We do not know well the structure of Jupiter but the giant planets were probably formed by accretion of hydrogen and helium gas around a core of rock and ice. Jupiter with a mass 318 times that of Earth, probably has a rocky core (silicates and iron) the size of Earth and about 10 times the mass of Earth.
This rocky heart could bathe in hydrogen and helium liquid at a temperature of about 16,000 K and a pressure of about 80 million atmospheres.
The global composition of Jupiter based on hypothetical models. The rocky core is so small compared to the size of the planet (7%) that scientists omit this core talking gaseous planet.
In fact 93% of the volume of Jupiter is made of gas, again it is not necessary to see gas of Jupiter as our gas of our atmosphere but rather as a highly compressed liquid by gravitational force of the planet and extremely dense. At center of the planet, the metallic hydrogen atoms are broken by the enormous pressure, it is called then ionized hydrogen. It is only up to the surface that this hydrogen gradually becomes liquid and gaseous. There would be no boundary between free states of the hydrogen layers, transition would gradually.
To surface from interior, we descend gradually into a thick fog more and more dense and opaque until a sea of liquid hydrogen, also more and more dense and hot to finish in metallic hydrogen always more and more dense and hot (16000 K), to the rocky core which would it to a temperature of the order of 25,000 K and a pressure of about 80 million atmospheres.
Spectroscopy carried by space probes has broken down the layers of the atmosphere of Jupiter. Jupiter is composed of 86% hydrogen (H) 14% helium (He), a small amount of methane (CH4), ammonia (NH3) and water vapor (H2O).
Image: At center, the small rocky core surrounded by a huge sea of metallic hydrogen and then liquid then towards surface helium is mixed with hydrogen in a gaseous atmosphere. The global composition of Jupiter based on speculative models.