The question asks about the key concept introduced by Niels Bohr's atomic model in 1913 regarding electron orbits. We need to identify the statement that accurately reflects Bohr's postulates, which addressed the shortcomings of Rutherford's model.
B) In specific, fixed (quantised) energy levels or orbits, without continuously radiating energy
This option perfectly aligns with Bohr's first and second postulates. Bohr proposed that electrons occupy discrete energy levels and do not radiate energy while in these stable orbits, thus explaining the stability of atoms and the discrete nature of atomic spectra.
A) Only in a single, fixed orbit, identical for every type of atom — This is incorrect. Bohr's model proposed multiple possible orbits (energy levels) for electrons within an atom, and these orbits (and their energies) are specific to the type of atom (due to different nuclear charges).
C) While constantly radiating away all of their energy until they spiral into the nucleus — This describes the prediction of classical electromagnetism for Rutherford's model, which Bohr's model was specifically designed to overcome. Bohr's key insight was that electrons do NOT continuously radiate energy in their stable orbits.
D) In a completely random and unpredictable fashion, with no defined orbits whatsoever — This contradicts the fundamental idea of Bohr's model, which introduced the concept of specific, well-defined, quantized orbits (energy levels). The idea of random electron behavior came later with quantum mechanics (Heisenberg's uncertainty principle), but even then, electrons occupy probability distributions (orbitals) rather than being completely random without any definition.
Bohr's model of the atom describes electrons orbiting the nucleus in specific energy levels or stationary states. The emission or absorption of light (photons) is directly related to transitions between these energy levels. We need to determine which type of transition leads to the emission of a photon.
Energy Levels: According to Bohr's model, electrons can only exist in discrete energy levels (orbits) around the nucleus. Each orbit has a specific energy associated with it. Orbits closer to the nucleus have lower energy, and orbits farther away have higher energy.
Energy Conservation: When an electron changes its energy level, the total energy of the atom must be conserved. This means that any change in the electron's energy must be compensated by the absorption or emission of energy in the form of a photon.
Photon Emission: For an atom to emit a photon, it must lose energy. This energy loss occurs when an electron moves from a higher energy orbit to a lower energy orbit. The energy of the emitted photon is equal to the difference in energy between the two orbits.
Photon Absorption: Conversely, for an atom to absorb a photon, it must gain energy. This happens when an electron moves from a lower energy orbit to a higher energy orbit, absorbing a photon with energy equal to the difference between the two levels.
Energy Difference Formula: The energy of the emitted or absorbed photon (\(E\)) is given by Planck's relation:
\[ E = h\nu = E_{\text{higher}} - E_{\text{lower}} \]where \(h\) is Planck's constant and \(\nu\) is the frequency of the photon.
D) Transitions (jumps) from a higher energy orbit to a lower energy orbit
When an electron moves from a higher energy state to a lower energy state, the atom releases the excess energy as a photon of light. This process is known as emission.
A) Transitions from a lower energy orbit to a higher energy orbit — This process requires the atom to absorb energy (e.g., by absorbing a photon) to move the electron to a higher energy level. It does not result in the emission of a photon.
B) Remains permanently fixed within a single, unchanging orbit — If an electron remains in a single, unchanging orbit, its energy level is constant, and there is no emission or absorption of photons. This is a stable state.
C) Is completely removed from the atom entirely (ionisation) — Ionisation is the process where an electron gains enough energy to completely escape the atom. While this involves energy absorption, it's a different phenomenon than photon emission due to an orbital transition. An atom becomes an ion, but it doesn't emit a photon in the process of an electron leaving it.
The question asks to identify the three fundamental subatomic particles that form the basic structure of an atom. This is a direct knowledge-based question from basic chemistry and physics.
Correct Option: D) Protons, neutrons, and electrons are the three fundamental subatomic particles that make up an atom. Protons carry a positive charge, neutrons are neutral, and electrons carry a negative charge. Protons and neutrons reside in the nucleus, while electrons orbit the nucleus.
The question asks for the definition of the atomic number of an element. This is a fundamental concept in chemistry and physics, defining the identity of an element.
C) Protons present in the nucleus of an atom of that element — This is the precise definition of the atomic number. The number of protons determines an element's chemical properties and its position in the periodic table.
The question asks for the definition of the mass number of an atom. To answer this, we need to recall the basic structure of an atom and the components that contribute to its mass.
D) The number of protons and the number of neutrons in its nucleus — This is the correct definition of the mass number. It represents the total count of nucleons (protons and neutrons) in an atom's nucleus, which accounts for almost all of the atom's mass.
This question asks for the definition of isotopes. To answer this, we need to recall the fundamental definitions of atomic number, mass number, protons, and neutrons, and how they relate to the identity and properties of an element.
C) The same number of protons (and hence the same atomic number) but different numbers of neutrons (and hence different mass numbers)
This statement accurately defines isotopes. They are atoms of the same element, meaning they have the same atomic number (number of protons), but they have different numbers of neutrons, leading to different mass numbers.