A Category of One's Own Trivia Quiz

Answer: Noble gas

It was long thought that the noble gases (helium, neon, argon, krypton, xenon, and radon) did not react at all with other chemicals, and they were called inert, meaning non-reactive. The family was also sometimes called the rare gases, but they are not actually all that rare, especially when compared to some others. Argon comprises over 1% of the mass of the gases forming the earth's atmosphere. The term noble gas was coined in reference to their low reactivity as a way of comparing them to the noble metals, which also have low reactivity compared to the elements around them. The noble metals, which include silver, platinum and gold, do not all belong to the same chemical family, however.

Before 1990, there were two different numbering systems used to identify the families of the Periodic Table, both based on using Roman numerals that had some correlation to the element's electron configuration, but they used a different basis for deciding when the letter A was added to the Roman numeral, and when it was B. This meant that the same symbols were applied to different groups in the two systems, which was more than a little confusing. The International Union of Pure and Applied Chemistry introduced the new system, using the Arabic numerals 1-18 to stand for the 18 columns seen in the traditional layout of the periodic table. The noble gases are in group 18, at the extreme right hand end of the table.

Answer: Alkali metals

The alkali metals are found in group 1 of the periodic table, at the very left-hand side. They are all metals, with a single electron in their outermost shell. They react readily (even explosively) with water to form a solution which contains a large amount of hydroxide ions, which is called an alkaline solution. Caesium, the element described in the question, will even react with solid ice! Its name (from the Latin 'caesius', meaning sky-blue) comes from the fact that its emission spectrum contains bright blue lines, and many of its compounds burn with a blue flame. Caesium is very soft (0.2 on Mohs scale, the lowest of any element), and has a low melting point of 28.4 °C (83.1 °F). This means that it will melt on a hot day, although it is not considered to be a liquid at room temperature because chemists use that term to refer to a specific temperature - 25 °C (77 °F). When exposed to small amounts of oxygen, the metal develops a silvery-gold patina, but exposure to larger amounts, such as found in air, causes it to spontaneously combust.

The gas that was harassing the caesium in the question was chlorine, whose compound with sodium, second-lightest member of the alkali metal family, is familiar to all as table salt.

Answer: Actinoids

Also called the actinides, the elements with atomic numbers from 89 to 103 are usually shown as one of the two groups of chemicals running horizontally under the main body of the periodic table. This is because adding them all in the main body would make the table much wider, and difficult to fit neatly onto the page. Also, they have enough similarity of chemical and physical properties to justify setting them out as a group of their own.

Only a few members of the family (thorium and uranium) are found in significant quantities in nature. The others are formed as short-lived decay products of one of these, or have been artificially produced. Because of its usefulness in producing nuclear weapons, the artificial element plutonium is the third-most abundant member of the family. Many modern smoke detectors use americium: the alpha particles emitted by americium are readily absorbed by smoke particles, so that the current flowing inside the chamber is reduced, and the alarm is triggered. But don't forget to change the batteries regularly!

Answer: Pnictogens or nitrogen family

Often called the nitrogen family, and still referred to by many physicists as Group V even though chemists have decided to call it Group 15, the pnictogens got their current name in the late twentieth century at the suggestion of the Dutch chemist Anton Eduard van Arkel.

It is based on the Greek root ('pnitka') for the German name for nitrogen - 'Stickstoff' can be translated as 'suffocating substance', and 'pnitka' means 'suffocated things'. 'Pnictogen' therefore means 'maker of suffocated things'. If the atmosphere were completely nitrogen, we would indeed have trouble breathing! The rest of the pnictogen family are solids under standard conditions. Nitrogen and phosphorus are essential for life; they are both non-metals, while the poisonous arsenic and the not-so-poisonous antimony are metalloids (with some properties of metals and some of non-metals), and bismuth is a metal.

This group provides a very nice example of the generalization that metallic properties increase as you go down a column in the Periodic Table.

Answer: Chalcogens

Oxygen, O2, is one of the main constituents of the Earth's atmosphere. Ozone, O3, provides essential protection against ultraviolet-radiation. Water, H2O, is also essential for most forms of life. The incorrect options are the forms used for compounds of three of the other chalcogens. Oxygen, the narrator of the question, forms oxides. For historical reasons, many still treat oxygen and oxides separately from the other members of the group, whose chemical behavior is more in line with the new name. 'Chalcogen' comes from Greek words that literally mean 'copper forming', but the reference is to their presence in ores, both copper compounds and others.

They tend to form insoluble compounds with many metals, so are found in a number of useful ores. Pyrite, often called Fool's Gold, is iron disulfide; galena, an important source of lead, is lead sulfide.

Answer: Rare earth metals

Despite their name, the seventeen rare earth metals are not all that rare (with the exception of promethium, which is radioactive and decays to form other elements). Their name comes from the fact that their compounds tend to be found spread in among other minerals, and are not often found in commercially-exploitable concentrations. Fifteen of them, with atomic numbers 57 (lanthanum) through 71 (lutetium) are generally shown on Periodic Tables as a row of their own, below the main body of the table.

The entire row should fit into the third column, underneath scandium and yttrium, which are also commonly found in the same ore deposits as the lanthanoids. The first commercially-viable deposit of a mineral containing them was gadolinite, also called ytterbite, discovered near the Swedish town of Ytterby.

A number of the rare earth metals have names referring to this location - scandium comes from Scandinavia; yttrium, terbium, erbium, and ytterbium from Ytterby.

Answer: Hydrogen

Hydrogen is usually considered to be in a family of its own, and is not shown as belonging to one of the 18 groups of the IUPAC system. With only one electron, it can lose the electron to form a positive ion, a property that would make it fit into Group 1.

However, the other Group 1 elements are all metallic solids, whereas elemental hydrogen is a diatomic gas. This molecular structure is formed because hydrogen atoms each need one more electron to have a filled outer electron shell, a property they have in common with the elements of Group 17. Since these two groups are at opposite sides of the Periodic Table, it's no wonder hydrogen feels a bit lost!

Answer: Alkaline earth

A high pH, which means a low concentration of hydrogen ions in the solution as compared to hydroxide ions, means the solution is alkaline. This family includes magnesium, calcium, strontium, barium and radium. (While some periodic tables include helium in Group 2 because it also has two electrons in its outer shells, it is not an alkaline earth metal, it is a noble gas.) The 'earth' part of the family name refers to the fact that their oxides have low solubility in water and a high melting point.

Magnesium and calcium are essential to all living organisms. Beryllium, on the other hand, is usually toxic. Strontium is added to some toothpastes designed to strengthen tooth enamel; barium is used for medical diagnostic purposes because it is opaque to X-rays, which means that a liquid containing it can be tracked through the digestive system; radium's radioactivity makes it highly toxic.

Answer: Metalloids

As is common in scientific terminology, the suffix -oid indicates a resemblance to a group to which the object does not actually belong. Metalloids (which include boron, silicon, germanium, arsenic, antimony and tellurium) have a mixture of metallic and non-metallic properties.

They look metallic, but have a much lower electrical conductivity than is typical of metals, and their chemical properties tend to be non-metallic. This has made them extremely valuable in the semi-conductor industry. Silicon dioxide, or silica, is found in nature as quartz or as sand, and is commonly used to make glass.

Answer: Halogens

The halogens are so named because they form salt-like compounds with metals. Table salt, of course, is sodium chloride, and fluorine, bromine, iodine and astatine form similar compounds with the alkali metals. The halogens also all form hydrogen compounds that are very strong acids, since they are very polar molecules, and readily dissolve into positive hydrogen ions and negative chloride ions.

Their high electronegativity makes halogens very reactive, and they are dangerous to living organisms because of this - hence the use of chlorine compounds to destroy microbes in your swimming pool.