Atomic mass of an element is equal to the sum of number of
protons and neutrons only
Understanding Atomic Mass Calculation
The atomic mass of an element is a fundamental property that tells us how much one atom of that element weighs relative to a standard (like Carbon-12). It's primarily determined by the particles found within the nucleus of an atom.
Atomic Particles and Their Masses
An atom is made up of three main subatomic particles:
- Protons: Positively charged particles located in the nucleus.
- Neutrons: Neutrally charged particles located in the nucleus.
- Electrons: Negatively charged particles that orbit the nucleus.
The mass of these particles differs significantly. Protons and neutrons have roughly the same mass, which is considerably larger than the mass of an electron.
| Particle | Location | Charge | Relative Mass (approx.) | Contribution to Atomic Mass |
|---|---|---|---|---|
| Proton | Nucleus | +1 | 1 atomic mass unit (amu) | Significant |
| Neutron | Nucleus | 0 | 1 atomic mass unit (amu) | Significant |
| Electron | Orbits nucleus | -1 | About \(\frac{1}{1836}\) amu | Negligible |
Because electrons have such a small mass compared to protons and neutrons, their contribution to the overall atomic mass of an atom is considered negligible for most calculations.
Calculating Atomic Mass
The vast majority of an atom's mass is concentrated in its nucleus, which contains the protons and neutrons. Therefore, the atomic mass of an element is essentially the sum of the number of protons and the number of neutrons in an atom's nucleus.
Mathematically, for a specific isotope of an element:
\(\text{Atomic Mass} \approx \text{Number of Protons} + \text{Number of Neutrons}\)
The number of protons is also known as the atomic number, which defines the element. The sum of protons and neutrons is called the mass number.
Analyzing the Options for Atomic Mass
Let's evaluate the given options based on our understanding of atomic mass:
- electrons and protons only: This is incorrect because neutrons contribute significantly to the mass, and electrons contribute very little.
- protons and neutrons only: This is correct because protons and neutrons are the primary contributors to the atomic mass, with electrons having negligible mass.
- electrons and neutrons only: This is incorrect because protons contribute significantly to the mass, and electrons contribute very little.
- electrons, protons and neutrons: While technically all three particles contribute to the total mass, the contribution of electrons is so small that it is typically ignored when calculating the approximate atomic mass based on particle count. The term 'atomic mass' usually refers to the sum of the particles in the nucleus (protons and neutrons).
Therefore, the atomic mass of an element is equal to the sum of the number of protons and neutrons only, as these are the particles that account for almost all of the atom's mass.
Revision Table: Atomic Structure Key Points
| Concept | Description |
|---|---|
| Atomic Number (Z) | Number of protons in the nucleus. Defines the element. |
| Mass Number (A) | Total number of protons and neutrons in the nucleus. Specific to an isotope. |
| Isotopes | Atoms of the same element (same Z) but with different numbers of neutrons (different A). |
| Atomic Mass | Average mass of atoms of an element, taking into account the relative abundance of its isotopes. (The question refers to the mass number of a specific atom/isotope). |
Additional Information on Atomic Mass
The term "atomic mass" can sometimes refer to the average atomic mass of an element as found on the periodic table. This average atomic mass is a weighted average of the masses of all naturally occurring isotopes of that element, based on their relative abundance. However, the question asks for the sum of the number of particles that equals the atomic mass, which points towards the definition related to the composition of a single atom or isotope, meaning the mass number.
For a specific atom or isotope, the mass number (sum of protons and neutrons) is a whole number. The actual mass of an atom (often measured in atomic mass units, amu) is very close to this mass number but not exactly equal due to the binding energy of the nucleus (mass defect).
However, in the context of introductory chemistry and physics questions asking for the sum of particles that constitute atomic mass, the intended meaning is the sum of protons and neutrons (the mass number), as electrons contribute negligibly to the mass.
Similar Questions
- The mass number of argon is 40. Which one of the following statements is correct?
Which one of the following is the largest composition in biogas?
Which one of the following is not a bio-mass energy source?
Important Questions from Nucleus
- The binding energy B of a nucleus is approximated by the formula B = a 1 A − a 2 A 2/3 − a 3 Z 2 A −1/3 − a 4 (A − 2Z) 2 A −1 where Z is the atomic number and A is the mass number of the nucleus. If \(\rm\frac{a_4}{a_3}\) ≃ 30, the atomic number Z for naturally stable isobars (constant value of A) is
What are the main constituents of biogas?
The energy equivalent of mass associated with the rest mass of an electron, is nearly:
- Considering that the radius of an atomic nucleus, $R$, can be approximated by the formula $R = R_0 A^{1/3}$, where $R_0 \approx 1.2 \times 10^{-15}$ m is the Fermi radius constant and $A$ is the mass number, and the average mass of a single nucleon is approximately $1.67 \times 10^{-27}$ kg. Calculate the approximate order of magnitude of nuclear matter density in $\text{kg/m}^3$.
- The mass number of argon is 40. Which one of the following statements is correct?
