What Are Atoms Made Of? | Azimuth
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What Are Atoms Made Of?
This post starts with the the slides of an elementary talk I gave at Sloans Bar and Grill, in Glasgow, as part of a wonderful series called A Pint of Science. At the end I include some fascinating details which I only had time to briefly touch on in my talk. If you already know plenty of physics, this is the juicy stuff.
Start with a hair:
A red blood cell is ten times smaller across:
A flu virus is 10 times smaller across than that:
The flagellum of this cell is ten times smaller across than that:
A molecule of hemoglobin is about ten times smaller than that:
A water molecule is ten times smaller than that:
A hydrogen atom is 5 times smaller than that:
This is an actual image of a single hydrogen atom, which is amazing. (Read more here.)
But how did people figure out that atoms even exist, back before we could see individual atoms? And how did we figure out what they’re made of?
It started with chemistry. Two liters of hydrogen burn with one of oxygen to form one liter of water vapor, so we guess water is made of 2 atoms of hydrogen and 1 of oxygen: H2O. But one cubic meter of oxygen is 16 times heavier than one of hydrogen, so we guess O is 16 times heavier than H.
And so on — this took a lot of detective work, back in the 1800s.
By 1871, Mendeleev created the periodic table, listing atoms in order of weight. Here’s a modern version:
But what are the numbers on these elements? They are called atomic numbers. They are not the atom’s weights. Now we know it’s the number of electrons they have! But how did we find out about electrons?
In 1869, William Crookes made a beam by evacuating a glass tube and running an electric current though it:
In 1897, J. J. Thomson showed that this beam is made of particles less than one thousandth the mass of hydrogen atoms! We now call these particle electrons though Thomson disliked this term. (Read more here.)
Electrons are negatively charged, but atoms have no charge, so there must be something positively charged in the atom. Thomson proposed that an atom is a ball of positive charge with electrons speckled throughout it. A journalist called this the plum-pudding atom, and the name stuck. (Read more here.)
But in 1886, Eugen Goldstein created a beam moving in the opposite direction from the electrons in an evacuated tube! (Read more here.)
In 1898 Wilhelm Wien showed this beam is made of positively charged particles with about the same mass as a hydrogen atom. It took a long time to realize the full importance of these particle. We now call them protons. (Read more here.)
In 1909 Rutherford’s team in Manchester showed that the positive charge in an atom is concentrated in a small part near its center: the nucleus.
His assistants Geiger and Marsden fired particles (which we now know are helium nuclei) at some gold leaf, and some bounced back. When Rutherford saw the results of this experiment, he wrote "it was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you". The plum-pudding model was out!
But there’s a big problem. The "atomic weight" is the little number at the bottom of each square here:
If protons account for almost all the mass of the atom, the atomic weight of an atom should be the number of protons it contains. But since the atomic weight is bigger than the atomic number, this would mean atoms would have more protons than electrons — hence be positively charged. They’re not!
Let’s not even worry now about the fact that the atomic weight is not always close to an integer. Let’s just worry about this: how could an atom have more protons than electrons, if it’s electrically neutral?
Here’s one possible solution: the nucleus contains extra electrons to cancel out the excess positive charge. Rutherford argued for this in 1920.
For example helium has atomic number 2, but atomic mass 4. With 2 electrons and 4 protons, helium would have charge +2. It doesn’t! But if it also had 2 extra electrons in the nucleus it would be neutral, as observed.
But what would make some of the electrons stay in the nucleus, while others orbit it?
In 1930, Marie Curie’s daughter Irène and her husband created a beam of electrically neutral particles by bombarding the metal beryllium
with radiation.
In Cambridge, James Chadwick carried these experiments further and proved there was a neutral particle with almost the same mass as a proton: the neutron. (Read more here.)
It became clear that the nucleus of an atom is made of protons and neutrons!
Later experiments showed a nucleus is about 1/60,000 as big across as an atom. If the hydrogen atom were a sphere 30 meters across, the proton would be a grain of salt at the center.
This raises another huge problem. If the protons are confined in the tiny nucleus, and they’re all positively charged, they must repel each other immensely! What holds them together?
But...