9.2.6 Atomic Number and Mass Number
This is the sixth lecture from Chapter 2: ‘Atomic Structure’ of the new book for Class 9 Chemistry (Punjab Board – PCTB). It discusses the atomic number (Z) and mass number (A) of atoms. The lecture includes a multiple-choice quiz, short-answer questions, and detailed long-answer notes.
MCQs Based Quiz
Short Questions
Q1. What is meant by the atomic number of an element? How is it represented?
Atomic number of an element means the total number of protons an atom has. It is represented by the letter ‘Z’ and mentioned on the bottom left side of an element’s symbol.
Example: $\ce{_{6}C}$, $\ce{_{8}O}$ and $\ce{_{9}F}$.
Q2. What is meant by the mass number or nucleon number of an atom? How is it represented?
Mass number, also called nucleon number, is the total number of protons and neutrons an atom has. It is represented by letter Z and mentioned on the top left side of that atom’s symbol.
Example: $\ce{^{12}C}$, $\ce{^{16}O}$ and $\ce{^{19}F}$.
Q3. What is the importance of the atomic number of an atom?
- The atomic number is the identity of an atom.
- Elements are arranged in the periodic table based on their atomic numbers.
Q4. What is the importance of the mass number (nucleon number) of an atom?
- The mass number helps identify different isotopes.
- It is also used to estimate the mass of an atom.
Q5. Which subatomic particle is found in the same number in all atoms of an element, and why?
The proton is the subatomic particle present in the same number in all atoms of an element. This is because the number of protons determines the identity of the atom.
Q6. How are elements arranged in the periodic table?
In the periodic table, elements are arranged in ascending order of their atomic numbers.
Q7. Why does the nucleon number give us an estimate of the mass of an atom?
The nucleon number is the total of protons and neutrons in an atom. Since 99.9% of an atom’s mass comes from these two particles, it gives a good estimate of the atom’s mass.
Q8. Calculate the number of neutrons, protons and electrons in barium ${^{137}_{56}Ba}$.
$\text{Number of electrons (neutral atom)} = \text{Number of protons} = \text{Atomic number }(Z)$
This barium atom contains 56 electrons and 56 protons.
$\text{Number of neutrons} = \text{Mass number }(A) – \text{Atomic number }(Z)$
$\text{Number of neutrons} = 137 – 56 = \boxed{81}$
This barium atom contains 81 neutrons in its nucleus.
Descriptive Question
Write an explanatory note on atomic number and mass number.
Atoms of different elements vary because they have different numbers of sub-atomic particles. Two important numbers help us identify and understand atoms: atomic number and mass number.
Atomic Number (Z):
The atomic number of an element is the number of protons in one atom of that element. It is represented by the letter Z and written on the bottom-left side of the element’s symbol.
For example: $\ce{_{6}C}$, $\ce{_{8}O}$ and $\ce{_{9}F}$.
Importance of Atomic Number:
- 1. It is unique for every element and helps to identify the atom.
- It is used to arrange elements in the periodic table in ascending order.
Mass Number (A):
The mass number is the total number of protons and neutrons in the nucleus of an atom. It is represented by the letter A and written on the top-left side of the element’s symbol.
For Example: $\ce{^{12}C}$, $\ce{^{16}O}$ and $\ce{^{19}F}$.
Importance of Mass Number:
- It is used to identify isotopes of an element.
- It gives an estimate of the mass of an atom.
The mass number is also called the nucleon number, because protons and neutrons are collectively known as nucleons.
Number of Neutrons:
We can calculate the number of neutrons in an atom using this formula:
$\text{Number of neutrons} = \text{Mass number }(A) – \text{Atomic number }(Z)$
For example:
$\text{Number of neutrons in } \ce{^{12}_{6}C} = 12 – 6 = \boxed{6}$