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The crisscross method is a way to determine the chemical formula of compounds by using the valence of the atoms. Atoms have a central core of protons and neutrons, with layers of electrons circling the core in layers called shells. The outermost shell may contain too many or few electrons, which is described by a plus or minus valence number. Compounds form by exchanging these electrons in reactions called ionic bonding, where the electrons are shared between two or more atoms.
When an atom with two extra electrons combines with another containing one less electron, it takes two of the second atom to form the compound. Silicon contains two extra electrons in its outer shell, while oxygen contains one electron short of a full shell. The ionic formula for the two atoms would be Si+2 and O−1, which shows the ionic valence for each.
Using the crisscross 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 crossed to the oxygen, and the −1 is transferred or crossed to the silicon. Crossing the valences between the two atoms when describing the molecule led to the term crisscross method.
The reaction of the two atoms eliminates any electron charge, because the atoms combine in the ratios needed to use all excess electrons. With a balanced number of electrons the valence is considered zero, and there is no plus or minus sign used in a molecular formula. Ionic molecules tend to be very stable, because the electrons are shared between the atoms, making a very strong chemical bond.
A process called reduction is used to create the proper molecular name 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 crisscross method, the valence of 2 would be crossed to the atomic name of the other atom, resulting in a molecule named Ba2O2. The subscripts can be divided by 2, so the correct molecule is BaO, and the valences have been reduced to the minimum needed.
The crisscross method will also work when the molecules are made from more complex groups, such as zinc acetate. Zinc (Zn) with a +2 valence, can chemically combine with an acetate molecule (C2H3O2) with a −1 valence. The method switches the two valences to show that one zinc atom will combine with two acetate molecules to form Zn(C2H3O2)2. As long as the molecule has a known valence, the crisscross method can be used to determine the correct molecular structure of any compound.