Formula Units (Empirical Formulas)
Let’s talk about building models of ionic compounds, using KembloX, which is a combination of 3D blocks and a chart of ions (see the post about the Kemblox™ presentation).
Chemistry is the science of matter and its transformations. Matter is made up of small entities, called “atoms”, which is Greek for “indivisible”, made of a positively charged nucleus surrounded by negatively charged electrons. Those atoms are held together by bonds, to form compounds. It is the electrons that participate in bond formation.
We are familiar with two main types of bonds: one in which atoms exchange electrons – the ionic bond, and one in which atoms share electrons (covalent bonds)
The type of compounds modelled by Kemblox is IONIC compounds. These are made up of ….. ions.
The atoms, as they are listed in the periodic table, are electrically neutral, meaning that they are neither positively, nor negatively charged.
In order to become ion, a neutral atom can acquire one or more electrons modeled in KembloX™ as little spiky additions on one side of the blocks. By acquiring an electron, the atom becomes negatively charged and becomes an ANION
Alternatively, the neutral atom may lose electrons, loss represented in KembloX™ as wells inside the blocks. By losing electrons, the atom becomes a CATION, or a positive ion.
Elements can acquire ….. or …..lose
Or four electrons
Following the color code, we’ll use a schematic, unambiguous representation of the KembloX™ blocks to build models for ionic compounds.
The positive and the negative ions are attracted to each other by electrostatic forces to form a formula unit, or empirical formula.
The formula units of ionic compounds are electrically neutral, which means that the number of positive charges MUST equal the number of negative charges, meaning that
…… is not a good model for a formula unit
However an electrically balanced model such as this ….
IS correct, and is illustrative of a formula unit of an actual compound, for example, magnesium iodide.
For building correct models of the formula unit, one can use the chart included in the KembloX™ system. The color-coded Kemblox chart comprises a list of common ions, both monatomic and polyatomic. It shows up to two most preferred charges of the various ions. In the modified periodic table, the elements resulting in monatomic anions are located in the shaded area of the table, so it is easy to decide by inspection whether an element will result in an anion or in a cation, while the charge is explicitly specified in the list of polyatomic ions.
For example, let us model the ionic compound resulting from bonding of ions originating in the elements calcium and chlorine. We check the KembloX™ chart and localize the two elements and their corresponding color code. First calcium, whose color code indicates a preferred ion with two positive charges, ….. and then chloride, whose color code and position indicate a charge of minus one.
As said, from the color code in the chart we ascertained that chlorine will result in a one minus ion, and that calcium will result in a two- positive ion. We can see that one Chloride ion (Cl–) does not provide the two negative charges necessary to balance the 2+ charge of the Ca ion, so a second chloride ion is necessary, and thus the correct formula MUST be ………….CaCl2
The Kemblox™ system covers formula units that contain one type of anion, and one type of cation. It does NOT limit, however, the number of ions involved, as per the electroneutrality requirement, as seen in the models for aluminum oxide, Al2O3 , or for lead(IV) phosphate, Pb3(PO4)4.