The structure of the carbon atom (1)

All atoms have a positively charged nucleus composed of one or more protons, each with a positive electrical charge of +1, and neutrons which are electrically neutral. Each proton and neutron has a mass of one and together account for practically the entire mass of the atom. The nucleus is surrounded by electrons, moving around the nucleus, each with a negative electrical charge of -1. The number of electrons is the same as the number of protons so that the positive charge of the nucleus is balanced by the negative charge of the electrons and the atom is electrically neutral.

As determined by Schroedinger, the behavior of the electrons in their movement around the nucleus is governed by the specific rules of standing waves. These rules state that, in any given atom, the electrons are found in a series of energy levels called orbitals, which are distributed around the nucleus. These orbitals are well defined and, in-between them, large ranges of intermediate energy levels are not available to the electrons since the corresponding frequencies do not allow a standing wave.

In any orbital, no more than two electrons can be present and these must have opposite spins as stated in the Pauli’s exclusion principle. A more detailed description of the general structure of the atom is given in Ref. 3, 4, and 5.

Nucleus and Electron configuration of the carbon atom: The element carbon has the symbol C and atomic number of 6, i.e., the neutral atom has six protons in the nucleus and correspondingly six electrons. In addition, the nucleus includes six neutrons. The electron configuration, that is, the arrangement of the electrons in each orbital, is described as 1S²2S²2P². This configuration is compared to that of neighboring atoms in Table 2.2.

The notation 1S² refers to the three quantum numbers necessary to define an orbital, the number “1” referring to the K or first shell. The letter “s” refers to the sub-shell s and the superscript numeral “2” refers to the number of atoms in that sub-shell. There is only one orbital in the K shell which can never have more than two electrons. These two electrons, which have opposite spin, are the closest to the nucleus and have the lowest possible energy. The filled K shell is completely stable and its two electrons do not take part in any bonding.

Table 2.2 Electron configuration of carbon and other atoms:

Note: The elements shown in bold (H, N and O) are those which combine with carbon to form most organic compounds.