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The cross method determines chemical formulas using atom valence. Electrons are exchanged in ionic bonding to form compounds. The valence of each atom is transferred using the cross method to write the molecular formula. Reduction is used to create precise molecular names. The cross method works for complex molecules with known valence.
The cross method is a way of determining the chemical formula of compounds using the valence of atoms. Atoms have a central nucleus of protons and neutrons, with layers of electrons surrounding the nucleus in layers called shells. The outermost shell can contain too many or too few electrons, which is described by a plus or minus valence number. Compounds are formed by exchanging these electrons in reactions called ionic bonding, in which electrons are shared between two or more atoms.
When an atom with two more electrons combines with another containing one less electron, it takes two of the second atom to form the compound. Silicon contains two more electrons in its outer shell, while oxygen contains one less electron than a full shell. The ionic formula for the two atoms would be Si+2 and O−1, showing the ionic valence for each.
Using the cross method, the resulting compound can be written by transferring the valence of each atom to the other and writing them as a subscript. The molecule resulting from the combination of silicon and oxygen is silicon dioxide, or SiO2. The +2 valence of the silicon atom is cross-fertilised with oxygen and the -1 is transferred or cross-fertilised with silicon. Crossing the valences between the two atoms when describing the molecule has led to the term cross method.
The reaction of the two atoms eliminates any electron charge, because the atoms combine in the ratios necessary to use up any excess electrons. With a balanced number of electrons the valence is assumed to be zero and there is no plus or minus sign used in a molecular formula. Ionic molecules tend to be very stable, because electrons are shared between atoms, creating a very strong chemical bond.
A process called reduction is used to create the molecular name precisely when the ionic valences are multiples of smaller numbers. Barium and oxygen can combine to form barium oxide, with each of the atoms containing a valence of two. Using the cross method, the valence of 2 would be crossed with the atomic name of the other atom, resulting in a molecule called Ba2O2. The subscripts can be divided by 2, so the correct molecule is BaO and the valences have been reduced to the minimum necessary.
The cross method will also work when the molecules are made up of more complex groups, such as zinc acetate. Zinc (Zn) with valence +2, can chemically combine with an acetate molecule (C2H3O2) with valence -1. The method switches the two valences to show that one zinc atom combines with two acetate molecules to form Zn(C2H3O2)2. As long as the molecule has a known valence, the cross method can be used to determine the correct molecular structure of any compound.
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