The Mass Number (A), is the total number of protons and neutrons in an atomic nucleus.
The Mass Number gives the approximate weight of the atom, because it counts the weight of the protons and neutrons, does not include the electrons weight because they are negligible. (See Atom Description for the values)
The Mass Number, does not identify an atom or element, in fact any element has different values of Mass Number. For example, the Hydrogen (Atomic Number, Z=1) has three values, A= 1, 2, 3.
Mathematically, the Mass Number is expressed as, A = Z + N
Where, N= number of neutrons
Thus, the Hydrogen has three Mass Numbers because some of them have zero neutrons, others have one neutron, or they could also have two neutrons.
Atomic Number is represented by the symbol Z, and is the number of protons found in the nucleus of an atom.
The atomic Number identifies a Chemical Element, that means adding or removing protons from the nucleus of an atom creates a different element. A nucleus with 1 proton belongs to the Hydrogen, if we have 3 protons in the nucleus, that is now a Lithium atom. Other important atomic numbers belongs to Carbon, Oxygen, Gold and Uranium: 6C, 8O, 79Au, 92U.
In an atom with neutral charge, the atomic number is also equal to the number of electrons.
The Atomic Number is the upper number in the Periodic Table of the Elements.
Atom is the smallest unit of matter that is composed of a Nucleus surrounded by the electrons. The atomic nucleus contains protons, positively charged, and neutrons with zero charge.
Every proton has a positive charge +1, and the electrons round the nucleus with a negative charge of -1.
Protons and Neutrons have a similar mass of 1.67×10-27 kg and 1.69×10-27 kg respectively, which is around 1836 times the weight of the electrons at 9.11×10-31 kg.
The electrons are attracted to the protons by electromagnetic forces, but they have properties of a particle, spinning around their own axis, and a wave, which defines a region where they can be found around the nucleus, called Orbital.
Orbitals can have one or more rings or node structures, and they differ from each other in size, shape and orientation. The first two electrons are located in the spherical orbital 1s, a third electron will be in a bigger orbital as the 2s.