SiO2 has a melting point of 1700°C, whereas CO2 has a melting point of -78°C. What is the reason for this huge difference between two elements in the same group?​

The difference is because SiO2 is solid while CO2 is gaseous

Silicon and carbon are two elements that belong to the same group (Group 4) of the periodic table. They, however, both form oxides. Silicon bonds to two oxygen atoms to form silicon dioxide (SiO2) while carbon also bonds to two oxygen atoms to form CO2. According to this question, SiO2 has a melting point of 1700°C, whereas CO2 has a melting point of -78°C. The huge difference in the melting point values can be attributed to the state at which both molecules exist. SiO2 is a solid, hence, will require great heat to break the strong intermolecular forces that hold it's molecules. On the other hand, CO2 is a gas.

The difference is the melting point values despite belonging to the same group is their STATE OF MATTER.

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I’d have to say no because without oxygen in the water the fish would all be dead. so if you want to fish for dead fish, pick the ponds with the lowest oxygen concentrations.