Block (periodic table)
A block of the periodic table is a set of chemical elements having their differentiating electrons predominately in the same type of atomic orbital. A differentiating electron is the electron that differentiates an element from the previous one. For example, sodium [Ne] 3s1 when compared to neon [He] 2s22p6 has a difference of one s-electron. The term appears to have been first used by Charles Janet. Each block is named after its characteristic orbital: s-block; p-block; d-block; and f-block.
There is an approximate correspondence between this nomenclature of blocks, based on electronic configuration, and sets of elements based on chemical properties. The s-block and p-block together are usually considered main-group elements, the d-block corresponds to the transition metals, and the f-block encompasses nearly all of the lanthanides (like lanthanum) and the actinides (like actinium). Not everyone agrees on the exact membership of each set of elements. For example, some scientists regard the group 12 elements Zn, Cd and Hg as main group, rather than transition group, because they are chemically and physically more similar to the p-block elements than the other d-block elements. The group 3 elements are sometimes considered main group elements due to their similarities to the s-block elements. Groups (columns) in the f-block (between groups 3 and 4) are not numbered.
The s-block is on the left side of the conventional periodic table and is composed of elements from the first two columns, the alkali metals (group 1) and alkaline earth metals (group 2), plus the nonmetals hydrogen and helium. Their general valence configuration is ns1–2. Helium is an s-element, but nearly always finds its place to the far right in group 18, above the p-element neon. Each row of the table has two s-elements.
The metals of the s-block (from the second row onwards) are mostly soft and have generally low melting and boiling points. Most impart colour to a flame.
Chemically, all s-elements except helium are highly reactive. Metals of the s-block generally form ionic compounds with nonmetals.
The p-block is on the right side of the standard periodic table and encompasses elements in groups 13 to 18. Their general electronic configuration is ns2 np1–6. Helium, though being the first element in group 18, is not included in the p-block. Each row of the table has a place for six p-elements except for the first row (which has none).
This block is the only one having all three types of elements: metals, nonmetals, and metalloids. The p-block elements can be described on a group-by-group basis as: group 3, the icosagens; 4, the crystallogens; 5, the pnictogens; 6, the chalcogens; 7, the halogens; and 8, the helium group, composed of the noble gases and oganesson. Alternatively, the p-block can be described as containing post-transition metals; metalloids; reactive nonmetals including the halogens; and noble gases.
The p-block elements are unified by the fact that their valence electrons (outermost electrons) are in the p orbital. The p orbital consists of six lobed shapes coming from a central point at evenly spaced angles. The p orbital can hold a maximum of six electrons, hence there are six columns in the p-block. Elements in column 13, the first column of the p-block, have one p-orbital electron. Elements in column 14, the second column of the p-block, have two p-orbital electrons. The trend continues this way until column 18, which has six p-orbital electrons.
They are mostly covalent in nature and the block is a stronghold of the octet rule, especially in groups 14–17. They show variable oxidation states usually differing by multiples of two. The reactivity of elements in a group generally decreases downwards. This is not case in group 18, where reactivity increases in the following sequence: Ne < He < Ar < Kr < Xe < Rn < Og.
The d-block is in the middle of the periodic table and encompasses elements from groups 3 to 12; it starts in the 4th period. Most or all of these elements are also known as transition metals because they occupy a transitional zone in properties, between the strongly electropositive metals of groups 1 and 2, and the weakly electropositive metals of groups 13 to 16. Group 3 or group 12, while still counted as d-block metals, are sometimes not counted as transition metals because they do not show the chemical properties characteristic of transition metals, for example, multiple oxidation states and coloured compounds.
The d-block elements are all metals mostly having one or more chemically active d-orbital electrons. Because there is a relatively small difference in the energy of the different d-orbital electrons, the number of electrons participating in chemical bonding can vary. The d-block elements have a tendency to exhibit two or more oxidation states, differing by multiples of one. The most common oxidation states are +2 and +3; Cr, Fe, Mo, Ru, W, and Os can have oxidation numbers as low as −4; Ir holds the singular distinction of being capable of achieving an oxidation state of +9.
The d-orbitals can contain up to five pairs of electrons; The block therefore includes 10 columns in the periodic table.
The f-block appears as a footnote in a standard 18-column table but is located at the center-left of a 32-column full width table. While these elements are generally not considered part of any group some authors consider them to be part of group 3. They are often called inner transition metals because they provide a transition between the s-block and d-block in the 6th and 7th row (period), in the same way that the d-block transition metals provide a transitional bridge between the s-block and p-block in the 4th and 5th rows.
The f-block elements come in two series in periods 6 and 7. All are metals. The f-orbital electrons are largely inactive in determining the chemistry of the period 6 f-block elements. Their chemical properties are mostly determined by a single d and two s-orbital electrons. Consequently, there is less chemical variability within this series of elements. Among the early period 7 f-block elements the energies of the 5f, 7s and 6d shells are quite similar; consequently these elements tend to show as much chemical variability as their transition metals analogues. The later f-block elements behave more like their period 6 counterparts.
The f-block elements are unified by mostly having one or more electrons in an inner f-orbital. The f-orbitals can contain up to seven pairs of electrons hence the block includes fourteen columns in the periodic table.
The two 14 member rows of the f-block elements are sometimes confused with the lanthanides and the actinides, which are names for sets of elements based on chemical properties more so than electron configurations. The lanthanides are the 15 elements running from La to Lu; the actinides are the 15 elements running from Ac to Lr.
- Charles Janet, La classification hélicoïdale des éléments chimiques, Beauvais, 1928