The outermost electrons of an atom that are mostly involved in bonding reactions are called valence electrons. These electrons are farthest from the nucleus and have high energy.
For example, the total number of electrons in Lithium is three distributed in two energy levels, closest to farthest from the nucleus- 1s2 2s1.
The redistribution of electrons in an atom, bond, or molecule creates two ends (or poles), one electron-rich negative and the other electron-deficient positive; such an atom, bond, or molecule is said to have a dipole (two poles).
Ion is different from an atom since an atom is an electrically neutral specie with an equal number of positive protons and negative electrons.
However, if an atom or a molecule loses or gains electrons, its electron number varies, creating a charged specie called an ion.
In molecules consisting of more than two atoms, the least electronegative atom (except Hydrogen) is the central atom. Due to its low electronegativity, the central atom will not hoard electrons but will share with other atoms, thereby forming a maximum number of bonds than the terminal atoms. So, the central atom is also the least numerous.
In a regular trigonal planar molecular geometry, a central atom is surrounded by three equally spaced substituents in one plane, so joining the three corners would give a triangle.
In a regular tetrahedral molecular geometry, a central atom is surrounded by four substituents that occupy the four corners of a tetrahedron. The substituents are called ligands if the central atom is a metal.
A benzyl, abbreviated as Bn, refers to the phenylmethyl group (C6H5-CH2-), consisting of a benzene ring attached to a methylene (-CH2-) group.
An allyl group is three-carbon substituted propene, also called a propenyl group (prop-2-en-1-yl).
It has a structural formula CH2=CH-CH2-X, where the X can be an alkyl/aryl group or a functional group.
A carbocation is a positively charged, trivalent carbon ion that acts as a reactive intermediate in many organic reactions.
With three bonds and only six electrons, carbocations have an incomplete octet and, therefore, electron deficient. It functions as an electron acceptor and an electrophile forming new Carbon-Carbon (C-C) bonds.
A carbanion is a negatively charged, trivalent carbon ion that acts as a reactive intermediate in many organic reactions.
With three bonds and 8 electrons, carbanions are electron rich and act as a base forming a C-H bond or as a nucleophile forming a C-C bond.