Atoms have a central nucleus of protons and neutrons.
The crossover 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 circling the nucleus in layers called shells. The outermost shell may contain many or few electrons, which is described by a positive or negative valence number. Compounds are formed by exchanging these electrons in reactions called ionic bonds, in which electrons are shared between two or more atoms.
When an atom with two extra electrons combines with an atom with one less electron, it takes two from the second atom to form the compound. Silicon contains two extra electrons in its outer shell, while oxygen contains one electron less than a full shell. The ionic formula for the two atoms would be Si & plus; 2 and O −1 , which shows the ionic valence for each.
Using the crossover 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 SiO 2 . The valence & plus; 2 of the silicon atom is crossed to oxygen, and the −1 is transferred or crossed to silicon. Crossing the valences between the two atoms when describing the molecule led to the term cross method.
The reaction of the two atoms eliminates any electronic charge, because the atoms combine in the proportions necessary to use up all the excess electrons. With a balanced number of electrons, 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 electrons are shared between atoms, forming a very strong chemical bond.
A process called reduction is used to create the proper molecular name when ionic valences are multiples of smaller numbers. Barium and oxygen can combine to form barium oxide, with each of the atoms having a valence of two. Using the crossed method, the valence of 2 would be crossed with the atomic name of the other atom, resulting in a molecule called Ba 2 O 2 . 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-over method also works when the molecules are made up of more complex groups, such as zinc acetate. Zinc (Zn) with a valence & plus; 2, can chemically combine with an acetate molecule (C 2 H 3 O 2 ) with a valence of -1. The method alternates the two valencies to show that a zinc atom will combine with two acetate molecules to form Zn (C 2 H 3 O 2 ) 2 . As long as the molecule has a known valency, the crossover method can be used to determine the correct molecular structure of any compound.