Isotopes have the same atomic number but a different mass number

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All atoms of a given element have the same number of protons, hence the same atomic number.They may however differ from one another in mass and therefore in mass number.

This can happen because, although the number of protons in an atom of an element is fixed, the number of neutrons is not, it may vary.Consider the element hydrogen (Z=1) . There are three different kinds of hydrogen atoms , the most common type one has zero neutrons (A=1), another type of hydrogen atom (deuterium) has one neutron (A=2) and third type (tritium) has two neutrons (A=3).

Such types of atoms are called isotopes.

Isotopes are atoms whose nuclei have the same atomic number but different mass numbers: that is, the nuclei have the same number of protons but different numbers of neutrons.

For exampls: Naturally occuring oxygen is a mixture of isotopes: it contains 99.759% oxygen-16,0.037% oxygen-17 and 0.204% oxygen-18.

Uranium has two isotopes Uranium-235 and Uranium -238 the atomic number of Uranium is 92.Find the number of neutrons and protons in each isotopes of Uranium?

Uranium-235  has mass number 235 and atomic number 92 so

number of protons in U-235 = 92  AND number of neutrons U-235 = 143 (mass number - no.of protons= 235-92)

Uranium-238 has mass number 238 and atomic number 92 so,

number of protons in U-238 = 92 and number of neutronns U-238 = 146 (mass number - no.of protons= 238-92) 

SO, both the uranium isotopes have same number of protons but different numbers of neutrons.