Thursday, June 14, 2012

Discovery of an electron

My previous post about the nuclear atom and its parent experiment occurs later than one I discuss in this post. Before the nuclear atom was  proposed in 1907, the electron was discovered.

This experiment rests on a fundamental understanding of attraction and repulsion between charges. Naturally it is logical to assume the positive and negative charges attract one another while the like charges (positive/positive and negative/negative) repel one another. Here is a diagram of that basic concept:

Millikan used an apparatus called a cathode ray tube. I've always found that apparatus very conceptually confusing. Although I understand it now I'd rather gloss over the actual appartus and focus on the concept of what he did with it.

Millikan basically used the concept of electricity to prove that electrons were negatively charged. He pointed a beam of electricity in a way that it could either bend toward a positively charged area or a negatively charged area. The beam was attracted to the positive plate. This proved that the electron had a negative charge.

Millikan was able to derive a charge/mass ratio from this experiment. Here is the apparatus.


Tuesday, June 12, 2012

Key Experiments in Chemistry: Part I- The Rutherford Gold Foil Experiment

Anyone who reads this blog knows that I stopped making entries this past month. I wasn't sure what direction I wanted to take the blog and wanted to think about it for awhile. Now that I'm preparing to teach again next semester I have been reviewing the first few chapters of my introductory chemistry textbook. The thought occurred to me that readers might find it helpful for me to describe the most important discoveries and experiments that have led up to our current understanding of atomic structure and basic chemistry. So for the next few entries (at least) I will present a key experiment in chemistry and discuss its significance.

I thought I'd start with the most important one from an introductory chemistry perspective. In my opinion, Rutherford's gold foil experiment is the most important experiment that has influenced the way we envision the modern atom.  Here is a picture of the apparatus:

Prior to this experiment, scientists thought that JJ Thompson's model of the atom was correct. In his model, electrons float around in a sea of positive charge. (See image to the left). Scientists believed that if they projected a stream of alpha particles (like helium atoms without the electrons) at an atom the alpha particles would stream straight through the "sea" of positive charge.

Instead, when Rutherford set up the apparatus above, the stream of alpha particles only partially penetrated the atoms (gold foil). Some were deflected backwards. This meant that there was a hard mass preventing the stream of particles from passing through the fold foil.

And this is where the idea of the nucleus was born.

We now know that the nucleus of the atom is a small, hard, dense center of the atom surrounded by electrons circling in orbitals. A comparison of scale is to envision a flea in the center of a domed stadium. This model of the atom dictates how we think of quantum science (behavior of electrons), nuclear science (protons/neutrons in nucleus), and even explains how the periodic table is structured (by atomic number or number of protons in nucleus). It also explains how we determine the mass of an atom (protons + neutrons).

The modern model of the atom changed the course of science.

Tuesday, May 29, 2012

Ideas for posts: Let me know what you want!

I've had some comments on this blog leading me to believe that these posts are helping students. I'd like to know more about who you are and what class you are taking. Is it a high school level class or other?

Email me at julieawissd@yahoo or leave a comment here.

Also: Do you have specific topics you would like me to write about here? What are they?`

Thursday, May 17, 2012

Graph for determining polarity based on shape for Lewis Dot/VSEPR theory

This by far the best diagram I've seen for determining polarity based on shape. I've taught from three different textbooks now and never seen it explained so well. See my previous post about determination of electronegativity. You need do that before you can determine shape.

Sunday, April 8, 2012

Electronegativity and gradation of ionic bond

The top figure here shows the trends of electronegativity on the periodic table. The concept of electronegativity can be most simply thought of as the ability of an atom to suck electrons toward it. Atoms on the left of the periodic table have low electronegativities. This makes sense since they actually tend to lose electrons to form positively charged atoms. The steps on the periodic table become steeper toward the right and top where atoms tend to have the highest ability to suck electrons into their centers.

When two atoms with different electronegativities come together as a molecule, the difference in their electronegativity values becomes the dipole moment of the molecule (see drawing). Basically, one side of the molecule feels a partial negative charge while the other side feels a partial positive charge. (The symbols that represent this are also included on the figure with + and - signs). A molecule with a dipole moment is a polar covalent compound.

You can actually calculate numerically the difference in the electronegativity. See the numerical values for each element on the periodic table at the top. The difference in electronegativity determines the degree of polarity of the covalent bond. When the difference in electronegativity becomes very great, the bond is no longer covalent (or polar covalent) but is considered an ionic bond. In an ionic bond the electrons are no longer considered shared between atoms. In the ionic bond the electrons are donated from atom to atom.

The Periodic Table and Ionic and Covalent Bonding

At this point a discussion of the periodic table is useful. A look at the periodic table helps you understand why certain elements share outer electrons while other elements donate their electrons to another atom. The two versions of the periodic table below are slightly different in that one shows the details of the elements on it while the other one shows large categories of elements only. Both convey the same information in a slightly different way.

The main group elements span both sides of the periodic table and form a sandwich on either side of the transition metals. The main difference between main group elements and transition elements is that the charges or partial charges on the main group elements are much easier to predict in any molecule. Many of the transition elements form species with varying charges. Often the only way to figure out the charge within the compound is to base it on the negatively charged atom, typically a main group element.

Generally, metals exist on the left side of the table and nonmetals on the right. The metals tend to lose electrons to form a + charge on an atom while the nonmetals tend to gain an electron to form a - charge on an atom.

In an ionic compound, the + and - charged atoms come together in an attractive bond. Thus typically this bond is made from one atom on the left side of the PT while the other atom is from the right of the PT.

The covalently bonded compounds, on the other hand, typically come from two nonmetals on the right hand side of the table. Since nonmetals tend to form - charged atoms, the attraction between two - charged atoms is obviously not a +/- charge. It is nothing like this because the outer electrons of both atoms rearrange themselves to form a more stable arrangement of shared electrons between the two atoms.

The top periodic table shows certain elements in yellow. These are elements that tend to form gases like H2, O2, F2, Cl2 and others. We call these diatomic gases (as the label indicates). These have very pure covalent bonds between atoms because the properties of each atom are exactly the same.

Visual picture of single ionic bond next to single covalent bond

 If we strictly talk about the base unit of an ionic compound and the base unit of a covalent compound we can show pictorially the physical difference between these two types of bonds. See figure below.

The melding of the two nuclei on the left (H-F bond) shows the sharing of outer electrons while the distinct nature of each atom of the NaCl demonstrates the +/- nature of the ionic bond.