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Atomic number & mass Number Let’s start ATOMIC NUMBER
It is the number of protons of an atom, which determines its atomic number It is denoted by ‘Z’ ATOMIC NUMBER
All atoms of an element have the same atomic number ATOMIC NUMBER
Elements are defined by the number of protons they possess For hydrogen, Z = 1, because in hydrogen atom, only one proton is present in the nucleus. ATOMIC NUMBER
the atomic number is defined as the total number of protons present in the nucleus of an atom Mass number
➢ mass of an atom is practically due to protons and neutrons alone. ➢ These are present in the nucleus of an atom. ➢ Hence protons and neutrons are also called nucleons. Mass number
➢ Carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u. Mass number
the mass of aluminium is 27 u (13 protons+14 neutrons). Mass number
The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom Notation for representation of Element
In the notation for an atom, Notation for representation of Element
, NotationMass for representation number of Element Isotopes
➢ atoms of some elements have which have the same atomic number but different mass numbers. Isotopes
For example, take the case of hydrogen atom, Isotopes
Some more examples :, Isotopes of Carbon Isotopes of Oxygen Isotopes of Chlorine How to define Isotopes ? How to define Isotopes ?
➢ isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers. ➢
Important points about Isotopes
➢ elements consist of a mixture of isotopes. ➢ Each isotope of an element is a pure substance. ➢ The chemical properties of isotopes are similar but their physical properties are different ➢
Isotopes
➢ Chlorine occurs in nature in two isotopic forms, with masses 35 u and 37 u in the ratio of 3:1. ➢
Isotopes
➢ what should we take as the mass of chlorine atom? ➢
Average atomic mass of isotopes
➢ The mass of an atom of any natural element is taken as the average mass of all the naturally occurring atoms of that element ➢
Atomic mass of a single isotope
➢ If an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it. ➢
Average atomic mass of isotopes
➢ But if an element occurs in isotopic forms, ➢ the percentage of each isotopic form and then the average mass is calculated. ➢
Average atomic mass of isotopes
➢ The average atomic mass of chlorine atom, on the basis of above data, will be ➢
Average atomic mass of isotopes ➢
Isotopes
➢ This does not mean that any one atom of chlorine has a fractional mass of 35.5 u. ➢ It means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass is 35.5 ➢
Applications of Isotopes
some isotopes have special properties which find them useful in various fields ➢
Applications of Isotopes
An isotope of uranium is used as a fuel in nuclear reactors ➢
Applications of Isotopes
An isotope of cobalt is used in the treatment of cancer ➢
Applications of Isotopes
An isotope of iodine is used in the treatment of goitre ➢
ISOBARS
➢ Let us consider two elements — calcium, atomic number 20, and argon, atomic number 18. ➢ The number of electrons in these atoms is different, but the mass number of both these elements is 40 ➢
ISOBARS
➢ Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.
1. Write 2 differences between Isotopes & Isobars. 2. What is the relation between the no of protons(p) , no of neutrons(n) & mass number(A). Today’s Session
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