Chemistry Lab/Periodicity

This page is related to the 2012 focus on periodicity for Chem Lab.

Basic Information
Periodicity is the backbone of the Periodic Table. It means "a repeating pattern that is at regular levels." The Periodic Table is ordered by certain chemical and physical properties. These periodic trends may increase or decrease by element, depending on the way one moves along a row or column.

The Periodic Table
For a picture of the Periodic Table, see Periodic Table.

The periodic table was first ordered by Dimitri Mendeleyev in 1869. It grouped the elements known at the time into families, based on similar chemical properties, and left spaces blank for future elements to be discovered.

The Periodic Table is arranged by rows (or periods) and columns (or groups). Periods are arranged by atomic number, and groups are ordered by similar chemical properties.

Groups
Groups are ordered from 1 to 18 (new IUPAC numbering), and these correspond to the number of valence, or outer, electrons that the element has.

Some groups have names, which are dependent on chemical properties.

Special Blocks
There are also certain blocks of elements that have special names.


 * The Lanthanoids and the Actinoids are the elements at the very bottom of the short form of Mendeleyev's Periodic Table.
 * The Lanthanoids are elements 57-71
 * The Actinoids are elements 89-103
 * The Lanthanoids and part of the Actinoids (up to Uranium) are known as Rare Earth Metals, based on the incorrect belief that they were extremely rare.
 * The Lanthanoids' general purposes involve magnetism mostly, as they are components of strong magnets, like neodymium-iron-boron. However, cerium has uses for the fact that it can create sparks, europium is used in computers and the euro, and terbium's ability to change shape in a magnetic field allows the element to turn a table into a loudspeaker.
 * As for the actinides, most of their purposes are not beyond experimental. However, thorium is used for purposes similar to transition metals (its half-life in 14 000 000 000 years), uranium-235 is used in nuclear reactors and weapons, uranium-238 is used in old merchandise (from the radiation-craze era), plutonium is used in weapons and old pacemakers, americium in smoke detectors, and californium in some cancer treatments.
 * Elements past Uranium are known as Trans-Uranic, and do not occur in nature (although neptunium and plutonium might occur in traces in uranium ore).


 * Boron, Silicon, Germanium, Arsenic, Antimony, and Tellurium are known as semi-conductors, because they can conduct electricity under certain conditions - that's why electronics use silicon.
 * With Polonium, these seven elements are known as metalloids - they have some metallic properties and some nonmetallic properties. However, some are more metallic than others, and these are to the left of this "divide" that starts just below boron and "stair-steps" down the periodic table towards the lower righthand corner. Elements to the left are more metallic; elements to the right are more nonmetallic.


 * Elements in Groups 13-16 that are not categorized are just simply known as "other metals/non-metals."

Some examples of Periodic Trends

 * Atomic Radius: distance from nucleus to outermost electron; increases as one moves down a period, and from right to left. These are due to more electron shells and lower electronegativity - except in the case of the noble gases, which have enough electrons already and so have no electronegativity, but the protons have enough electromagnetivity to pull electrons in.
 * Ionization Energy: amount of energy required to move one electron from an atom; increases as one moves up a period, and from left to right
 * Redox properties: the possibility the element will be involved in a redox equation; increases as one moves outward from the center of the table

Bonding Trends
Elements' place on the Periodic Table also governs the elements' bonding habits.

Ionic vs. Covalent
An ionic bond is a chemical bond where one ion loses one or more electrons and gives it to the other ion.

A covalent bond is a chemical bond where one or more electrons from one element leave that element and fill the outermost electron shell of the other element. In most simple cases, this results in both elements having full outer shells of electrons.

Solubility Trends
For information about solubility trends, please see Chem Lab/Aqueous Solutions.

Links

 * Chemistry and Mineral Terms section on GMOA Notes
 * Nice explanation of a few periodic properties