�Before You Begin:
To master this material you will need your a copy of the periodic table. You will need to be familiar with the symbols for the elements and have a basic understanding of atomic theory.
Types of elements
Semi-metals or metalloids
The elements boron, silicon, germanium, arsenic, antimony, tellurium, and astatine have properties similar to both metals and non-metals. These elements are located along the semi-metal line of the periodic table. To find the semi-metal line, locate boron, atomic symbol B, on a periodic table. Draw a stair step line down the left side of the boron square and across the bottom. Extend the line down the next horizontal row between aluminum, Al, and silicon, Si. Extend the line horizontally to the right below silicon, then down the next row. Continue in this pattern until you have drawn a zig-zag from boron to astatine. All the elements that share a straight side on this line except for aluminum and polonium are semi-metals.
Elements to the left of the semi-metal line on the periodic table are malleable (can be hammered into a shape), ductile (can be stretched into a wire) and good conductors of heat and electricity. These elements tend to lose electrons to form cations.
Elements to the right of the semi-metal line on the periodic table (and hydrogen) are brittle and insulators of heat and electricity. These elements tend to gain electrons to form anions or share electrons as bonds to form molecules.
4Concept Check: What type of element is cesium, Cs?
Answer: Cesium is a metal. It can be found on the lower left of the periodic table.
Regions of the Periodic Table
Groups and Periods
Groups or families
The elements in vertical columns of the periodic table have similar chemical properties. The sets of elements are called groups or families. Some of the families have historical names that pre-date the modern periodic table, since their chemical similarities were noted before the table was widely accepted.
· Alkali metals—the metallic elements in group 1 are called the alkali metals because they react with water to form alkaline solutions. Note that hydrogen, though it is in group 1, is not an alkali metal.
· Alkaline earth metals—the elements in group 2 have properties similar to those of group 1 but are slightly less reactive, hence “earthy.”
· Chalcogens—the non-metals and semi-metals in group 16 are called chalcogens from the Greek word for ore because a great many ores are oxides, sulfides or telurides. Note that polonium is a metal and has very different properties from the other members of group 16. This is the only traditional group name that you won’t be expected to know.
· Halogens—the elements in group 17 are known as the halogens (‘salt former’ in Greek) because they easily form negative ions (which are found in the class of compounds called salts).
· Noble or inert gases—the elements in group 18 are known as noble gases because they do not readily form compounds. The word noble, in this context, is the opposite of the word base, or easily corruptible.
Groups are numbered from left to right across the periodic table. Older designations used Roman numerals and the letters A and B to identify the groups. Unfortunately, while these systems are still in use, the letter designations in the United States and in Europe are NOT the same.
The elements in horizontal rows of the periodic table have atoms with the most similar sizes. Periods are numbered from 1 through 7. The two horizontal rows found at the bottom of the periodic table are actually inside periods 6 and 7.
4Concept Check: What group and period is tin, Sn, in?
Answer: Tin is in group 14 and in period 5.
Main group or representative elements
Elements found in the tallest groups of the periodic table are the main group or representative elements. Elements in this region of the periodic table are the most reactive (except the noble gases) and the most predictable with respect to periodic properties.
The transition elements are the elements found in the middle region of the periodic table in a rectangle formed with the elements scandium, zinc, actinium, and ununbium at the corners. Elements in this region of the periodic table are metals with less predictable properties with respect to the periodic properties.
Inner transition elements
Elements found in the bottom two rows of the periodic table in the lanthanide and actinide series. These elements are similar to the transition elements. In fact, if you scan the atomic numbers from 58 to 72 and from 89 to 104, you will see that the inner transition elements actually belong inside the transition area. The top row of inner transition elements is also called the lanthanide series, because these elements follow lanthanum, atomic number 57. The bottom row of inner transition elements is also called the actinide series, because these elements follow actinium, atomic number 89.
Ionic charge—the number of electrons an element tends to gain or lose is a periodic property. The alkali metals always form +1 ions. Hydrogen can form either +1 or -1 ions. The alkaline earth metals always form +2 ions. Nitrogen and phosphorus in group 15 usually form -3 ions. The chalcogens usually form -2 ions. The halogens always form -1 ions. The noble gases don’t readily form ions. Many of the transition metals form more than one type of ion.
4Concept Check: What is the most common charge for selenium, Se?
Answer: Selenium forms a negative two charged ion like oxygen and sulfur do.
Electron affinity—the energy released when an atom gains an electron. In general, this increases, from left to right, across a period and at the top of a family. A notable exception to the periodic trend is group 18, which resists gaining electrons. The elements that form positive ions the most readily are at the lower left of the periodic table.
Ionization energy—the energy needed to remove the outermost electron from an atom or ion. This trend also increases from right to left across a period and from top to bottom of a group on the periodic table. A notable exception is the noble gas group. The elements most resistant to being converted to negative ions are at the lower left and the easiest are at the upper right.
Atomic Radius—the distance between the nucleus and the outermost electrons of an atom is the atomic radius. The distance between the nucleus and the outermost electrons of an ion is the ionic radius. This trend increases from top to bottom of a group and from right to left across a period. The largest atoms are at the lower left. Ionic radii are always smaller than the corresponding atomic radii.
Valence—the valence is the number of outermost electrons that an atom has available to lose, gain, or share. The valence electrons are the ones that take part in chemical changes, so the number of valence electrons determines an atom's chemical behavior.
4Concept Check: What is the valence of calcium, Ca?
Answer: Calcium has a valence of 2.
Electronegativity—the strength with which an atom pulls on the electrons it shares in a bond (the attractions that hold molecules together) is its electronegativity. The trend is the same as the electron affinity trends.
© Copyright 2005, Kelley Whitley, ChemProfessor. All rights reserved.
This site was last updated 05/12/05