The mass number is different for each different isotope of a chemical element. The total number of protons and neutrons in the nucleus of an atom is called the atomic mass number (or the mass number) of the atom and is given the symbol A. Since electrons are almost massless (in comparision to the nucleons), the total number of protons and neutrons in the nucleus of an atom determines the atomic mass. This fact has key implications for the building up of the periodic table of elements. The ordering of the electrons in the ground state of multielectron atoms, starts with the lowest energy state (ground state) and moves progressively from there up the energy scale until each of the atom’s electrons has been assigned a unique set of quantum numbers.
It is the Pauli exclusion principle that requires the electrons in an atom to occupy different energy levels instead of them all condensing in the ground state. Each electron is influenced by the electric fields produced by the positive nuclear charge and the other (Z – 1) negative electrons in the atom. The total electrical charge of the nucleus is therefore +Ze, where e (elementary charge) equals to 1,602 x 10 -19 coulombs. The number of electrons in an electrically-neutral atom is the same as the number of protons in the nucleus. The total number of protons in the nucleus of an atom is called the atomic number (or the proton number) of the atom and is given the symbol Z. Atomic physics also help to understand the physics of molecules, but there is also molecular physics, which describes physical properties of molecules. This includes ions as well as neutral atoms and, unless otherwise stated, for the purposes of this discussion it should be assumed that the term atom includes ions. It is primarily concerned with the arrangement of electrons around the nucleus and the processes by which these arrangements change. While the term atomic deals with 1Å = 10 -10m, where Å is an ångström (according to Anders Jonas Ångström), the term nuclear deals with 1femtometre = 1fermi = 10 -15m.Ītomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus.
The nuclear physics deals with the nucleus as a system consisting of a nucleons (protons and neutrons). The atomic physics deals with the atom as a system consisting of a nucleus and electrons. However, physicists distinguish between atomic and nuclear physics. The term atomic physics is often associated with nuclear power, due to the synonymous use of atomic and nuclear in standard English. Note that, atomic and nuclear physics are not the same. The science, which deals with atoms is called atomic physics. Inside the protons and neutrons, we find the quarks. The nucleus itself is generally made of protons and neutrons but even these are composite objects. The electrons are spinning around the atomic nucleus. The atoms are the smallest constituents of ordinary matter, which can be divided without the release of electrically charged particles.
All matter except dark matter is made of molecules, which are themselves made of atoms. These are the smallest building blocks of matter. The physical world is composed of combinations of various subatomic or fundamental particles.
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