The name of the Periodic Table was derived from the fact that element properties tend to repeat themselves periodically when aligned in rows.
In any table, the items included are arranged in order of a particular property. A property, which can be expressed by a number (such as size) is better than a property which cannot (such as color). Before the chemical elements could be intelligently arranged there needed to be such definition.
Early chemists tried to use weights that had been estimated, but they were not only slightly inaccurate, faulty reasoning had led to some being a half or a third of the correct value. In 1860 a conference produced a much more accurate list of atomic weights than previously available. Alexandre Béguyer de Chancourtois was the first person to list the known elements in order of increasing weight of their atoms.
Today, elements are listed in order of increasing atomic number (i.e. The number of protons in the atomic nucleus).
While Dmitri Mendeleev stated
"...if all the elements be arranged in order of their atomic weights a periodic repetition of properties is obtained.", it does not appear that the element boxes in the illustration above are all connected, due to large gaps, but at least all the elements in a period are in the same row - indicated by color.
The layout of the periodic table above demonstrates recurring ("periodic") chemical properties. Elements at the beginning of a period, left side, and those at the end of a period, right side, while only one number apart, are quite different in their behavior and other properties. This provides a logical break point for purposes of 2-D charting.
Bohr's theory of the atom tells us that electrons are not located randomly around an atom's nucleus, but they occur in specific electron shells. Each shell has a limited capacity for electrons. As lower shells are filled, additional electrons reside in more-distant shells. Elements at the end of a period have a lack of space in the outer shell of the element, making it most difficult to combine with another element. That column is called "Noble" because of this refusal to mix with the others.
You may also notice that there are fewer elements to a row/period at the top of the table than at the bottom. This jump in the number of elements in the periods - few at top, more at bottom - is because the outer shell is larger as the number of protons and electrons become greater - hence a longer period. Every period, however, includes all of the groups above it.
On the periodic table, a period is from the far left (on a flat representation as above - the very long form) to the far right, which are only a number away, and on a 3-D form, are generally found adjacent. The elements in each period that are similar form groups (see above left), also known as a families, more often that not (except for the f-block - blue & gray) in vertical columns. There are 18 columns in the standard periodic table, and 32 in the extended table illustrated above.
Groups are considered the most important method of classifying the elements. In some groups, the elements have very similar properties and exhibit a clear trend in properties down the group - these groups tend to be given trivial (unsystematic) names, e.g. The alkali metals, alkaline earth metals, halogens and noble gases. Some other groups in the periodic table display fewer similarities and/or vertical trends (for example Groups 14 and 15), and these have no trivial names and are referred to simply by their group numbers, and below, also using the name of the top element.
▪ Group 1 (IA,IA): the alkali metals or hydrogen family/lithium family
▪ Group 2 (IIA,IIA): the alkaline earth metals or beryllium family
▪ Group 3 (IIIA,IIIB): the scandium family
▪ Group 4 (IVA,IVB): the titanium family
▪ Group 5 (VA,VB): the vanadium family
▪ Group 6 (VIA,VIB): the chromium family
▪ Group 7 (VIIA,VIIB): the manganese family
▪ Group 8 (VIII, VIIIB): the iron family
▪ Group 9 (VIII, VIIIB): the cobalt family
▪ Group 10 (VIII, VIIIB): the nickel family
▪ Group 11 (IB,IB): the coinage metals (not an IUPAC-recommended name) or copper family
▪ Group 12 (IIB,IIB): the zinc family
▪ Group 13 (IIIB,IIIA): the boron family
▪ Group 14 (IVB,IVA): the carbon family
▪ Group 15 (VB,VA): the pnictogens or nitrogen family
▪ Group 16 (VIB,VIA): the chalcogens or oxygen family
▪ Group 17 (VIIB,VIIA): the halogens or fluorine family
▪ Group 18 (Group 0): the noble gases or helium family/neon family
Families that are more than single columns are shown on the periodic table icon above. They are Alkaline Metals, Alkaline-earth Metals, Transition Metals, Semi Metallics, other Metals, Non-Metals, Halogens, Inert (Noble) Gases, Rare Earths, and Radioactive Rare Earth Elements.
There are progressively longer periods further down the table, grouping the elements into s-, p-, d- and f-blocks to reflect their electron configuration.
The s-block of the periodic table of elements consists of the first two groups/columns: the alkali metals and alkaline earth metals, plus hydrogen and helium. These elements are distinguished by the property that in the atomic ground state, the highest-energy electron is in an s-orbital. Except in hydrogen and helium, these electrons are very easily lost to form positive ions. The helium configuration is chemically exceedingly stable and thus helium has no known stable compounds; thus it is generally grouped with the noble gases.
The p-block of the periodic table of the elements consists of the last six columns minus helium (which is located in the s-block). In the elemental form of the p-block elements, the highest energy electron occupies a p-orbital. The p-block contains all of the nonmetals (except for Hydrogen and Helium which are in the s-block) and semimetals, as well as some of the metals.
The d-block of the periodic table of the elements consists of those periodic table groups that contain elements in which, in the atomic ground state, the highest-energy electron is in a d-orbital. The d-block elements are often also known as transition metals.
Although Lutetium and Lawrencium are in the d-block, they are not considered transition metals but lanthanide and actinide according to IUPAC. Group 12 elements are also in the d-block but are considered post-transition metals if their d-subshell is completely filled.
The f-block of the periodic table of the elements consists of those elements (sometimes referred to as the inner transition elements) for which, in the atomic ground state, the highest-energy electrons occupy f-orbitals. Unlike the other blocks, the conventional divisions of the f-block follow periods of similar atomic number rather than groups of similar electron configuration. Thus, the f-block is divided horizontally into the lanthanoid series and the actinoid series.