What are atoms composed of?
In order to understand atoms, one must first look at their individual parts. Atoms are made of three main parts that determine what type of element the atom is. The first two parts, protons and neutrons, reside in the nucleus of the atom, a central location that is much smaller than the entire atom itself. The number of protons present in an atom is what is used to define what type of element it is. This measure is used to classify atoms by their atomic number. For example, hydrogen, the first element of the periodic table, has 1 proton giving it an atomic number of 1. Neutrons are important for determining the atomic mass of an element. The atomic mass number is calculated by adding together the mass of all the neutrons and protons present. It is important to note that an atom with the same atomic number can have a different number of neutrons, giving it a different atomic mass. When this occurs it is known as an isotope of the element.
The third type of particle in an atom is the electron. Electrons reside outside the nucleus of the atom. In fact, the distance of electrons from the nucleus is immense. Electrons are also responsible for determining the charge of an atom. Today, quantum physics is focusing on looking at how electrons function in the atom and where exactly they are located at any given time. Hope this helps!