Why is the Earth's core hot? What caused it to heat up? Is it still heating, or now cooling?

Scientists estimate that temperature at the Earth's core is about 5538{+0}C.

Much of the heat inside the Earth today comes from elements that were present when the planet was first formed billions of years ago. One theory is that radioactive decay of the primordial elements inside the Earth, U-238, Th-232, and U-235 and their radioactive products generate thermal energy (heat).

A nucleus — the central core of an atom — contains both protons and neutrons. Elements, such as the ones mentioned above, have a fixed number of protons but may exist with various numbers of neutrons. The sum of the protons and neutrons makes up the mass number of an element. Isotopes of an element have the same chemical properties but different weights (indicated by the mass number). Radioactive elements are isotopes with an unstable nucleus.

The isotopes decay by emitting energetic alpha and beta particles until stability is reached. Alpha particles are the nuclei of ordinary helium atoms, which consist of two protons and two neutrons. Beta particles are electrons or positrons. The half-life of an isotope is the amount of time it takes for half of the atoms to decay into a more stable form. Within the Earth, the released particles from the elements are slowed by friction through interaction with Earth material, thereby generating heat. The primordial radioactive elements have half-lives on the order of a billion years. Hence, since the Earth formed, their abundance is decreasing over time as a function of their half-life. Therefore, Earth's core is not heating up, it's cooling down. Courtesy : The Hindu