There were many theories on the structure of an atom. Here are some of the most important ones.
J.J. Thomson's Model
Using an ampoule similar to Crooke's but with the perforated anode and a set of equipment that formed an electric field and a magnetic field, Thomson managed to discover the relationship load/mass of the electron .
Thomson laid the fundamental concept of the structure of the atom. He referred to it as the plum pudding model, in which the nuts showed the negatively charged particles.
He proposed the following two key points:
- An atom consists of a positively charged sphere, and the negatively charged particles are embedded in it.
- The negatively charged particles and the positively charged particles are equal in magnitude, i.e. an atom is electrically neutral.
This model had serious drawbacks; it was proved that this was a wrong concept by the next model as it couldn't explain the results of the gold foil experiment carried out by Ernest Rutherford in 1911.
He conducted a very famous experiment, the gold foil experiment. In this experiment, he bombarded alpha particles on a thin gold foil. He expected that the α particles would be deflected by the gold atoms, as he believed that the atom was a positively charged sphere. Moreover, he thought that matter was composed of atoms, in which there was no space. Imagine it like a row of marbles, one next to the other. With this idea in mind, all alpha particles should bounce back.
Astonishingly, only one out of every -particles bounced back. And very few got deflected by some angles. So he concluded that
- The positively charged sphere, the nucleus , is very small and densely packed, situated in the center.
- The electrons (negatively charged particles) revolve around the center in circular paths.
- The size of the nucleus is very small, i.e. most of the space in the atom is empty. One Angstrom for the atom, m for the nucleus, which means the nucleus is 100000 times smaller than the atom
But later his second conclusion was not accepted, because if the electrons revolve in a circular motion, they must spend energy and finally will fall into the nucleus, which means the atoms will be unstable. After a few years, Niels Bohr came up with a new model explaining the stability correctly. Although this model got the basics right, there are fundamental problems with it. For one, it doesn't explain why there are only 2 electrons in the K-level.
He explained the drawbacks of the previous model. His proposed the following:
- The electrons revolve in discrete orbits, which help them in overcoming the energy loss.
- They don't radiate energy in these orbits. Hence they are stable.
- Angular momentum is quantized.
What are Orbitals?
Quantum mechanical models are the modern concept to explain the structure of atoms and describe the state of electrons in an atom in terms of probability of finding electrons in the space around the nucleus .This leads to the concept of orbitals.
It is defined as "the three-dimensional region in space around the nucleus where the probability of finding the electron is maximum."
The classical concept of orbits, like planets revolving a sun, for example, is given up in the quantum mechanical description of the atomic world. We only know where there is a high chance that the electron is to be found. We can no longer pinpoint the position of the electron (nor the proton nor the neutron...), but we can define a probability function which gives us a good idea of where it should be. Thus the orbitals are the region where the probability of finding an electron is high. An electron can move anywhere in an atom, even inside the nucleus, or theoretically even at the other side of the universe, but 90% probability is that it is found in a small specific region of space around the nucleus, and this region is nothing but the"orbital."
This idea radically changes our view, to the extent that even Einstein himself doubted the model. He was willing to consider it a correct, yet incomplete model. The fundamental notion that we can only statistically gain information about quantum mechanical properties is a radical thought changer from the deterministic view Newton used to uphold. Even the theory of relativity is utterly deterministic.
However, all experiments conducted so far strengthen the quantum mechanical and we even have strong reasons to believe the great master himself was wrong in his doubting.
So for example, all atoms of sodium have 11 protons in the nucleus.
And if you look
on the periodic table, the atomic number for
sodium, the symbol is Na, remember, is eleven.
Your probably going to need a periodic table to
follow this lecture and do some of the in-video excersises.
So, go ahead and pause the video to get out a periodic table if you need to do so.
So, for example, in the last lecture, I noted, I noted that the
alkali metals, such as lithium, sodium, and potassium, tend to lose one electron.
For example, if a sodium atom has only 10
electrons, and not 11, then its charge is +1.
And because it has a positive charge, more
protons than electrons, it is called a cation.
The cation is a species that has a positive charge.
The first thing is that when a species forms an
ion, it does so by gaining or losing electrons, not protons.
If the number of protons is changing, then the identity of the element would change.
But the number of electrons can change and
the identity of the element remains the same,
it just changes into an ion.
An anion always has more electrons than protons.
It could be a monatomic anion, meaning there's a single
atom that has more electrons than protons, or it could
be a collection of atoms where the total number of
electrons is greater than the total number of protons, and
that's called a polyatomic ion. Again,
anions form whenever a neutral atom gains electrons.
Non-metals tend to be more attractive to electrons than metals.
So, non-metals tend to form anions more readily than metals do.
So, in this table, which we're going to fill
out together, we're going to start by being given a species,
and then we're going to determine the number of
protons, electrons and neutrons than that, that that species has.
If we look at the mass of a zinc atom we can see it's 65.39 atomic
mass units, which seems a little strange, so let's just round it down to 65.
If the mass of zinc is 65 atomic mass units and
we subtract the number of protons, which should weigh 30 atomic mass
units, we see that there are probably 35 atomic mass units
left, and those all come from the 35 neutrons that are present.
Well, in this case, you have to look out
for, remember, you have to look up the atomic mass.
Yttrium's atomic mass is 89. So, if the mass is 89, and I
You probably remember from one of the previous lectures, that
all atomic masses are based on carbon-12 as a standard.
We, as humans, arbitrarily decided to do that.
We decided that carbon that has six neutrons and six protons has
a mass of exactly 12. Everything
else is relative to the carbon. But if you have a pretty
decent periodic table and you look at it, you might notice, that the
average atomic mass reported even for carbon which is our standard, is not
exactly 12.00000. In fact it's 12.01 atomic
mass units for the average atomic mass.
Remember, that's the average atomic mass of all of the carbon on Earth.
Isotopes are atoms of the same element.
So, they have the same number of protons,
but there are different in their number of neutrons.
So, for example, I might have two different isotopes of carbon.
One of the carbons is called carbon-12. It has a mass number of exactly twelve.
So, it's got six neutrons and six protons.
But there's another stable form of carbon called carbon-13.
And you might have heard of yet another type of carbon called carbon-14.
Carbon-14 gets used in radioactive dating
of archaeological samples but that one's not
actually stable, so since we're saying stable
isotopes, I'm going to cross Carbon-14 out.
Let's do our calculation using just the isotopes
that don't decay over time, Carbon-12 and Carbon-13.
To calculate the number of neutrons, recall that we just take the
mass number and subtract the number of protons,
because the electrons don't have, a mass that matters.
The mass of the electrons is negligible.
So, Carbon-12 then, has six neutrons, and Carbon-13 has seven neutrons.
So, there's two different isotopes of carbon, that are stable on Earth.
Most of the carbon on earth has six neutrons.
But there's a little bit of carbon on earth,
about 1% of the carbon on earth, and that is in the plants we eats, in the
carbon dioxide in the air, has this extra neutron
and weighs 13 atomic mass units instead of 12.
Average atomic masses do vary a little bit from
place to place on the face of the planet Earth.
And that can be very useful sometimes when
we're trying to determine the origin of a sample.
In fact, the relative amount of Carbon-13 in a sample
depends not only on where it was found on Earth
but what time of day it is, if it is a plant.
When plants are actively breathing in carbon dioxide during the day they have a
different amount of Carbon-13 in their system
than they do at night when they're resting.
Isn't that interesting?
We can use the amount of Carbon-14 in the sample to determine
how old the sample is because once a species is no longer animate,
it's no longer consuming any other Carbon-14, samples like eating food,
and so the amount of carbon 14 in that sample decays extremely slowly over time.
The chemist can use that to determine how long the sample has been inanimate.
So, this is used all the time to date
things like mummies or plant fossils that are found.
Another time that isotopes comes in handy, this is part of the why do I care
portion of chemistry class Is when we're trying
to determine if someone has taken some artificial hormones.
For example, one of the things that can make you build muscle
mass more quickly is having testosterone in your system.
Testosterone is a naturally occurring steroid, that your body can synthesize
and men have more testosterone in their system, generally, than women do.
As it turns out, you can look at a sample of someone's blood and determine
if the testosterone in the sample is testosterone that their body manufactured
because then it has a certain ratio of Carbon-12 to Carbon-13.
Or, is it testosterone that was manufactured
in a laboratory, not by a human body?
That sample has a slightly different ratio of Carbon-12 to Carbon-13.
Now, we had a student at Duke many years ago, Derrick Lowe, who wrote a
wonderful blog post about this. Around the time that Floyd Landis
got in trouble for taking artificial testosterone, when he had the
fastest time in the Tour de France, which happened in 2006.
Now, shortly thereafter they stripped him of the title.
They determined that some of the testosterone in
his bloodstream was not testosterone that his body
had created and they did that using mass
spectrometry to look at the ratio of Carbon-12
to Carbon-13. Isn't that interesting?
If you are really interested in that, I encourage you to read more.
You can read more on, on Dr. Lowe's blog or you
can read more on other sites, but it is really interesting.
Let's do another example of isotopes.
We've talked a lot about carbon, but carbons,
most of the carbon on earth is one isotope.
Most of it's Carbon-12.
99% is Carbon-12.
There are other types of elements that have
a bigger split between different types of isotopes.
For example, chlorine.
a mass of 35 atomic mass units and some of the chlorine has a mass of 37
atomic mass units.
Remember, this is the mass number of a specific atom.
So, if we wanted to determine the number of neutrons.
Well, we know the number of protons because that's the atomic number.
We can just look that up on the periodic table.
For chlorine it's 17.
If we take 17 and subtract it from
35 we can determine that Chlorine-35 has 18 neutrons.
Chlorine-37 then, has two more neutrons. It's got 20 neutrons.
It's pretty simple math to do.
abundant, but this time the abundance isn't 99%, it's 70%.
So, these are things we've determined by
making observations of the world around us.
The abundance of Chlorine-37 is 30%.
Well, that makes the calculation for the
average atomic mass of chlorine a little strange.
It's the same
calculation that we did before. I'm going to take the average abundance.
If I had a 100 and 70% of them are Chlorine-35 then the abundance
of chlorine-35 is 0.7. That's 70 divided by 100, right?
And this is the mass Chlorine-35.
I have 30% Chlorine-37, so I take 30 divided by 100.
Multiply that times the mass of Chlorine-37.
I add those two numbers together, and I get that the average