2.1. Introduction to the atom. Its structure


Life, at its most fundamental level, is made up of matter that can be defined as any substance that takes up space and has mass.
We will begin our study of biology with the most basic element of matter: the atom, we will get to know its structure, types, properties and how they arrange, by means of bonds, to form configurations with a higher level of complexity, which are named molecules.
Atoms are the basic structural components of chemical elements, which can be described as unique forms of matter with specific physical and chemical properties that cannot be broken down (by ordinary chemical reactions) into smaller substances.

Elements are named by a capital letter or two letters, when the first one has already been taken by another element, these letters are called chemical symbols. Thus, for example: H is the chemical symbol for hydrogen and He is the chemical symbol for helium.
The four most common elements in living organisms, including human being, are oxygen (O), carbon (C), hydrogen (H) and nitrogen (N), whose percentages are:

Element Percentage in living organisms
Oxygen 65%
Carbon 18%
Hydrogen 10%
Nitrogen 3%

As I have just mentioned, the structural components in elements are called atoms, which are the smallest units of matter that retain all the chemical properties of an element.
We can distinguish two different areas in an atom:

         The nucleus which is the centre of the atom made up of protons (p) and neutrons (n).

         The electrons (e-) in orbit around the nucleus, this is the outermost region in an atom.

Protons and neutrons have, approximately, the same mass: 1.67*10-24 g, but they differ in their electrical charges, whereas protons are positively charged, neutrons are uncharged. Electrons, however, have a very low mas: 9.11*10-28 g, around 1/800 of an atomic mass unit[1]. Therefore, their largest contribution is not to the atomic mass but to its charge, since this charge is equal to protons’ but with opposite sign (negative).
Interestingly, due to the tiny size of all these particles, most of the atomic volume is empty space ( > 99%), to give you an idea about what this means let’s imagine that our atom is a huge sphere around The Eiffel Tower (301 metres height), proportionally, the nucleus would be represented by a cherry stone and the electrons would be pinpoints around it. 

As a result of this enormous vacuum, you can wonder why solid objects are impenetrable and this is on account of the fact that the electronic shells repel (negatively charged) each other.

[1] Atomic mass unit (u) or dalton (Da) is approximately the mass of one nucleon, either a single proton or neutron and is defined as one twelfth of an unbound neutral atom of carbon-12 and has a value of 1.660*10-27 kg.

Source: OpenStax College, Biology. OpenStax College. 30 May 2013.

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