Discussion

intro

Seekers of ye knowledge. Behold! The periodic table. Go now and use it wisely.

Are you still here? Well, I guess I should tell you something about the periodic table — like how to read it.

symbols

Each element name is represented by an element symbol. It could be…

• the capitalized first letter of the name of the element in English, Latin, or German. For example…
  • C for carbon
  • K for kalium (potassium in Latin)
  • W for wolfram (tungsten in German)
• the capitalized first letter followed by another lowercase letter from the name of the element in English or Latin. For example…
  • Si for silicon
  • Sn for stannum (tin in Latin)

• Systematic naming rules

  digit	symbol	root
  0	n	nil
  1	u	un
  2	b	bi
  3	t	tri
  4	q	quad
  5	p	pent
  6	h	hex
  7	s	sept
  8	o	oct
  9	e	en
  suffix	none	-ium

  a systematic element name and symbol — essentially a placeholder. These are only used for elements that are very heavy, very unstable, and very hard to make. Systematic element names are built from three roots, one for each decimal digit in the atomic number, with the suffix -ium added to the end. The corresponding symbol is built from three letters, one for each root, with the first letter capitalized. For example…
  • Uuo for ununoctium (the systematic name for element 118). In 2016 it was named oganesson after Yuri Oganessian, a pioneer in superheavy element research. Uuo and ununoctium were then retired.
  • Uts for untriseptium (the systematic name for element 137). If and when it is discovered it will be called this for a while. Some have proposed naming it feynmanium (Fy) after the American physicist Richard Feynman who predicted it would be the heaviest element possible. How this scenario plays out is open to some speculation.

This last convention arose during the Transfermium Wars of the late 20th century, which, despite sounding like a science fiction battle for supremacy of the galaxy, was actually nothing more than an academic argument. At this time, nuclear chemists in the United States and the Soviet Union were synthesizing elements heavier than fermium (thus the adjective "transfermium") and the governments of the United States and the Soviet Union were embroiled in the Cold War (thus the noun "wars"). Put the two together and you get Transfermium Wars — a bunch of chemists arguing about whose laboratory (and by proxy, whose superpower nation) was the best.

Naming rights go to the lab with priority in a discovery. The International Union of Pure and Applied Chemistry (IUPAC), which establishes rules for things like naming chemical elements, invented systematic names to impose a kind of armistice on warring chemists. Naming elements is a much slower process now as a result. This gives the scientific community time to verify claims of discovery. If laboratory A says it's found a way to synthesize element X and laboratories B and C can't synthesize element X using the same process, no one gets to name an element that day. As a pleasant side effect, the systematic naming convention also makes it possible to name elements that haven't been synthesized yet (and even those that may never be synthesized). I can fill my periodic table with placeholder names and symbols and then replace them with real names and symbols as reality reveals itself to science.

Two numbers are usually added to each cell.

The atomic number is the number of protons inside the nucleus of an atom. It is represented by the symbol Z in equations because Z is the first letter in the word atomic number (definitely not true). Each element can be identified from this number alone. It is always a whole number greater than zero when used to identify an element. The atomic number can also be applied to things like electrons and isolated neutrons in nuclear reactions. In these situations, electrons get Z = −1 and neutrons get Z = 0. This topic is discussed across several sections of this book.

The atomic mass is the mass of the average nucleus in an element and is stated in atomic mass units (1 u = 1.6605 × 10⁻²⁷ kg). It is represented by the symbol A in equations because A is the first letter in the word atomic mass (probably true, but who knows). This number is roughly equal to the total number of protons and neutrons in the typical nucleus of an element. By definition, this number is exactly equal to the number of protons and neutrons in the nucleus of your garden variety carbon atom — carbon 12 as it's called, which has a nucleus with 6 protons and 6 neutrons and a mass of exactly 12 u. Real carbon is a blend of isotopes, nuclei of the same element with different numbers of neutrons. Your typical collection of carbon atoms is 98.9% carbon 12 with 6 neutrons and a mass of 12 u (by definition), 1.1% carbon 13 with 7 neutrons and a mass of 13.00335 u (measured), and a minute but detectable trace of carbon 14 with 8 neutrons and a mass of 14.003241 u (measured). This averages out to 12.011 u for carbon on the Earth taken from non-living (or never alive) sources. Because living things prefer the lighter forms of carbon, the average is slightly lower than this in living and dead plants and animals. Then there's carbon on other planets, and other solar systems, and other galaxies, and other times in the history of the universe. The blend of isotopes in a sample of any element tells you something about where and when that sample came from and what kind of experiences it's had. This is what makes isotopes a subject of study unto itself.

Values in parentheses are used for radioactive elements whose atomic weights cannot be determined without knowing the origin of the element. The value given is the mass number of the most stable isotope of that element. The mass number is the number of protons and neutrons present in one nucleus. It is always a whole number. Elements of this type are formed in one of two basic ways — naturally or artificially. (More on this later.)

Some elements have never been produced in earthly experiments and no evidence for them exists on or off the Earth. These undiscovered elements are purely hypothetical. The tradition in this case (a tradition I invented right now) is to write the atomic number, followed by the systematic name, followed by three question marks in parentheses. I don't put a border around the cell and I use a faint gray that makes the text nearly invisible. When the discovery of a new element is announced, I'll add a border, change the text color to black, and replace the three question marks with the reported mass number.

blocks

The general layout of the periodic table is four different blocks of cells; inexplicably labeled s, p, d, and f. The names come from the obscure terminology of 19th century spectroscopy: sharp, principal, diffuse, and fine (or fundamental). Make sense now?

The cells are numbered consecutively starting at hydrogen on the upper left and continuing down and across in order of atomic number. There are two peculiarities about the table arranged this way.

Helium (He) is in the s block, but its placed like a shampoo horn on top of the p block. That's because the chemistry of helium is more like neon (Ne), argon (Ar), etc. — the noble gases. In terms of electronic structure, which is what the blocks really relate to (see the section on orbitals below), helium definitely belongs with beryllium (Be), magnesium (Mg), etc. I guess the periodic table is chemistry's baby. The chemists get to decide how to raise it. Let's see how they do.

There is a discontinuity in the sequence after barium (Ba) and radium (Ra) — atomic numbers 56 and 88, respectively. That's where the f block should go, but it doesn't have to. The f block is pulled out and placed below the rest of the elements. It's a formatting convention that works well for pieces of paper bound into books (and by extension, displays on book shaped electronic devices). This is something like maps of the United States with Alaska and Hawaii placed below the 48 contiguous states. It uses the space more wisely without losing much of the meaning.

Each block is double an odd number of cells wide (2, 6, 10, 14). The Austrian physicist Erwin Schrödinger explained the mathematical origin of this pattern in 1926. More on that later. For now realize that the chemical behavior of the elements are related in a way that can be derived mathematically from physical principles. Chemistry detected the patterns. Physics explained them.

groups

Elements in the same column or group on the periodic table have similar or related chemical properties. This kind of behavior is probably best illustrated through the law of definite proportions. I don't know how to say what this law is in a sentence, so let me illustrate through examples. (The actual law of definite proportions is stated in terms of masses, but I will use the much simpler notion of "parts", where a "one part" is an arbitrary number of atoms, "two parts" is twice as many, and so on.)

Everybody knows this molecular formula.

H₂O

It says if you want to make water, you take 2 parts hydrogen and 1 part oxygen and then get them to do their thing (try adding some heat). Then hydrogen (H₂) plus oxygen (O) equals water (H₂O). That's maybe not the best example for a book written for people who want to learn physics instead of chemistry. It's actually 2 parts hydrogen (2H₂) and 1 part oxygen (O₂) produce 1 part water (H₂O).

2H₂ + O₂ → 2H₂O

What if I took 3 parts of hydrogen and added it to one part of oxygen? Could I make H₃O? Would this super water give me super powers? That would be awesome. Let's see.

3H₂ + O₂ → 2H₂O + H₂

Crap. My extra hydrogens did nothing but stay hydrogens. Super water and super powers will have to wait. It appears that atoms only combine in definite proportions — not in whatever proportion I feel like. Such are the laws of nature.

Let's try some examples that see how the law of definite proportions reveals itself in the groups of the periodic table. Take two elements from the first column on the periodic table (group 1), sodium and potassium, and one from the second to last column (group 17), chlorine. The natural way to combine these elements turns out to be one sodium atom (Na) for every one chlorine atom (Cl). That gives us sodium chloride, which we write it like this…

Na + Cl → NaCl

Similarly one part potassium (K) will combine with one part chlorine (Cl) to form potassium chloride.

K + Cl → KCl

Elements selected from group 1 combine with those in group 17 in a similar proportion (1:1).

Let's change things up and try it again. Take two elements from the second column (group 2), magnesium and calcium, and one element from the second to last column (group 17), chlorine again. Magnesium chloride is a compound that has one magnesium atom (Mg) for every two chlorine atoms (Cl₂).

Mg + Cl₂ → MgCl₂

Compare that to calcium chloride, which is made from one calcium atom (Ca) and two chlorine atoms (Cl₂).

Ca + Cl₂ → CaCl₂

Elements selected from group 2 combine with those in group 17 in a similar proportion (1:2).

groups with trivial names

In general, elements in the same group on the periodic table have similar chemical behavior. There are 18 numbered groups. The 14 groups of the f block (the two separated rows near the bottom) are not numbered. Groups can be identified by the topmost element in a systematic fashion. Some of them are also known by nonsystematic, trivial names. The trivial names for groups (and periods, discussed next) seem seem anachronistic — relics of some obscure forgotten age.

Group 1, the lithium group, a.k.a. the alkali metals

Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs) are all shiny, soft, highly reactive metals that are normally found attached to elements from the opposite side of the periodic table to form salts. The salt used in cooking is sodium chloride (NaCl) also known as table salt. The related compound potassium chloride (KCl) is sometimes used as a salt substitute by people on low sodium diets. Lithium chloride (LiC) was also used as a salt substitute for a while until it was found to be toxic. Hydrogen (H) may be a group 1 element, but it's not an alkali metal. If you decided to by some hydrogen right now, it'd be delivered to you as a diatomic molecular gas (H₂) in a pressurized tank not as a lump of shiny, soft, highly reactive metal nor attached to chlorine as a salt. When hydrogen gets together with chlorine you get hydrogen chloride (HCl), which seems superficially similar to table salt (NaCl), salt substitute (KCl), and toxic salt substitute (LiCl) except for the fact that it's a gas, not a salt. Bubble hydrogen chloride gas in water and you get hydrochloric acid. Hydrogen is an oddball in the periodic table. Where does it really belong?

Group 2, the beryllium group, a.k.a. the alkali earth metals

Calcium (Ca) is an important element in your bones. Strontium (Sr) is a significant component of the radioactive fallout from nuclear weapons. Both are group 2 elements. That means that strontium from nuclear fallout can easily replace calcium in your bones and nerves. This is bad news for everyone who lived through the bad old days of regular nuclear explosions. (Your humble author was an infant back in the day when nuclear weapons were tested every two to three days on average somewhere on Earth. Thankfully, it did not kill him.) The strontium in fallout is radioactive. It is quietly carried on the winds until some of it settles on agricultural land. The strontium is absorbed by crops, which are then eaten by people. Some of it lands on pasture where it is absorbed by grass, which is eaten by livestock, which is eaten by people. Voila! Steak frites avec strontium. Strontium is nearly indistinguishable from calcium chemically, so your digestive tract extracts the radioactive strontium as if it was highly valuable calcium. Now the strontium is a part of you. Bon appétit!

Group 11, the copper group

Copper (Cu), silver (Ag), and gold (Au) are the best conductors of heat and electricity. They are all group 11 elements. Nothing else needs to be said.

Group 16, the carbon group

Carbon (C) is a key element to life on Earth. The adjective "organic" in chemistry refers to any molecule that contains at least one carbon atom. Living things are nothing but bags of water mixed with organic molecules (and some mineral salts). Since silicon (Si) is directly below carbon in group 16 on the periodic table, silicon based life is an interesting possibility — if not on Earth, then perhaps somewhere in outer space. Unfortunately, silicon is not as reactive as carbon. When silicon does get together with another element, it tends to be oxygen. Silicon is the second most abundant element in the Earth's crust. The first is oxygen. What? Oxygen in the Earth's crust? Surely that's a mistake. I don't recall being able to breathe rocks. Oxygen is found in the Earth's crust bound to silicon to make silicon dioxide (SiO₂) also known as quartz, also known as what sand is, also known as what nearly every rock on the planet is made from. Carbon is the fifteenth most abundant element in the Earth's crust. Even titanium is more abundant (ninth) and chances are good most people on Earth have never touched a piece of titanium. Carbon is easy to find. It's called charcoal. Fossilized carbon is called coal. If life could have been made from silicon, why isn't it? While silicon likes to bond with oxygen and not much else, carbon likes to bond with nearly everything. If silicon is monogamous, carbon is promiscuous. It'll hook up with anyone, anytime, anywhere. Large silicon molecules tend to be monotonous, repeating units like SiO₂, SiO₂, SiO₂, over and over again. Carbon is much more creative — single bonds, double bonds, triple bonds; bind to oxygen, bind to nitrogen, bind to more carbon. Life arose out of carbon because carbon chemistry is more versatile and thus more adaptable. Silicon based life isn't impossible, and some have proposed that silicon clays may have been the first lifeforms, but life built on carbon will surely prevail.

Group 17, the fluorine group, a.k.a. the halogens

The elements in group 17 are often the second element in salts. They are also known as the halogens — from the Greek άλας (alas), salt and γεννω (genno), born. Like an elemental mother, the halogens "give birth to salt", but like a fairytale stepmother, the elements in group 17 are wicked. Diatomic fluorine (F₂) is a highly toxic pale yellow gas. Hydrofluoric (HF) acid can dissolve glass. Fluorine hates your beauty and wants to kill you. Diatomic chlorine (Cl₂) is a highly toxic yellow-green gas. Chlorine compounds are added to water to kill bacteria. We call them bleaches. In small concentration, chlorine bleaches are of little concern, but in larger concentrations, bleach will kill you too. Chlorine doesn't like you any more than fluorine does. Diatomic bromine (Br₂) is a fuming red-brown corrosive and toxic liquid at room temperature. It's fuming and brown. What more do you need to know? Bromine is not your friend either. Iodine (I) is necessary for life. It's the friendly member of group 17. In solid form, its dark brown. As a vapor, it's violet or purple. You eat straight iodine and your life will be short. You consume iodine in small amounts as a part of a healthy diet and your thyroid gland will be happy. Iodine is occasionally nice to humans.

Group 18, the helium group, a.k.a. the neon group, a.k.a. the noble gases

Every chemist's worst nightmare. Not because they live to see you die like the elements in group 17. Chemists dislike elements in group 18 because they're boring. They might as well not be called chemical elements. Helium (He) and neon (Ne) don't appear to participate in any chemical reactions at all. In 1962, xenon (Xe) was finally coaxed into combining with fluorine to make several compounds, most notably xenon difluoride (XeF₂) and xenon tetrafluoride (XeF₄). Both of these compounds are used to etch integrated circuits in semiconductor fabrication plants. (Fabs as they're called in the biz.) Combining highly corrosive fluorine with nearly inert xenon basically makes it easier to get the fluorine to go where you want it to. Then the fluorine leaps off the xenon and kicks chemical ass. Fluorine is also the only element capable of doing any chemistry with argon (Ar), krypton (Kr), and radon (Rn). The elements in group 18 are nonreactive gases. In some sense, they're like the corrosion resistant metals prized by nobility — gold, platinum, and silver — thus their trivial name, noble gases. Group 18 is a castle at the end of the periodic table where the noble gases exercise their rights of inertia.

periods