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Chemistry - Atomic Theory and Chemical Symbols

The three “elementary” particles of matter comprising all atoms are electrons, protons, and neutrons. Combinations of these three particle types in various whole-number quantities constitute every type of atom. These fundamental particles are absolutely miniscule in comparison to the macroscopic existence of human beings. Just to illustrate, the mass of a single proton is approximately 1.67×10-27 kilograms: written without scientific notation, it would be 0.00000000000000000000000000167 kg. An electron is even smaller: weighing in at 9.11 × 10-31 kg (about 1800 times less mass than a proton!). Being far smaller in size than a wavelength of visible light1, we cannot see these particles even with the most powerful optical microscope.

Protons and neutrons are very tightly bound together in the nucleus (center) of an atom. The bind is so tight that only extraordinary forces are able to pry an atom’s nucleus apart. Suffice it to say, one cannot disturb the stability of an atomic nucleus by rubbing, cutting, grinding, heating, smashing, or any other macroscopic physical process. The force binding protons and neutrons together in the nucleus is known as the strong nuclear force.

Electrons “orbit” the nucleus of atoms, and are held in proximity to those nuclei by electrostatic attraction (the so-called electromagnetic force), which is many orders of magnitude weaker than the strong nuclear force. Thus, electrons can be dislodged from or added to atoms through the agency of macroscopic forces such as rubbing, cutting, grinding, heating, etc. It is the changeable configurations of electrons that accounts for different atoms joining together to form molecules.

The chemical identify of any atom is a simple and direct function of how many protons that atom has in its nucleus. Each nitrogen atom, for example, has seven (7) protons in its nucleus. This quantity is called the atomic number of an atom. In order for an atom to have a net neutral electric charge, there must be as many electrons orbiting the nucleus as there are protons in the nucleus, since protons carry equal and opposite electric charges. Therefore, a neutral atom of nitrogen will have seven electrons orbiting around the nucleus, electrically balancing the seven protons within the nucleus.

The number of neutrons within the nucleus of an atom does not affect the atom’s chemical identity, but it may affect its nuclear properties (e.g. whether or not it is radioactive; to what degree it captures certain forms of particulate radiation, etc.). For example, most nitrogen atoms have seven neutrons along with seven protons in their nuclei, giving a total nuclear particle count of fourteen – the atomic mass of the atom, sometimes called the atomic weight. However, it is possible for a nitrogen atom to have eight neutrons (an atomic mass of fifteen) and still be “nitrogen,” with all the same chemical properties.

A tremendously simplified model of a common nitrogen atom is shown here, with 7 protons and 7 neutrons in the nucleus, and 7 electrons in “orbit” around the nucleus:


The atomic number of this atom (the number of protons in the nucleus) is seven, which is what makes it nitrogen. The atomic mass of this atom (the sum of protons and neutrons in the nucleus) is fourteen. The chemical symbol for this atom is shown here:


The atomic number is redundant to the letter “N” for nitrogen, since only the element nitrogen can have an atomic number of seven. The atomic mass is only relevant when we need to distinguish one isotope of nitrogen from another (variations of elements having the same number of protons but different numbers of neutrons), and this is seldom because the chemical properties of isotopes are identical – only their masses differ. For these reasons, you will usually find no left-hand subscripts or superscripts placed near chemical symbols of elements in chemical expressions.

By contrast, subscripts and superscripts placed to the right of a chemical symbol have very important meanings in chemistry. A right-hand subscript refers to the number of atoms bound together to form a molecule. A right-hand superscript refers to the electrical charge possessed by an atom (or by a molecule) by virtue of the number of electrons not matching the number of protons:


An N2 molecule may be represented simplistically as follows, the two nitrogen atoms joined by a mutual sharing of the highest-energy (valence) electrons, shown in this illustration as those electrons residing in the largest-diameter “orbits:”



An N3ion is an atom of nitrogen having three more electrons than it normally would when electrically balanced:


A chemical formula is a written description of a molecule’s composition. Ethanol (ethyl alcohol), for example, is a conglomerate of two carbon atoms, six hydrogen atoms, and one oxygen atom. One way to express this structure is to write the following formula for ethanol, the right-hand subscripts showing the relative quantities of atoms in each ethanol molecule:


This is called a molecular chemical formula, because it shows the proportions of atom types comprising each molecule.

A more common way to write the formula for ethanol, though, is this:


Here, an attempt is made to show the physical structure of the ethanol molecule, where one of the hydrogen atoms is found distant from the others. This is called a structural formula. If more detail is needed, a semi-graphic representation called a displayed formula may be used in lieu of a structural formula in lieu of a structural formula:

Chemical engineers often deal with processes where mixtures of similar compounds exist. Wastewater treatment is but one example, where an array of organic compounds must all be treated through oxidation (chemical reaction with oxygen). In such cases, it is common to write formulae expressing the average ratios of elements. Primary sludge clarified from municipal wastewater, for example, may be represented by the compositional formula C22H39O10N. This does not suggest the existence of some monstrous molecule consisting of twenty-two carbon atoms, thirty-nine hydrogen atoms, ten oxygen atoms, and a lone nitrogen atom somewhere in a sample of sludge, but rather that the combined average carbon, hydrogen, oxygen, and nitrogen quantities in that sludge exist in a variety of molecular forms in these approximate proportions. This aggregate formula expression helps the engineer calculate the gross chemical characteristics of the sludge, such as the amount of oxygen needed to completely oxidize the sludge.

Sometimes, compositional formulae are written with non-integer subscripts. An example of this would be the compositional formula C4.8H8.4O2.2, which also happens to be an average composition for wastewater sludge (ignoring nitrogen content). The same formula could just as well have been written C48H84O22, or even C24H42O11, because these subscript values all express the exact same proportions.


1In order for a wave of light to be influenced at all by an object, that object must be at least the size of the wave’s length. To use an analogy with water waves, it would be comparing the interaction of a water wave on a beach against a large rock (a disturbance in the wave pattern) versus the non-disturbance of that same wave as it encounters a small buoy.

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