Page 1 - 3. LESSON NOTES-(AOMIC NUMBER AND MASS NUMBER)

SAI International School
Subject-Chemistry

Ch-Structure of the Atom
Topic- Atomic Number & Mass number
Lesson Notes

Atomic Number

 The total number of protons in the nucleus of an atom gives us the atomic
number of that atom.

 It is represented with the letter ‘Z.’
 All the atoms of a particular element have the same number of protons, and
hence the same atomic number.

 Atoms of different elements have different atomic numbers.
 For example, all carbon atoms have the atomic number of 6, whereas all
atoms of Oxygen have 8 protons in their nucleus

The atomic number of an atom is equal to the number of protons in the nucleus of
an atom or the number of electrons in an electrically neutral atom.

Atomic number = Number of protons= Number of electrons

Z= p = e

Atomic mass number or Mass Number

 The number of protons and neutrons combined to give us the mass number of
an atom.
 It is represented using the letter ‘A.’
 As both protons and neutrons are present in the nucleus of an atom, they are
together called nucleons.

Atomic mass number or mass number = Number of protons + Number of
neutrons

A = p + n

 For example, an atom of carbon has 6 protons and 6 neutrons. Thus, its mass number is
12.
Calculating Number of Neutrons:
Number of neutrons= Mass number -Atomic number
n= A-Z
or, n= A-p
or n= A-e

Notation of atom:

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