Final answer:
The question lacks sufficient context to provide an accurate response, as it appears to mix elements with potentially chemical compounds and other unrelated topics, such as solar system properties.
Step-by-step explanation:
The question seems to be regarding similarities between different elements or compounds, as indicated by the notation '7C, 15, and 18.' However, without additional context, it's challenging to provide an accurate comparison. If '7C' pertains to a carbon compound with seven carbon atoms and '15 and 18' represent atomic numbers of elements, for example phosphorus (15) and argon (18), the question would involve comparing properties across different categories such as alkanes, nonmetals, and noble gases respectively. Since this context is not provided, and also considering the tail clarifications that direct us to review questions about the solar system, Venus-Earth-Mars similarities, periodic table electron configurations, and chemical naming, it appears there might be a mix-up. As a result, I cannot confidently provide an accurate answer.