Which of the following pairs of 'number – composition' is correct? I. Atomic number – number of protons II. Mass number – Sum of number of neutrons and protons
Both I and II
Understanding Atomic and Mass Numbers
Let's break down the two statements provided regarding the composition of an atom based on its atomic number and mass number.
| Term | Definition / Composition | Symbol |
|---|---|---|
| Atomic Number | Number of protons in the nucleus of an atom. This defines the element. | \(Z\) |
| Mass Number | Total number of nucleons (protons and neutrons) in the nucleus of an atom. | \(A\) |
Analyzing Statement I: Atomic number – number of protons
Statement I says that the atomic number is the number of protons. This is the fundamental definition of the atomic number (\(Z\)). The number of protons uniquely identifies a chemical element. For example, any atom with 6 protons is carbon, and its atomic number is 6.
Mathematically, this relationship is expressed as:
\(Z = \text{Number of Protons}\)
Based on the definition, Statement I is correct.
Analyzing Statement II: Mass number – Sum of number of neutrons and protons
Statement II states that the mass number is the sum of the number of neutrons and protons. The mass number (\(A\)) represents the total count of particles in the nucleus, which are protons and neutrons (collectively called nucleons). Electrons are not included in the mass number because their mass is negligible compared to protons and neutrons.
The relationship is given by:
\(A = \text{Number of Protons} + \text{Number of Neutrons}\)
Based on the definition, Statement II is also correct.
Conclusion on the Correct Pairs
We have analyzed both statements:
- Statement I: Atomic number corresponds to the number of protons. This is correct.
- Statement II: Mass number corresponds to the sum of the number of neutrons and protons. This is correct.
Since both statements accurately describe the composition related to atomic number and mass number, the pair 'Both I and II' is correct.
Revision Table: Atomic Structure Basics
| Property | Definition | Location in Atom | Determines |
|---|---|---|---|
| Atomic Number (\(Z\)) | Number of protons | Nucleus | Element identity |
| Mass Number (\(A\)) | Number of protons + Number of neutrons | Nucleus | Isotope of an element |
| Number of Neutrons | Mass Number (\(A\)) - Atomic Number (\(Z\)) | Nucleus | Varies among isotopes |
| Number of Electrons (in neutral atom) | Equal to the number of protons (Atomic Number) | Orbiting the nucleus | Chemical behavior / Bonding |
Additional Information: Isotopes and Atomic Mass
The concept of mass number is closely related to isotopes. Isotopes are atoms of the same element (same atomic number, meaning the same number of protons) but with different numbers of neutrons. This difference in neutron count leads to different mass numbers for isotopes of the same element.
For example, Carbon-12 (\(\text{}^{12}\text{C}\)) has 6 protons and 6 neutrons (\(A=12\)), while Carbon-14 (\(\text{}^{14}\text{C}\)) has 6 protons and 8 neutrons (\(A=14\)). Both are carbon because they have 6 protons (\(Z=6\)).
The atomic mass (or atomic weight) listed on the periodic table is typically a weighted average of the masses of all naturally occurring isotopes of an element, taking into account their relative abundance.
Important Questions from Atomic Structure
In 1893, which Swiss chemist was the first to understand the molecular structures of inorganic substances – chemical compounds that do not contain carbon?
- What is the atomicity of Phosphorus?
Which are the four quantum numbers for an electron present in 4f orbital?
What is the atomic number of Bohrium which is named after physicist Niels Bohr, one of the founders of quantum theory?
The quantum numbers n and l for four electrons are given below.
(i) n = 4, I = 1
(ii) n = 4, l = 0
(iii) n = 3, l = 2
(iv) n = 3, l = 1
The order of their energy from lowest to highest is:
