Venus has a tick atmosphere of CO₂. Because of that, any sunlight that strikes the surface of Venus and heats the rocks of the surface re-radiates in the infrared part of the spectrum. Now CO₂ gas ABSORBS infrared light. When CO₂ does that, the CO₂ acquires more energy and we sense that as a rise in temperature. This is called a runaway greenhouse effect. The temperature on surface of Venus is 900 degree Fahrenheit or 500 degree centigrade. The oven your mom uses goes to a MAXIMUM of about 500 degree Fahrenheit!

Mercury, being a small planet, only 5% the mass of Earth, does not have enough gravitational pull to hang onto an atmosphere. Any gasses that come from any volcanic emissions on Mercury just drift off into space!!! Hence the side of Mercury facing the sun is super hot, whereas at THE SAME TIME, on the dark side, it is super cold.

Venus' atmosphere has more CO₂ (carbon dioxide) than all of the CO₂ on Earth (Earth's atmosphere, in the mantle, in volcanoes, and even in rocks)!!! As I'm sure you know, CO₂ is a VERY powerful greenhouse gas, and Venus' atmosphere is full of CO₂ and CH₄ (methane, which is an even stronger greenhouse gas than CO₂). CO₂ and CH₄ within Venus' atmosphere act to trap heat in, just like a blanket. This is why even though Venus is further away from the Sun than Mercury, it is much, much hotter!!!

When you say that Venus is full of carbon dioxide, that is actually what makes up its atmosphere. So Venus does have one, it's just much thicker than what we have here on Earth (and Mercury for that matter). Because carbon dioxide is a really good greenhouse gas (gases that trap heat and don't let it escape), Venus gets really really hot compared to Mercury. Mercury also has an atmosphere, but it's so thin that it can't keep it warm despite being really close to the Sun. I hope I answered your question. Good luck with your studies!

Venus is hotter than Mercury because of the greenhouse effect, yes - the thick carbon dioxide atmosphere of Venus is the reason why it is so hot.

Incidentally, if Venus didn't have that much carbon dioxide, it would be about the same temperature as the Earth. It's closer to the sun, but reflects almost all of the sun's light due to its clouds, so the total amount of heat going in is about the same as that on Earth.