Atoms, Ions and Molecules. What’s the Difference?


When you view an atom, the first question you might ask is ‘what kind of atom is it?’ What you are really asking is ‘which element am I looking at?’ You can answer this by counting the number of protons in the atom. The number of protons, neutrons and electrons is the same in a neutral atom- that is, an atom with no charge. So, in a regular Calcium atom, there are 20 protons, 20 neutrons and 20 electrons. The electrons orbit around the nucleus and have a negative charge.  An isotope is a version of the element with a different number of neutrons.

When the number of protons differs from the number of electrons, you have an ionic charge. A Calcium atom with 20 protons and 18 electrons has a charge of 2+ because it has 2 extra protons, which have a positive charge. A charged atom is called an ion. When the charge is positive, it is called an cation. If the Calcium gained two more electrons it would have a charge of 2- because it has 2 extra electrons and electrons have a negative charge. A negatively charged ion is called an anion.

Remember, if the number of protons were to change, the element itself would be different. Since opposites attract, the charges on an ion can create bonds between elements. Na+, for example, may bond with Cl- to create sodium chloride. This is called an ionic bond.

carbon_atomIonic bonds are not the only bonds that form between atoms. The arrangement of the elements on the periodic table chart tells you how many electrons the element has in its outermost orbit. What an atom really ‘wants’ is to have 8 electrons in its outermost orbit. This is the most stable configuration it can achieve. The elements on the right side of the chart are the closest to achieving this octet and thus are more greedy for electrons than other elements. This ‘greed’ is called electronegativity and it is a defining characteristic of an element.

When an element needs to achieve an octet, it can do so by sharing an electron with another element in a covalent bond. Carbon has four electrons in its outermost orbit. It needs to make four bonds to achieve an octet. Oxygen needs only two. When atoms group together, they form a molecule. H²O is a molecule of water. It is made of 2 hydrogen atoms and a single Oxygen atom all covalently bonded to one another. Single atoms rarely occur in nature. When a pure element occurs, it is sometimes bonded to itself in a diatomic molecule, such as O² or H². The elements that form diatomic molecules are hydrogen, nitrogen, fluoride, oxygen, iodine, chloride and bromine.

The phrase Building Blocks of the Universe is a cliche but, it is overused for a reason. It is an apt description of the role that atoms, ions and molecules play in the cosmos. This is why chemistry, physics and biology textbooks tend to start at the atomic level. Everything is made of them and understanding them goes a long way toward success in the study of nature.

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