Chemistry Lab/Periodicity
Periodicity was a topic for the event Chemistry Lab in 2012 and 2013.
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
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.
| Group Number | Group Name |
|---|---|
| Group 1 | Alkali Metals |
| Group 2 | Alkaline Earth Metals |
| Groups 3-12 | Transition Metals |
| Group 11 | Coinage Metals |
| Group 13 | Boron group |
| Group 14 | Carbon group |
| Group 15 | Pnictogens (Nitrogen Family) |
| Group 16 | Chalcogens (Oxygen Family) |
| Group 17 | Halogens |
| Group 18 | Noble Gases |
- The alkali metals are extremely light, reactive, and soft metals. Some spontaneously burn in air, and they all react violently on contact with water.
- Alkaline earth metals are slightly less reactive than the alkali metals, but are still reactive and flammable, but less so than their neighbors.
- Transition metals are what most people think of when they hear the word "metal." They are generally tough, stable, and hard to melt, with some exceptions. Technetium (Tc, 43) is radioactive, and mercury (Hg, 80) is liquid at room temperature.
- The coinage metals are a subgroup in the transition metals. They are very good conductors of electricity - silver (Ag, 47) is the best conductor of any element.
- The boron group is generally reactive, but properties vary by element, although all can react as +3. As you go down the column, the likelihood of reacting as +1 increases.
- Properties of the carbon group elements vary widely.
- The pnictogens' properties vary widely as well.
- Like the boron group, the chalcogens are generally pretty reactive, but have variable properties.
- The halogens are all very reactive, because they have seven electrons in their outer shell and desperately need eight. Fluorine (F, 9) is the most reactive of all and can even form compounds with noble gases.
- The noble gases are the most inert group of all. Under normal circumstances, they will not form compounds. However, in 1962, scientists made xenon (Xe, 54) compounds with ultra-reactive fluorine (F, 9).
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." Hydrogen is also considered in the"other nonmetals."
Periodic Trends
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, with the exception of the inert noble gases.
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. These are generally seen in metal-nonmetal combinations.
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. These are usually seen in nonmetal-nonmetal bonds.
Ionic Charges
If an atom gains or loses electrons, it is known as an ion. When it loses electrons, it has a positive charge and is known as a cadion. When an ion gains electrons, it has a negative charge and is called an anion.
Other Types of Bonding
Metallic bonds
Two or more metals forming a compound make a metallic bond. It's like the metallic equivalent of a covalent bond.
Polar covalent bonds
When the atoms in a covalent bond do not share the electrons equally, it is called a polar covalent bond.
Solubility Trends
For information about solubility trends, please see Chem Lab/Aqueous Solutions#Solubility Rules.