An extract on #trends
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. 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.
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
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 (n1) 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.