Why are electrons not included in the calculation of an atom’s mass number?
Electrons have negligible mass compared to protons and neutrons
The mass number (A) of an atom is the total number of protons and neutrons (nucleons) in its nucleus: \(A = p + n\).
A proton and a neutron each have a mass of about 1 u, whereas an electron has a mass of only about 1/1836 of a proton (about 0.00055 u). Even all the electrons of an atom together add a negligible amount to its mass, so practically the entire mass of the atom is concentrated in the nucleus.
The other statements are wrong: in a neutral atom the number of electrons equals the number of protons, so electrons do not outnumber them; electrons do decide the charge and chemical behaviour, but that is not why they are left out of the mass number; and electrons are far lighter, not heavier, than protons and neutrons.
Hence, electrons are excluded because electrons have negligible mass compared to protons and neutrons.
Similar Questions
Which of the following would change if a neutron is added to the nucleus of an atom, but not a proton or electron?
The statement that electrons are embedded in a positive sphere is explained by which model?
An atom of phosphorus has an electronic configuration of 2, 8, 5. What is its valency and how does it attain stability?
What is the charge and location of an electron in an atom?
Why does the number of protons in an atom never change during chemical reactions?
According to Rutherford's model of the atom, where are most of the atom's mass and positive charge concentrated?
The mass of an electron is approximately:
Which statement best describes a neutron in the structure of an atom?
If an atom of magnesium loses a proton, what happens to its atomic number and identity?
Consider the following statements regarding Thomson's model of an atom.
Statement I: The atom is a sphere of positive charge with electrons scattered inside it.
Statement II: Thomson's model could explain the presence of the nucleus in an atom.
Which of the following is TRUE?
Important Questions from Structure of Atom
- According to Bohr's model, electromagnetic radiations possess dual character properties behaving like:
- Which of the following is NOT an isotope of hydrogen atom?
- The atomic number of an element is equal to which of the following?
- Which of the following factors primarily determines the size of an atom?
- The atomic radius of hydrogen atom is
