Organic chemistry is the study of carbon compounds.

• The fact that most organic substances are composed of carbon and only a few additional elements—most notably, hydrogen, oxygen, and nitrogen—is perhaps its most surprising trait.

Chemists have found or created over 10 million compounds including carbon and the three other elements.

Organic compounds can be found in a variety of products.

• They can be found in items including foods, flavors, and fragrances; medicines, toiletries, and cosmetics; plastics, films, fibers, and resins; paints and varnishes; glues and adhesives; fuels and lubricants; and, of course, our bodies and the bodies of all living things.

A compact, dense nucleus of neutrons and positively charged protons make up an atom.

The nucleus of an atom contains the majority of its mass.

• An extranuclear region containing negatively charged electrons surrounds the nucleus. An atom's nucleus has a diameter of 10214 to 10215 meters (m).

• The primary quantum numbers are 1,2,3, and so on are used to identify the electron shells.

The electrons inhabit a substantially bigger space, with a diameter of around 10210 m, as seen in the accompanying picture.

• Each shell may hold up to 2n2 electrons, where n is the shell number.

• Thus, the first shell can have two electrons, the second eight, the third eighteen, the fourth 32, and so on (Table 1.1, as shown in the image attached below).

Electrons in the first shell are closest to and most tightly gripped by the positively charged nucleus; these electrons have the lowest energy.

• Electrons in higher-numbered shells are retained less firmly because they are further away from the positively charged nucleus.
  

Shell: a region of space around a nucleus that can be occupied by electrons, corresponding to a principal quantum number