Periodic table periodicity: Fundamental to corrosion chemistry

Periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number (number of protons). Atomic number is the number of protons. Atomic mass is mass of protons plus neutrons. Rows of the table are called periods. The columns are called groups. There are seven periods, generally metals are on the left and non-metals are on the right. There are eighteen groups, contain elements with similar chemical behaviours. There are 118 elements. Electron configuration, and recurring chemical properties are fundamental to the positioning of elements in periodic table. The structure of the table represents periodic trends.

Periodic table periodicity : Recurring chemical properties

Groups:

A group is a vertical column in the periodic table. Groups usually have more significant periodic trends than periods. Elements within the same group generally have the same electron configurations in their valence shell. Elements in the same group tend to show patterns in atomic radius, ionization energy, and electronegativity. From top to bottom in a group, the atomic radii of the elements increase. Since there are more filled energy levels, valence electrons are found farther from the nucleus. From the top, each successive element has a lower ionization energy because it is easier to remove an electron since the atoms are less tightly bound.

What is Atomic radius: 

The atomic radius of a chemical element is a measure of the size of its atoms, usually the mean or typical distance from the center of the nucleus to the boundary.

Role in corrosion:

The atomic radius of the alloying elements like Ni, Cu, and Mo play major role in generating strain in a metal alloy.

What is Ionization energy

It is the energy needed to remove an electron from a neutral atom is the ionization energy of that atom.

Role in corrosion:

Ionization energy is an indicator how tightly the valency electrons are held by protons in nucleus of a metal and therefore its oxidation – reduction potential.

What is Electronegativity

It is the affinity of an element to attract electrons.

Role in corrosion

It is an indicator of oxidation – reduction potential of a metal

Periods

A period is a horizontal row in the periodic table. Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electronegativity. Moving left to right across a period, atomic radius usually decreases. This occurs because each successive element has an added proton and electron, which causes the electron to be drawn closer to the nucleus. This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period. The more tightly bound an element is, the more energy is required to remove an electron. Electronegativity increases in the same manner as ionization energy because of the pull exerted on the electrons by the nucleus. Electron affinity also shows a slight trend across a period. Metals (left side of a period) generally have a lower electron affinity than non-metals (right side of a period), with the exception of the noble gases.

Metals, metalloids and non-metals

According to their shared physical and chemical properties, the elements can be classified into the major categories of metals, metalloids and non-metals. Metals are generally shiny, highly conducting solids that form alloys with one another and salt-like ionic compounds with non-metals (other than noble gases). A majority of non-metals are coloured or colourless insulating gases; non-metals that form compounds with other non-metals feature covalent bonding. In between metals and non-metals are metalloids, which have intermediate or mixed properties. Metal and non-metals can be further classified into subcategories that show a gradation from metallic to non-metallic properties, when going left to right in the rows. About 91 of the 118 elements in the periodic table are metals; the others are non-metals or metalloids.

Alloying metals/elements

Periodicity in periodic table at a glance

Atomic Radius

Atomic radius decreases across a period as nuclear charge increases but shielding effects remain approximately constant, resulting in electrons being drawn closer to the nucleus. Atomic radius increases down a group as valence electrons become increasingly distant from the nucleus, and shielding also increases. This leads to an increase in atomic radius despite the increasing nuclear charge down a group.

Melting Point

Metallic bonded and macromolecular substances tend to have high melting points. For both, this is due to the fact that the bonds require a lot of energy to break. The majority of non-metals have a simple molecular structure. Simple molecular substances have low melting points as only weak intermolecular forces must be overcome in order to melt them. Strength of these is determined by the size of the molecule.

Electronegativity

Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Generally, electronegativity increases moving towards the top right of the Periodic Table. This increase in electronegativity across a period is due to the increased nuclear charge and approximately constant shielding effects resulting in a greater force of attraction to the nucleus of the atom felt by the bonding electrons.

Ionisation Energy

The first ionisation energy generally increases from left to right across a period, as the electron is drawn closer to the nucleus by the increased nuclear charge and becomes harder to remove.