Question

What is true of all atoms?

Answer 1

An atom is the smallest particle in an atom until we find there are more fundamental particles of matter. Bohr was the first one to devise that the atoms can further be differentiated into subatomic particles and these particles have charge and the nucleus is at the centre of it and around the nucleus electrons revolve.

Complete answer:
For a long time people thought that atoms are the smallest part of a matter which on combining with its further small particles form matter.
Faraday performed the cathode ray tube experiment and found negatively charged rays which were then decided to be something that is smaller than the atom and were named electrons and therefore, it was believed every atom has an electron.
But if there were negative charged then there must be positive charge as well,
Protons were discovered by Rutherford in $1919$ and it started to appear in Rutherford atomic models first. Then an electrical discharge carried out in the modified cathode ray tube led to the discovery of canal rays which were basically positively charged rays, confirming the presence of positively charged subatomic particles.
Neutrons were discovered by Chadwick in $1932$, while on bombarding a thin sheet of beryllium by alpha particles, neutrally charged particles were discovered which had mass much greater than that of the positive particle.
The positive subatomic particle was named proton while the neutrally charged particle is called neutron.
For the smallest atom of the smallest atom that is hydrogen
We have one electron in its outside and one proton in its nucleus, there are no neutrons as the mass number and atomic number are the same.
The atom has a nucleus, which will always have at least one proton and if the atom is neutral which it always is, it will have an electron orbiting outside the nucleus.

Note:
Atomic number of an atom is the number of protons or electrons and the mass number of the atom is the number of protons and neutrons present in the nucleus.