The Handy Chemistry Answer Book (2014)

ATOMS AND MOLECULES

TRENDS IN REACTIVITY AND THE PERIODIC TABLE

What was the law of triads?

A scientist named Johann Döbereiner discovered trends in the reactivity of groups of elements. Certain sets of three elements, such as lithium, sodium, and potassium, showed similar chemical properties, and Döbereiner noticed that the average of the atomic masses of the atoms of the heaviest and lightest elements in the triad gave the atomic mass of the midweight element. For example, the mean of the atomic masses of lithium and potassium is (3 + 19)/2 = 11, which is the atomic mass of sodium. Due to differing numbers of neutrons in each element and the existence of different isotopes, this law isn’t always strictly true, but it does tend to work well, especially for lighter elements. For reasons we hope to explain in the coming questions, this trend plays an important role in the structure of the periodic table.

What was the law of octaves?

The law of octaves was put forth by the British chemist John Newlands. He noticed that, when the elements were listed in order of increasing atomic weight, elements with similar properties occurred every eight elements. The trend was named “the law of octaves” by analogy to musical scales, and it was the first realization of the relationship between atomic masses and a repeating pattern of elemental properties. This periodicity has since been explained in detail as chemists have gained a better understanding of atomic structure, and the law of octaves played a crucial role in the development of the periodic table chemists use today.

How was the modern periodic table developed?

A French geologist named Alexandre Béguyer de Chancourtois is actually the first on record to list all of the elements in order of increasing atomic mass. His first version contained sixty-two elements and they were placed in columns that wrapped around a cylinder; however, there were a variety of issues with this first attempt that were later improved upon. Newlands made the next significant advance, publishing the elements in columns of those with similar properties, which brought the description close to the version chemists use today.

The modern version of the periodic table was proposed by a Russian scientist named Dmitri Mendeleev in 1869. His table was the first to lay out the elements in order of increasing atomic mass in columns of elements with similar reactivity. Elements on the table appeared periodically, essentially in accordance with the law of octaves, hence the name “the periodic table.” Mendeleev’s table had to include some blank spaces so that the elements were each in the proper column according to reactivity. As more elements were discovered, the blank spaces in the table were eventually filled in, validating Mendeleev’s table.

Which elements are most abundant?

Element

Abundance
(ppm atom fraction)

Hydrogen

909,964

Helium

88,714

Oxygen

477

Carbon

326

Nitrogen

102

Neon

100

Silicon

30

Magnesium

28

Iron

27

Sulfur

16

Image

The Periodic Table of Elements.

What are the different groupings for elements on the periodic table?

There are several ways of classifying the elements on the periodic table. One is by the periods, or the horizontal rows, in each of which the properties of the elements change going from left to right. Another common classification is by groups, or vertical columns on the table. All elements in the same group are expected to have similar properties, which is the periodic property, originally noted in the law of the octaves, after which the table is named.

Yet another classification is by blocks, meaning the elements are classified by the type of orbital in which the highest energy electrons reside (see below). The logic behind this type of classification is that the type of orbital in which the highest energy electron resides strongly influences the reactivity of the element, thus elements in the same block usually have similar properties. There are even more ways still of clustering the elements on the table, but these three are the most commonly used.

What is scientific notation?

Scientific notation is a commonly used method for expressing large numbers. The number is written as a product of a decimal number and a power of 10. See the following question for an example of where this is useful.

What do the numbers on the periodic table mean?

The periodic table lists elements in boxes containing their name, atomic number, chemical symbol, and atomic mass (averaged over the natural abundances of the various isotopes). A typical arrangement for a given element looks something like this:

Image

How many elements are there, and will more be discovered?

As of the writing of this book, 118 elements have been discovered. The lightest element, with only one proton and an atomic mass of 1.00794 g/mol, is hydrogen. The heaviest is ununoctium with 118 protons and an atomic mass of 294 for the only detected isotope. Considering that five new elements have been discovered since the year 2000, it seems very likely that more elements will be discovered. It is getting harder and harder for scientists to discover, or synthetically create, new elements, though, because the heaviest elements that have been observed to date are usually unstable and decay extremely quickly.

How are elements named?

The names of elements often have interesting origins. They have been named after people, places, colors, mythological creatures, or for a variety of other reasons. Some are named after scientists, such as Curium (after Marie and Pierre Curie), Lawrencium (Ernest Lawrence), Seaborgium (Glenn Seaborg), Mendelevium (Dmitri Mendeleev), Einsteinium (Albert Einstein), and Bohrium (Niels Bohr). Others are named after places, such as Lutetium (Lutetia means Paris in Greek), Californium, Berkelium (Berkeley, California), Americium, Dubnium (Dubna, Russia), Hassium (Hessen, Germany), Yttrium, Ytterbium, Terbium, and Erbium (these last four being named after Ytterby, Sweden).

Tantalum (Tantalus), Niobium (Niobe), Promethium (Promethius), Uranium (Uranus), Neptunium (Neptune), Plutonium (Pluto), Palladium (Pallas), and Cerium (Ceres) are all named after mythological creatures.

Though elements can take on different names in different countries, the commonly accepted names are those agreed upon and assigned by the International Union of Pure and Applied Chemistry (IUPAC).