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Atomic number, We know that, the nucleus is at the centre of the atom and contains the protons and neutrons. , Elements differ from one another according to the number of protons in their atomic 'nuclei'. This value is called the element's atomic number and is denoted by the letter Z. , Atomic number is the number of protons in the nucleus of an atom., Atomic mass number, The mass of an atom which is a characteristic of an atom depends on the number of neutrons and protons its nucleus contains. , Number of protons in a nucleus is denoted by Z (atomic number) and number of neutrons of a nucleus is denoted by N. , The number of nucleons, is the total number of protons and neutrons in an atom. It is called the atomic mass number and is denoted by the letter A. Atomic mass number = atomic number + neutron number A = Z + N

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Writing symbols of atoms In standard notation to represent an atom, the atomic number, mass number and symbol of the element are written as:, F is the symbol of the element fluorine, its atomic number is written at bottom left. It tells us that this atom has 9 protons. , The mass number is written at top left. It tells us that the fluorine has 19 nucleons (protons + neutrons). , Therefore the number of neutrons present in fluorine is equal to 19 - 9 = 10 neutrons. (N = A - Z).

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Isotopes , Every element has a unique atomic number, or number of protons. , What about mass number? , Does every element has a unique mass number, which is different from the mass number of other elements? , No, the mass number of an element is not unique because there are more than one type of atoms of the same element present in nature in certain cases., Observe the following figure of different Hydrogen atoms. , An atom of hydrogen has one nucleon in its nucleus, an atom of deuterium has two nucleons in its nucleus, and tritium has three., Since atoms of hydrogen, deuterium and tritium have only one proton in their nuclei, they only have one electron. , But number of neutrons present in hydrogen atom is not same in all cases.

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What is the main difference among the isotopes of the same element?, Fill in the missing information in the following table.