has a 4, and not a 6. This is alright. The superscripts should add up to the number of electron in the atom. In this case, it adds up to 8, which is correct. 

When writing electronic configurations for negative ions, simply add the appropriate number of electrons from where the neutral atom left off. 

When writing electronic configurations for positive ions, take away electrons from the outermost shell first. (aka. the higher energy shell/larger n-value) 

Another method of writiting configurations is Core Notation.
The core is the part of the configuration of the nearest noble gas that comes before the given element. (Ex/ Ar is the noble gas that comes before Br)
The outer part consists of all the electrons outside of the core. 

The core notation for Rubidium would be... [Kr]5s^1 

Do you see how all the numbers and letters that should've been before 5s^1 was replaced by Kr? 

Another example... the core notation for Chlorine would be... [Ne]3s^2 3p^5 

Electron configuration can also tell me the number of valence electrons that an atoms has. Valence electrons are electrons in the outermost open shell. By open, we mean that the shell contains less than its maximum number of electrons. Whereas, a closed shell is a shell containing exactly its max number of electrons.

In electron configuration, valence electrons are those that aren't in the core, or in d- and f- subshells.

Ex/ Determine number of valence electrons by looking at core notation of Selenium.

[Ar] 4s^2 3d^10 4p^4

There are 6 valence electrons because of the 4s^2 and the 4p^4 part of the configuration. 3d^10 doesn't count because its a d- subshell.

Monday, April 18, 2011

Atomic Structure

Protons and Neutrons are both located in the nucleus, and weigh about the same. The weight of the protons and neutrons make up the atomic mass.

Electrons are located in a cloud around the nucleus, floating in their orbits. Electrons weigh almost zero.

The number of protons (positive) and electrons (negative) are the same in a NEUTRAL atom. The number of protons is the atomic number, so if a proton is added, a new element would be formed.

If an atom has a charge, meaning the number of protons and electrons AREN'T the same, then it is called an ION. Atoms that have gained or lost electrons become ions.

ANION: a negatively charged ion, meaning it GAINED elections. (non-metals gain electrons)
CATION: a positively charged ion, meaning it LOST electrons. (metals tend to give away electrons)

The mass number is the total number of protons + neutrons.
The atomic mass of the periodic table is the average mass of an elements isotopes. The atomic mass should be close to the mass number.
If a NEUTRON is added to an elements nucleus, then a heavier version of the SAME element is formed. This is called an ISOTOPE.

So to review:
When a proton is added, a new ELEMENT is formed.
When a neutron is added, an ISOTOPE is formed.

Let's move on the natural mixtures. There can not be half of a proton/neutron, so when the periodic table states that the atomic mass of chlorine is 35.5, we know that it must be the average mass of the isotopes.

When given the percentage of each isotope in a mixture, and a precise mass for each of those isotopes, we can calculate the average mass of it. This can be done simply by multiplying the percentage and the mass of each, and adding the products together.

Ex/ Given the following information, calculate the average mass of this oxygen mixture.

Oxygen-16: 99.763%, 15.99491463g
Oxygen-17: 0.038%, 16.9991312g
Oxygen-18: 0.020%, 17.9991603g

0.99763 x 15.99491463  = 15.9570067 g
0.00038 x 16.9991312 = 0.00645967 g
0.00020 x 17.9991603 = 0.003599832 g

Now add all the products together to get your average mass:

15.9570067 + 0.00645947 + 0.004599832 = 15.967 g 

Thursday, April 14, 2011

Atomic Theory

A long time ago, Greek philosophers believed that all matter was made of atoms, which was the smallest piece of matter.

Aristotle believed that matter was made of combinations of earth, fire, water, and air. That led Alchemists to experiment with matter to try to turn common metals into Gold.
Aristotle's 4-element theory lasted for about 2000 years! However, it was not a scientific theory because it could not be tested.

Here are some significant scientists who studied the atom:

Democritus: A greek philosopher who believed that atoms were indivisible spheres that could not be seen with the naked eye.

Lavoisier: Discovered conservation of mass, and the law of definite proportions.

Proust: Discovered that if a compound is to be broken down, that the products would exist in the same ratio as the original compound- proving Lavoisier's laws.

Dalton: Still believed that atoms were solid indestructible spheres, but states that every element has a different type of atom. Stated that atoms of one type of element can combine with the atoms of another to form chemical compounds, and that atoms cannot be created nor destroyed.

JJ Thomson: Created the "raisin bun" model: solid positive spheres, with negative particles embedded within. This was the first atomic theory to have positive and negative charges! He used the cathode tube experiment to demonstrate the existence of electrons.

Rutherford: Showed the atoms have a dense positive center (nucleus), with electrons outside of it. He discovered this with the gold leaf experiment, which resulted in the planetary model of the atom. It explains the reason why electrons spin around the nucleus. It also suggested that atoms were mostly empty space.

Niels Bohr: Studied gaseous samples of atoms, which glowed when an electric current passed through them. Niels Bohr proposed that electrons surround the nucleus in different energy levels, and can jump from one to another.

Of course, now we know that the modern atom is made of three kinds of particles called SUBATOMIC particles, which include:

NEUTRONS (neutral)

The atom on the left is what scientists thought they looked like- solid indivisible spheres. This changed when JJ Thomson discovered the existence of protons and electrons! 

On the right is the modern atom. 

Wednesday, April 6, 2011


Hi there.
Imagine if you had a bag of chips. Yes I just made you hungry. I bet 50% of you just went into the kitchen to check if you had chips. Anyways, back to my point. Imagine the bag is big, and by big, I don't mean those baby bags you find in vending machines. By big, I mean momma sized chips, like them big ones you get in Costco. Appealing, I know. Would you be able to finish them? Perhaps some of you could, but most of you most likely can't without getting heartburn. This is the same with chemical reactions; sometimes not all of the reactants or products get used up; they can only do so much. What if we wanted to know how much of the compound was used up? (or how much of the bag of chips we finished) Thus now, ladies and germs, I present to you the PERCENT YIELD.

And yes I'm aware the chip explanation was uncalled for. But it's a good analogy no?

Percent Yield can be generally calculated using this formula:
Percent Yield= mass of product actually formed
                        --------------------------------------------------     x 100%
                        mass of product expected to be formed
This formula is actually not necessary when you use logic. It is like calculating a test mark. When you calculate a test mark, you divide what you got, by what you possibly could've gotten. Ahhhh logic feels good when you use it, doesn't it?

Here are a few examples:
EX/ What is the percent yield for a reaction if you predicted the formation of 21. grams of C6H12 and actually recovered only 3.8 grams?
-21g is expected, but you only got 3.8 grams. Bummer.
You would figure it out like so:    (3.8g/21g)x 100% = 18% (don't forget sig figs)

Now that was easy, wasn't it?
Now here, I will throw a curve ball at your way.

EX/ Consider the reaction: U+3Br2--> UBr6. What is my actual yield of uranium hexabromide if I start with 100 grams of uranium and get a percent yield of 83%?
First, we would use basic stoichiometry calculations to figure the amount of uranium hexbromide that is expected. Like so:
100g U x  1 mol Mg  x   1 mol H2   x 2.0g 
                --------------     -------------     ------------   = 301.4g (don't round yet! This is NOT your final ans)
                24.3g Mg       1 mol Mg     1 mol H2 

Since we know 2 variables (percent yield and expected amount), Let x= actual produced amount.
   x/301.4g=0.83. Therefore, x=250g=200g (S.F.)

Yes I know I'm an enthusiastic person when it comes to chemistry.

So long, young disciples.

^GG is all I have to say to that.

Monday, April 4, 2011

Percent Purity

Reactants that are used in experiments/reactions may not be 100% pure, so we use percent purity calculations to calculate how much of the pure substance is in a reactant.

% Purity = mass of pure substance
                -------------------------- X 100
                 mass of impure sample

It is basically the ratio of mass of pure substance to the mass of the impure sample.

Ex/ If a 2.50g sample of iron ore contains 0.90g of Iron metal, what is the % purity?

0.90g iron
                                      --------------- x 100% = 36.0% Purity 
2.50g iron ore

Ex/ 2Na + 2H20 -> 2NaOH + H2

If a 7.5g sample of impure Na that is 75% pure is reacted, how many grams of H2 is produced?

75% = pure Na
          ----------            X 100
           7.5g impure Na

75 x 7.5g
----------- = grams of pure Na = 5.6g pure Na

      5.6g Na x 1 mole Na     1 mole H2        2.0g H2
                          ------------- x   ------------- x ------------- = 0.24 g H2 produced
                     23.0g Na       2 mole Na       1 mole h2

By the way, if your percent purity ends up being over 100%, you either did something incorrectly, or you have stumbled upon a trick question. Percent yield cannot be over 100% either, by the way.

Don't fry your brain. Enjoy this cute little illustration.