What does this represent?
This is a cartoon of the periodic table of elements. It comes from xkcd, a website that makes cartoons about science, and often expects you to understand the underlying science.
What does it mean by “slightly fancy protons”?
Hydrogen is just a proton with an electron attached to it.
What does it mean by “you are here”?
The most important elements in biology are carbon, oxygen, nitrogen, and phosphorus (and hydrogen).
What does it mean by “lawful neutral”?
This is the location of the noble gases. “Lawful neutral” is a concept from Dungeons and Dragons. A character who is lawful neutral follows the rules whether it hurts others or helps others.
| Good | Neutral | Evil | |
| Lawful | Gandalf | Noble gases | Orcs |
| Chaotic | Robin Hood | Fish | Giant Spiders |
What does it mean by “safety goggles required”?
These elements are the most reactive. Fluorine, in particular, is the most reactive element. It forms the strongest bond with carbon (which is why Teflon is so slippery and why chlorofluorocarbons are so bad). Hydrofluoric acid is the only acid that will dissolve glass.
Other elements in this region also form acids, like hydrochloric acid.
What does it mean by “regular dirt”?
Calcium, magnesium, sodium and potassium are all found in dirt. A lot of what is left after wood burns (ashes) are oxides of these metals. However, a lot of the elements in dirt are not found here, including oxygen, silicon and iron.
What does it mean by “weird dirt”?
Lithium and beryllium are similar to the “regular dirt” elements but they are less common.
The elements in the second column (group 2) are called the alkaline earth metals.
What does it mean by “$ $ $ $”?
This is where you find gold, platinum and silver.
What does it mean by “weird metals”?
The only liquid metal, mercury, is here.
What does it mean by “murder weapons”?
Arsenic and thallium have both been used in murders and assassinations. Lead is also poisonous.
What does it mean by “ends in a number, let it slumber”?
Cesium-137 and Strontium-90 are radioactive isotopes of elements in this region of the periodic table. Although all atoms with the same name have the same number of protons, when a number is attached to the name, it means the number of neutrons in the nucleus is important. Usually, the isotope is specified because that isotope is unstable and radioactive.
Remember, the number of protons in an atom determines the number of electrons. The number of electrons, and particularly, the number of electrons in its outer shell, determines the chemical properties of the atom. Therefore, all atoms of the element are chemically identical.
The number of neutrons does not have an effect on the chemical behavior of the atom. But it greatly affects the stability of the nucleus. The study of this is the field of nuclear physics rather than chemistry.
The number, such as 137 or 90, shows the total number of nucleons (protons and neutrons) in the nucleus.
What does it mean by “they’re not staying long”?
These are the shortest-lived, most unstable elements. None of them occur naturally. They do not exist long enough to be useful, except for learning more about the structure of the atomic nucleus.
Why is the periodic table shaped the way that it is?
The elements are arranged in order based on their atomic number.
They are lined up in columns based on the number of electrons in their outer shell. All elements in column 1 have only one electron in their outer shell, for instance.
The first electron shell can only hold 2 electrons. Therefore, the first row has only two elements: hydrogen and helium. Helium is lined up with the other noble gases, which all have completely filled outer shells.
The second electron shell can hold 8 electrons. The second row has 8 elements. Neon has atomic number 10 and has 2 electrons in its first shell and 8 in its second shell.
The third electron shell can actually hold 18 electrons, but it is happy to have 8 electrons (this is the octet rule). The third row also has 8 elements.
Beyond this, the filling of the electron shells becomes more complicated, following the Aufbau rule.
In general, it looks like this:
This diagram also shows why there are traditionally two rows of elements that are shown separate from the rest of the table. They are shown in green in the diagram above. They are part of the regular periodic table; they just take up too much space.
You don’t need to understand this diagram. Just know that the periodic table is the shape that it is as a direct consequence of the way that the electron shells get filled.
CoMo Science 
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