Most of the time, however, we can get by with two pieces of information. 

 

Generally, it is the largest atom that assumes the central position. This is reasonable as it is easier to position smaller atoms around a larger one. Consider the compounds ammonia, water, chlorine trifluoride, methane, and beryllium difluoride. In each of these compounds the largest atom is in the central position. They can each be shown with the skeleton structures below.

 

 

The above rule (#1) does not always work. Take, for instance, carbon tetrachloride, CCl₄. In this compound, the smaller of the two  atoms, carbon, assumes the central position. To explain this we need  to look at the number of valence electrons that each of the component atoms possesses. The atom with the fewest valence electrons ‑ this is the atom that requires the greatest number of bonds to complete  its octet ‑  assumes the central position.

 

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