Atoms are the basic building blocks that determine the peculiarities of matter.
As already mentioned, the ancient Greeks had the idea of a smallest indivisible particle of matter.
The atoms of e.g. gold, iron and oxygen are different.
Then there is additional possibility of putting together other substances from atoms which have other properties.e.g. CO2 consists of carbon and oxygen. CO2 is a gas at room temperature, just like oxygen. Carbon is different in strength. Very loosely arranged it is known as soot in technical sense. A little more compact is graphite and if you put the atoms together very tightly in a fixed arrangement, it is a diamond and becomes transparent. Also black, but regularly arranged you get nanotubes or balls.
As has already been written, the nucleus consists of protons and neutrons and around it the electrons.
Same loads come off and unequal ones attract. Since the protons have a positive charge, they attract the negatively charged electrons.
The protons in the core would actually repel themselves, but the charge-neutral neutrons act as glue to hold them together.
The simplest atom is hydrogen, which consists of a proton and an electron, and since there is only one proton in the nucleus, it does not require a neutron.
Since electrons and protons have the same charge, an atom usually has the same number of protons and electrons.In addition to hydrogen, you usually have the same number of neutrons. What makes an atom electrically neutral.
Now, of course, there are always mistakes in nature.
In ions you have at least one electron too much or too little and the atom is no longer electrically neutral.Basically, however, an ion is anxious to get rid of a superfluous electron or to get a replacement for a missing one somewhere. In other way, the chemical properties of an ion do not really change. So gold remains gold, even if the electron number is not correct.
Over 82 protons and neutrons each, most atoms are unstable.Most neutrons and protons, each with 103 pieces, have the lawrencium, but depending on the isotope it disintegrates after 1s to 11h and does not occur in nature.
Another fault lies with the neutrons, again here the error does not change anything on the atom.Isotopes have too many or too few neutrons.
Physically, however, such formations are mostly unstable and spontaneously disintegrate into two parts.These formations are called isotopes.
Since the early 1970s, it has been known that neutrons and protons are composed of smaller building blocks, the quarks.These consist of 3 different quarks each. The quarks also make the difference between matter and antimatter and other building blocks.
In the meantime, however, it looks like the quarks are also composed of smaller building blocks.Among other other, CERN is currently trying to prove this.