Beginning Atomic Theory

This chapter is about atomic theory and how we got there.



Beginning Atomic Theory


Two hundred years ago the fundamental discovery was that all elements are made up of atoms. Each sample of the same element had an identical atomic structure. This is what made each element distinctive. You can always see a difference between one element and another. This is how we started identifying individual elements to catalog them and one of the first chemistry concepts to learn..

Compounds are formed when certain atoms for combined with each other. Any distinct compound will always have the same general number of atoms and ratios. Chemical reactions are changes in the way they are bound together.

An atom is mostly empty space that contains a very small nucleus in its center. Electrons move randomly around this nucleus far away from it. The nucleus has both protons and neutrons in it too.

To continue, a nucleus is very dense compared to the rest of the atom. It accounts for almost all of the atom's mass.

When atoms are combined, it is the electrons that join together in interesting ways. Their charges are very important in these interactions. The number of electrons that an atom contains directly influences its ability to interact with other atoms. The greater amount of electrons increases its chances of joining with other atoms. This is one of the fundamentals of chemistry that is so interesting.

Chemical compounds are collections of different atoms. The force that holds a compound together is called a bond. Bonds are formed by the sharing of electrons. These chemical bonds are known as covalent bonds. The compound that is formed by this process is a molecule.

Another type of chemical bond results from ions joining together. An ion is usually a group of atoms that has a distinct positive or negative charge. A bond is formed when an electron from one atom is stolen by another atom. This is because of the charges I mentioned earlier. It is attracted to the atom.

This ion can be either positive or negative. A positive ion is called cation. The negative version is called an anion. As you can see, they have opposite charges. Opposite charges attract. The force of attraction between oppositely charged ions is called ionic bonding.


The Periodic Table


The periodic table can be very helpful in understanding the elements. The more you learn about chemistry, the more help it will be. The letters are the symbols for elements. The number shown above each element is the atomic number of that element. This is the number of protons it has.

Most elements are metals. They all have relatively similar characteristics. Nonmetals are in the upper right of the table, except hydrogen, which is by itself in the top left.

Metals usually lose electrons to form positive ions. Nonmetals are the opposite and tend to gain electrons to form negative anions.

The table is arranged so that elements in the same columns have similar chemical properties.


Naming Compounds


Inorganic binary compounds are made up of two elements. They can be either ionic or covalent. Cations are always written first in the formula. Anions follow after. A cation with one type of atom takes its name from that element.

An anion with one type of element uses the first part of the element and then add "-ide" at the end. An example is fluoride.

I want to point out there is a lot more to naming compounds. You can have multiple types of atoms. Depending on what kind and the number, there are many more rules. For right now though, I just wanted to mention the very basics.



Atoms and their Internals


The electron was discovered first by J.J. Thomson. He found out that electrons had a charge and were everywhere. Electrons actually have a negative charge. Why don't atoms have a negative charge then? That is because of the protons in atoms. They balance out the negative charge from an electron. There are also neutrons in an atom but they do not have a charge.

In the most popular model of an atom, all of the positive charge and most of the mass is centered in the nucleus. The negatively charged electrons are in varying spots around the nucleus. The atomic number of this object is the number of protons in the nucleus.


Atomic Radiation


The main way that atoms can be studied is by heating them up or applying a charge to them. When either of these are done the atoms will glow. Scientists study this light and its properties. This is the field of atomic spectroscopy.

Light is just another form of radiation. It moves very quickly. I am sure everyone has heard the term "speed of light". This is where its from. The important thing to remember about any form of radiation, is that it moves energy from one place to another.

Electromagnetic radiation moves as waves through a medium. As all waves do, they have an amplitude, intensity, and a wavelength. The amplitude is the height of the wave above the center. The wavelength if the distance between peaks. The intensity of a wave is the square of its amplitude.

If the wavelength of a type of radiation is short then its frequency is high. Also, if the wavelength is long then the frequency will be low.


Spectrum of an Atom


Each type of wavelength has different characteristics. Distances and frequencies are all different from one another. This is the electromagnetic spectrum. This includes visible light, x-rays, radio waves, gamma rays, and ultraviolet.

All these types of radiation deserve their own area of study. Each has important roles. So in another point in time we will come back and take a look at them too.

Now we need to talk about equations. Math is fundamental to most subjects and this is no exception. The first one we need to be familiar with is the relationship between wavelength and frequency. It looks like this.

\[ wavelength * frequency = speed of light \]

This equates to the "c" constant that is always referenced when talking about the speed of light.

The next important equation concerns the spectral lines themselves. It contains the Rydberg constant after the famous person that discovered and put together this expression.

\[ v = R \left\{\frac{1}{n^2} - \frac{1}{n^2}\right\} \]

The Rydberg constant has been determined to have an exact value. It took many experiments to do but we have a nice consistent value for it now.

\[ R = 3.29 * 10^15 \]

When we send pure white light through an element we can see its spectrum. This looks like a bunch of black lines. The absorption lines have the same frequencies as the lines in the emission spectrum. When you think about this it means that atoms can absorb radiation only of those certain frequencies. The takeaway is that an electron can only have certain energies.


Photons and Radiation


As mentioned before, when something is heated up a lot it will start to glow. Atoms, elements, and many other objects do this. This is known as incandescence. Its color will change depending on how high the temperature is. White is generally the hottest.

We now know that electromagnetic radiation is made up of particles. These particles are called photons. Each photon is a packet of energy and that energy is related to the frequency of the radiation. Another important fact to remember is that the intensity is related to how many photons are present.

This leads us to the next important equation to be familiar with. It is Planck's constant.

\[ E = hv \]

The "h" is Planck's constant. It has a value of :

\[ h = 6.62 * 10^-34 \]

Assume electromagnetic radiation is a stream of constant photons. Meaning, they are a continuous. Therefore, the kinetic energy of electrons changes linearly with the frequency.

This tells us a few things. An electron can be driven away from a nucleus if a photon hits it with enough energy. If it does have enough energy, the electron that it hit will be ejected. Finally, the energy of the electron increases linearly with the frequency of the radiation involved.

Studies of radiation led to Planck's idea of the quantization of electromagnetic radiation. The photoelectric effect provides evidence of the particulate nature of electromagnetic radiation.