The Periodic Table

On 6 March 1869 the Russian chemist Dmitri Mendeleev presented a paper to the Russian Chemical Society, and according to Wikipedia's article on him it was titled "The Dependence between the Properties of the Atomic Weights of the Elements". Its central claim was that elements, "if arranged according to their atomic weight, exhibit an apparent periodicity of properties". That sentence is the idea behind the periodic table: the chemical elements are not a random list but fall into a pattern. This chapter shows how the modern table is laid out, how to sort its elements into metals, non-metals and metalloids, what three well-known families have in common, how Mendeleev used his pattern to predict elements nobody had found, and how to read one element's box. It builds on Chapter 2, where the atomic number was defined as the number of protons.

How the Table Is Laid Out

Wikipedia's Periodic table article describes it as "an ordered arrangement of the chemical elements into rows ("periods") and columns ("groups")", with the elements "listed in order of increasing atomic number". So the atomic number from Chapter 2 is the ordering key: hydrogen (1) comes first, and each step along the sequence adds one proton.

The same article says: "Today, 118 elements are known, the first 94 of which are known to occur naturally on Earth." The remaining 24, "americium to oganesson (95-118)", occur only when synthesised in laboratories. (The subtraction 118 - 94 = 24 was run in Python and agrees with the page.) This agrees with the figure of 118 elements used in Chapter 1.

Why a Group Behaves Alike

The Periodic table article says: "Elements with similar chemical properties generally fall into the same group in the periodic table." That is the practical value of the table: knowing one element tells you a lot about the elements above and below it. For the reason, the article points to electron arrangement: elements with the same number of electrons in a particular subshell share a column. The Alkali metal article makes the same point for group 1, saying the elements all have an outermost electron in an s-orbital and that "this shared electron configuration results in them having very similar characteristic properties".

The explanation in terms of electrons needs ideas (shells, subshells, orbitals) that this course reaches in a later chapter, so for now treat it as a stated fact: same group, similar chemistry, and the cause lies in the electrons.

Metals, Non-metals and Metalloids

The Periodic table article describes two trends: "Metallic character increases going down a group and from right to left across a period", and "Nonmetallic character increases going from the bottom left of the periodic table to the top right". Wikipedia's Nonmetal article agrees on position: nonmetals "nearly all occupy the right-hand side of the periodic table".

Pages read: Metalloid and Nonmetal were each about 234,000 and 259,000 characters long, and I read the first 100,000 of each. Hydrogen is a special case: the Alkali metal article says hydrogen is included in group 1 ("together with hydrogen they constitute group 1"), though hydrogen is not itself one of the alkali metals it lists. That hydrogen is a non-metal is my general knowledge (check), and I did not read further into why it is placed there.

Three Families

Alkali metals (group 1)

From the Alkali metal article: lithium, sodium, potassium, rubidium, caesium and francium, with hydrogen sharing the group. They are "shiny, soft, highly reactive metals" that "can all be cut easily with a knife". "All the alkali metals react with water, with the heavier alkali metals reacting more vigorously than the lighter ones." Because of this they are "usually stored in mineral oil or kerosene (paraffin oil)".

Going down the group, the page lists increasing reactivity and decreasing melting point alongside increasing atomic radius. The table shows the melting-point trend. The densities do not fall in a simple order (sodium is higher than potassium), so not every property is a smooth trend. I read the first 100,000 of about 295,000 characters of this page.

Halogens (group 17)

The Halogen article says: "In the modern IUPAC nomenclature, this group is known as group 17." It lists fluorine, chlorine, bromine and iodine, plus radioactive astatine and tennessine (some authors exclude tennessine). The name means "salt former" or "salt maker", and the page says that halogens reacting with metals produce salts such as sodium chloride and potassium iodide. One picture caption states: "Chlorine is a gas, bromine is a liquid, and iodine is a solid." The page says "the reactivity of elements decreases because of the increasing size of the atoms", so reactivity falls going down the group, the opposite of the alkali metals.

I read about 100,000 of 207,600 characters of this page.

Noble gases (group 18)

The Noble gas article calls them "the members of group 18". Under standard conditions they are "colorless, odorless, tasteless, and nonflammable", and the first six are "monatomic gases with very low chemical reactivity" (monatomic means one atom per particle). The page attributes the low reactivity to a full outer electron shell. Boiling points from its table are helium 4.4 K, neon 27.3 K and argon 87.4 K. Older layouts, the page says, labelled this column group 0 or group 8. Uses it lists include helium in balloons, argon shielding welding and glowing discharge lamps such as neon signs. I read about 100,000 of 215,600 characters.

Mendeleev's Table and His Predictions

Wikipedia's Periodic table article says: "The first periodic table to become generally accepted was that of the Russian chemist Dmitri Mendeleev in 1869." He framed the periodic law around atomic mass, and because some elements were still unknown, he used the law to predict properties of the missing ones. The Dmitri Mendeleev article says he arranged elements by atomic weight (now called relative atomic mass, as in Chapter 2) and valence, and that his predicted elements were called "ekasilicon, ekaaluminium and ekaboron", corresponding to germanium, gallium and scandium. Gallium and germanium were found in "1875 and 1886 respectively". The Wikipedia article on Mendeleev's predicted elements explains that the prefix "eka" comes from a Sanskrit word meaning "one", and that Mendeleev used eka-, dvi- or tri- depending on how many places down the group the missing element lay below a known one.

All values are from the Mendeleev's predicted elements page. The differences (for example 72.630 - 72 = 0.63 and 6.0 - 5.91 = 0.09) are my own arithmetic in Python. The page also says gallium was discovered "soon after Mendeleev predicted its existence, in 1875 by Paul Emile Lecoq de Boisbaudran". The Mendeleev article gives no discovery year for scandium and no predicted property values, so this chapter gives none. It adds that Mendeleev was wrong to assume atomic weight must always increase in order: for tellurium he said the weight "must lie between 123 and 126", while the observed value is 127.6.

The Periodic table article also notes that "Group 18, the noble gases, had not been discovered at the time of Mendeleev's original table", so the table had a column that did not exist yet. I read all of the Mendeleev's predicted elements page and 100,000 of about 151,300 characters of the Mendeleev biography.

Reading One Element's Box

The Periodic table article says each tile shows the symbol and a standard atomic weight, with the atomic number printed in front of the symbol. Its example reads "Hydrogen 1 H 1.0080". Reading it:

• 1 is the atomic number: the number of protons (Chapter 2).
• H is the symbol.
• 1.0080 is the standard atomic weight, the weighted average over isotopes described in Chapter 2 as relative atomic mass.

The article also says that a bracketed value such as "[209]" is the mass number of the most stable isotope: where an element has no stable isotopes, there is no sensible average, so the table gives one isotope's mass number instead. The relation N = A - Z from Chapter 2 then gives that isotope's neutron count (a worked case is in Exercise 1).

Where to Read More

• Encyclopaedias. The Wikipedia articles Periodic table, Alkali metal, Halogen, Noble gas, Metalloid, Nonmetal, Dmitri Mendeleev and Mendeleev's predicted elements are the sources used here. Apart from the predicted-elements page, each was read only up to its first 100,000 characters, as noted above.
• A school textbook. A printed periodic table shows every box for all 118 elements.
• Practical. Take a periodic table and colour in the metalloids, then check how they follow a rough diagonal line.

Exercises