Molecules and Ions
Although atoms are the smallest unique unit of a particular element, in nature only the noble gases can be found as isolated atoms. Most matter is in the form of ions, or compounds.
Molecules and chemical formulas
A molecule is comprised of two or more chemically bonded atoms. The atoms may be of the same type of element, or they may be different.
Many elements are found in nature in molecular form – two or more atoms (of the same type of element) are bonded together. Oxygen, for example, is most commonly found in its molecular form “O2” (two oxygen atoms chemically bonded together).
Oxygen can also exist in another molecular form where three atoms are chemically bonded. O3 is also known as ozone. Although O2 and O3 are both compounds of oxygen, they are quite different in their chemical and physical properties. There are seven elements which commonly occur as diatomic molecules. These include H, N, O, F, Cl, Br, I.
An example of a commonly occurring compound that is composed of two different types of atoms is pure water, or “H2O”. The chemical formula for water illustrates the method of describing such compounds in atomic terms: there are two atoms of hydrogen and one atom of oxygen (the “1” subscript is omitted) in the compound known as “water”. There is another compound of Hydrogen and Oxygen with the chemical formula H2O2 , also known as hydrogen peroxide. Again, although both compounds are composed of the same types of atoms, they are chemically quite different: hydrogen peroxide is quite reactive and has been used as a rocket fuel (it powered Evil Kenievel part way over the Snake River canyon).
Most molecular compounds (i.e. involving chemical bonds) contain only non-metallic elements.
Molecular, Empirical, and Structural Formulas
Empirical vs. Molecular formulas
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- Molecular formulas refer to the actual number of the different atoms which comprise a single molecule of a compound.
- Empirical formulas refer to the smallest whole number ratios of atoms in a particular compound.
|
Compound |
Molecular Formula |
Empirical Formula |
| Water | H2O | H2O |
| Hydrogen Peroxide | H2O2 | HO |
| Ethylene | C2H4 | CH2 |
| Ethane | C2H6 | CH3 |
Molecular formulas provide more information, however, sometimes a substance is actually a collection of molecules with different sizes but the same empirical formula. For example, carbon is commonly found as a collection of three dimensional structures (carbon chemically bonded to carbon). In this form, it is most easily represented simply by the empirical formula “C” (the elemental name).
Structural formulas
Sometimes the molecular formulas are drawn out as structural formulas to give some idea of the actual chemical bonds which unite the atoms.
Ions
The nucleus of an atom (containing protons and neutrons) remains unchanged after ordinary chemical reactions, but atoms can readily gain or lose electrons.
If electrons are lost or gained by a neutral atom, then the result is that a charged particle is formed – called an ion.
In general, metal atoms tend to lose electrons, and nonmetal atoms tend to gain electrons.
Na+ and Cl– are simple ions, in contrast to polyatomic ions such as NO3– (nitrate ion) and SO42- (sulfate ion). These are compounds made up of chemically bonded atoms, but have a net positive or negative charge.
The chemical properties of an ion are greatly different from those of the atom from which it was derived.
Predicting ionic charges
Many atoms gain or lose electrons such that they end up with the same number of electrons as the noble gas closest to them in the periodic table.
The noble gasses are generally chemically non-reactive, they would appear to have a stable arrangement of electrons.
Other elements must gain or lose electrons, to end up with the same arrangement of electrons as the noble gases, in order to achieve the same kind of electron stability.
Example: Nitrogen
Nitrogen has an atomic number of 7; the neutral Nitrogen atom has 7 protons and 7 electrons. If Nitrogen gained three electrons it would have 10 electrons, like the Noble gas Neon (10 protons, 10 electrons). However, unlike Neon, the resulting Nitrogen ion would have a net charge of N3- (7 protons, 10 electrons).
The location of the elements on the Periodic table can help in predicting the expected charge of ionic forms of the elements.
This is mainly true for the elements on either side of the chart.
Ionic compounds
Ions form when one or more electrons transfer from one neutral atom to another. For example, when elemental sodium is allowed to react with elemental chlorine an electron transfers from a neutral sodium to a neutral chlorine. The result is a sodium ion (Na+) and a chlorine ion, chloride (Cl–):
An ionic compound contains positively and negatively charged ions
It should be pointed out that the Na+ and Cl– ions are not chemically bonded together. Whereas atoms in molecular compounds, such as H2O, are chemically bonded.
Ionic compounds are generally combinations of metals and non-metals.
Molecular compounds are general combinations of non-metals only.
Pure ionic compounds typically have their atoms in an organized three dimensional arrangement (a crystal). Therefore, we cannot describe them using molecular formulas. We can describe them using empirical formulas.
Therefore, the concentration of ions in an ionic compound are such that the overall charge is neutral.
In the NaCl example, there will be one positively charged Na+ ion for each negatively charged Cl– ion.
What about the ionic compound involving Barium ion (Ba2+) and the Chlorine ion (Cl–)?
1 (Ba2+) + 2 (Cl–) = neutral charge
Resulting empirical formula: BaCl2
Researcher:
Vea Ezir Duenas
Sources:
http://www.mikeblaber.org/oldwine/chm1045/notes/Atoms/Molecule/Atoms05.htm (1996 Michael Blaber)
Senior Chemist 
