There is a tiny core at the centre of an atom, called the nucleus . It consists of particles called protons and neutrons . Protons and neutrons have about the same mass.
Protons have a positive charge.
Neutrons have no charge. A hydrogen nucleus is just a single proton. All the other nuclei contain protons and neutrons. The nucleus gets bigger and heavier as we move down the list of elements, until we come to element 92 - uranium. This has 92 protons and 146 neutrons.