Constituents of an Atom:After establishing the atomic nature of the matter, scientists became interested to know about the internal structure of atom. It is led to discovery of an additional particle called 'neutron'.Rutherford's experiment on the scattering of α- rays let us to conclude that an atom has a tiny central hard core. Nearly the whole mass of the atom is concentrated in this core.This central core is named as nucleus.Every atomic nucleus contains basically two types of particles protons and neutrons. The total number of protons in atomic nucleus is equal to total number of electrons in atom.The electrons revolve around the nucleus is different orbits in the same manner in which planets of our solar system revolve around the sun. SoAtoms consist of three fundamental particles. They are
1. Electron
2. Proton
3. Neutron
Electron: An electron is negatively charged particle. Its mass is negligibly small as compared to the mass of a proton or neutron.
Proton: A proton is positively charged particle with mass 1836 times the mass of an electron. Its charge is equal but opposite to that of an electron.
Neutron: A neutron is an electrically neutral particle with almost the same mass as that of proton.
Atomic number: The atomic number of an atom is the number of protons it contains.Atomic number is also called proton number. It is denoted by Z
All the atoms of a particular element have the same atomic number number of protons. The atoms of different elements have different numbers of protons. For example, all oxygen atoms have 8 protons and all sodium atoms have 11 protons.
Atomic mass number:The mass number of an atom is the total number of protons (Z) and neutrons (N) it contains. It is denoted by A
Thus, A = Z+ N. For example, in the case of Oxygen, the atomic number Z = 8, the number of neutrons N = 8, Atomic mass number A = 8 + 8 = 16. The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger.
Full chemical symbols Nucleus symbols: We are able to calculate the number of each sub-atomic particle in an atom if we have its atomic number and its mass number. The full chemical symbol for an element shows its mass number at the top, and its atomic number at the bottom.This symbol tells us that the chlorine atom has 11 protons. It will also have 11 electrons, because the number of protons and electrons in an atom is the same.The symbol also tells us that the total number of protons and neutrons in the sodium atom is 23. By this symbol, we can find out the number of neutrons from the mass number and atomic number. In Sodium, number of neutrons = 23 -- 11= 12 neutrons.

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