Periodic trends

The Periodic Trends

Periodic trends are specific patterns that are present in the periodic table that illustrate different aspects of a certain element, including its size and its electronic properties. Major periodic trends include: electronegativity, ionization energy, electron affinity, atomic radius, melting point, metallic character, and ionic radius. Periodic trends, arising from the arrangement of the periodic table, provide chemists with an invaluable tool to quickly predict an element's properties. These trends exist because of the similar atomic structure of the elements within their respective group families or periods, and because of the periodic nature of the elements.

Periodic law

These periodic trends are based on the Periodic Law which states that if the chemical elements are listed in order of increasing atomic number, many of their properties go through cyclical changes, with elements of similar properties recurring at intervals.^[1] For example, after arranging elements in their increasing atomic numbers, many of the physical and chemical properties of Lithium are recurred into Sodium such as its vigorous reactivity with water, which again recurs in the next cycle starting with Potassium.

This principle was discovered after number of investigations done by scientists in nineteenth century such as Lothar Meyer and Dmitri Mendeleev. Initially, no theoretical explanation for the Periodic Law was available and it was used only as an empirical principle. But, with the development of electronic theory of atomic structure, it became possible to understand the theoretical basis for the Periodic Law. From the modern periodic table, it is evident that the periodic recurrence of elements with similar physical and chemical properties, when the elements are listed in order of increasing atomic number, results directly from the periodic recurrence of similar electronic configurations in the outer shells of respective atoms.

Discovery of Periodic Law constitutes one of the most singularly important events in the history of chemical science. Almost every chemist makes extensive and continued use of Periodic Law. Periodic Law also led to the development of the periodic table, which is widely used nowadays.

Atomic radius

The atomic radius is the distance from the atomic nucleus to the outermost stable electron orbital in an atom that is at equilibrium. The atomic radii tend to decrease across a period from left to right. The atomic radius usually increases while going down a group due to the addition of a new energy level (shell). However, atomic radii tend to increase diagonally, since the number of electrons has a larger effect than the sizeable nucleus. For example, lithium (145 picometer) has a smaller atomic radius than magnesium (150 picometer).

Atomic radius can be further specified as:

• Covalent radius: half the distance between two atoms of a diatomic compound, singly bonded.
• Van der Waals radius: half the distance between the nuclei of atoms of different molecules in a lattice of covalent molecules.
• Metallic radius: half the distance between two adjacent nuclei of atoms in a metallic lattice.
• Ionic radius: half the distance between two nuclei

Ionization energy

The ionization potential is the minimum amount of energy required to remove one electron from each atom in a mole of atoms in the gaseous state. The first ionization energy is the energy required to remove two, the ionization energy is the energy required to remove the atom's nth electron, after the (n−1) electrons before it has been removed. Trend-wise, ionization energy tends to increase while one progresses across a period because the greater number of protons (higher nuclear charge) attract the orbiting electrons more strongly, thereby increasing the energy required to remove one of the electrons. Ionization energy and ionization potentials are completely different. The potential is an intensive property and it is measured by "volt"; whereas the energy is an extensive property expressed by "eV" or "kJ/mole".

As one progresses down a group on the periodic table, the ionization energy will likely decrease since the valence electrons are farther away from the nucleus and experience a weaker attraction to the nucleus's positive charge. There will be an increase of ionization energy from left to right of a given period and a decrease from top to bottom. As a rule, it requires far less energy to remove an outer-shell electron than an inner-shell electron. As a result, the ionization energies for a given element will increase steadily within a given shell, and when starting on the next shell down will show a drastic jump in ionization energy. Simply put, the lower the principal quantum number, the higher the ionization energy for the electrons within that shell. The exceptions are the elements in the boron and oxygen family, which require slightly less energy than the general trend.

Electron affinity

The electron affinity of an atom can be described either as the energy gained by an atom when an electron is added to it, or conversely as the energy required to detach an electron from a singly charged anion. The sign of the electron affinity can be quite confusing, as atoms that become more stable with the addition of an electron (and so are considered to have a higher electron affinity) show a decrease in potential energy; i.e. the energy gained by the atom appears to be negative. For atoms that become less stable upon gaining an electron, potential energy increases, which implies that the atom gains energy. In such a case, the atom's electron affinity value is positive.^[2] Consequently, atoms with a more negative electron affinity value are considered to have a higher electron affinity (they are more receptive to gaining electrons), and vice versa. However, in the reverse scenario where electron affinity is defined as the energy required to detach an electron from an anion, the energy value obtained will be of the same magnitude but have the opposite sign. This is because those atoms with a high electron affinity are less inclined to give up an electron, and so take more energy to remove the electron from the atom. In this case, the atom with the more positive energy value has the higher electron affinity. As one progresses from left to right across a period, the electron affinity will increase.

Although it may seem that Fluorine should have the greatest electron affinity, the small size of fluorine generates enough repulsion that Chlorine has the greatest electron affinity.

Electronegativity

Electronegativity is a measure of the ability of an atom or molecule to attract pairs of electrons in the context of a chemical bond. The type of bond formed is largely determined by the difference in electronegativity between the atoms involved, using the Pauling scale. Trend-wise, as one moves from left to right across a period in the periodic table, the electronegativity increases due to the stronger attraction that the atoms obtain as the nuclear charge increases. Moving down in a group, the electronegativity decreases due to the longer distance between the nucleus and the valence electron shell, thereby decreasing the attraction, making the atom have less of an attraction for electrons or protons.

However, in the group 13 elements electronegativity increases from aluminium to thallium, and in group 14 electronegativity of lead is lower than that of tin.

Valence electrons

Valence electrons are the electrons in the outermost electron shell of an isolated atom of an element. Sometimes, it is also regarded as the basis of Modern Periodic Table. In a period, the number of valence electrons increases (mostly for light metal/elements) as we move from left to right side. However, in a group this periodic trend is constant, that is the number of valence electrons remains the same.

However, this periodic trend is sparsely followed for heavier elements (elements with atomic number greater than 20), especially for lanthanide and actinide series.

It is also important to consider the core electrons when speaking about the valence electrons.

Metallic and non-metallic properties

Metallic properties increase down groups as decreasing attraction between the nuclei and the outermost electrons causes the outermost electrons to be loosely bound and thus able to conduct heat and electricity. Across the period, increasing attraction between the nuclei and the outermost electrons causes metallic character to decrease.

Non-metallic property increases across a period and decreases down the group due to the same reason.

See also

• List of elements by atomic properties
• History of the periodic table

Further reading

• ChemWiki - Periodic Trends

References