Content Reviewers:Rishi Desai, MD, MPH
One of the first models of the atom was the “plum pudding” model, which was proposed by the physicist J. J. Thomson in the nineteenth century.
In the “plum pudding” model, atoms were thought to have a cloud of positive charge, and within that cloud there were tiny electrons with negative charge.
Sort of like the mid-nineteenth century british dessert — plum pudding --- where the plums were the electrons sitting within a delicious positive charge of pudding.
However, in the 20th century the physicist Ernest Rutherford realized that the plum pudding model wasn’t quite right.
He did a famous experiment where he took positively-charged alpha particles, and shot them at a piece of gold foil.
Most of these alpha particles passed straight through the gold foil and hit the detector, but some of them actually bounced off of the gold foil and hit the sides of the detector.
His explanation for this was that the atoms are mostly empty space, and that the majority of the alpha particles travel straight through the foil and strike the back wall of the detector, but that on very rare occasions, the alpha particles bounce off of something.
He visualized a very small but very dense object in the middle of the atom, that he called the nucleus, which the alpha particles were bouncing off of on those rare occasions.
Since the alpha particles are positively charged, Rutherford figured that the nucleus must also be positively charged since like charges repel.
So Rutherford gave us the idea that the modern atom consists of mostly empty space containing electrons, which are miniscule in size compared to the atom overall.
But in the middle of that empty space there is a very small but very dense nucleus.
Later, scientists showed that the nucleus actually consists of two types of particles: protons and neutrons.
Protons have a positive charge but neutrons have no charge.
Now, when looking at the periodic table you can find the simplest atom, which is hydrogen.
Hydrogen has the symbol H, and it has an atomic number equal to one.
Atomic numbers tell you the number of protons in the atom which means that Hydrogen has one proton in its nucleus.
The number of protons in the nucleus of an atom is unique to each element.
A typical hydrogen atom has a nucleus with one proton and one electron outside the nucleus.
Hydrogen is an electrically neutral atom because the electron and proton have the same magnitude of charge, but different signs, so their charges cancel out.
However, there are variants of hydrogen atoms called “isotopes,” which all have one proton and one electron, but varying numbers of neutrons in their nuclei.
The most common isotope of hydrogen is protium, which does not have any neutrons in its nucleus. Another isotope, deuterium, has one neutron in the nucleus.
And a third isotope is tritium, which has two neutrons in the nucleus.
We can distinguish between these isotopes by writing 11H for protium, 21H for deuterium, and 31H for tritium.
Here, the subscript one refers to the atomic number. For an electrically neutral atom, it also tells you the number of electrons as well.
The superscript refers to the mass number, which is the number of protons plus neutrons.
Since protium has one proton and no neutrons, the mass number is one. Deuterium has one proton and one neutron, so the mass number is two.
Tritium has one proton and two neutrons, so the mass number is three.
These rules apply to any element in the periodic table. For example let’s look at carbon on our table.
Carbon has an atomic number of six, which tells you that it has six protons, and it’s electrically neutral, so it has six electrons as well.
An example of a carbon isotope can be written as 136C.
The superscript of 13 is a mass number, which equals the total number of protons and neutrons.
If we subtract the number of protons, which is 6, then we’re left with 13 minus 6 or 7 neutrons.
Another way to represent isotopes is to write the name of the element followed by a hyphen and then the mass number. So carbon-13 refers to this carbon isotope.
Let’s get our periodic table back up again. Now as you can see, hydrogen has an atomic number of one, which means one proton and one electron.
The electron for hydrogen can be symbolized by drawing a dot next to the H symbol. This is called Lewis dot notation.
The next element on the periodic table, helium, has an atomic number of 2, which means that it has two protons and two electrons.
Since the neutral helium atom has two electrons, the Lewis dot notation for helium has two dots next to the He symbol.
Atoms have shells around them that they like to have “filled” with electrons.
Both hydrogen and helium have a shell that can hold a maximum of two electrons.
Since helium has two electrons, Helium already has a full shell and is chemically unreactive.
In fact, helium is an example of a “noble gas”, which are elements on the far right side of the periodic table that are unreactive because their shells are filled with electrons.