What are atoms?
An atom is the smallest thing a chemical element can be divided into. Atoms can be broken down into smaller parts, but then it would no longer be the same chemical element.
An atom is made up of a nucleus, around which are some number of electrons. The nucleus is made up of a number of protons, whose positive charges attract a similar number of electrons, which have negative charges. How many protons an atom has is its atomic number, and determines what chemical element the atom is.
Hydrogen is an element with a single proton. It attracts a single electron. That electron can be taken away, leaving a hydrogen ion. An ion is an atom that is either missing one or more electrons, or which has one or more extra electrons.
Hydrogen can also have one or two neutrons in the nucleus, along with the proton. Neutrons don’t change the atomic number (the number of protons), so the atom is still a hydrogen atom. But they change its weight, so an atom with a proton and a neutron is called heavy hydrogen, or more commonly, deuterium. A hydrogen atom with two neutrons is called tritium.
Chemistry deals mostly with the effects of protons and electrons. The weight of an atom affects how it reacts with other atoms, but to a far smaller degree than the atomic number.
Atoms are extremely small. The nucleus of an atom is even smaller, so much so that an atom is mostly empty space. The electrons in the atoms are negatively charged, so they repel one another, and they also repel the electrons in other atoms. The reason you don’t fall through your chair is that the electrons in your pants repel the electrons in the chair. But the atoms in your pants and the chair are still mostly empty space.