Protons & neutrons
Protons & neutrons (the nucleons) are the biggest particles in the nucleus. In the nuclear physics have been discovered many more particles.
f.e. the nuclear particles are built up of quarks. More information can be found on the internet
Those small particles stay out of this course.
Protons and neutrons have about the same mass, but different charge: protons are positive and neutrons are neutral.
The positive protons attract the negative electronshells around the atom. Ehe neutrons take care for the stability in the nucleus. That is needed because equally charged particles to repell (the positive protons).
The very short distance forces of masses appear toe be stronger thant the repelling electric forces.
The proportion between the number of protons and neutrons in the nucleus is extremely important for the needed stability.
Atoms with a very high atom number (>92) cannot longer be kept stable. The amount of positive protons is getting too big to save.