Problem 18 - Chemistry in 30 days

Question

The line spectra of two elements are not identical because (a) The elements do not have the same number of neutrons (b) They have different mass numbers (c) Their outermost electrons are at different energy levels (d) They have different valencies

Step-by-Step Solution

Answer

(c) Their outermost electrons are at different energy levels.

Step 1: Understanding Line Spectra

The line spectra of elements are unique to each element. This uniqueness is due to the energy levels of the electrons in an atom. When electrons jump between energy levels, they emit or absorb light at specific frequencies, leading to line spectra.

Step 2: Analyzing the Options

Let's evaluate the options based on our understanding of line spectra: (a) The number of neutrons affects isotopic identity, not line spectra. (b) Mass number differences indicate isotopes which also do not affect line spectra directly. (c) Different outermost electron energy levels will affect the line spectra, because they determine the energy transitions possible. (d) Valency is related to bonding behavior, not directly to line spectra.

Step 3: Identifying the Correct Option

The differences in outermost electron energy levels directly affect the line spectra. When these energies vary between elements, the spectra will differ because the electronic transitions occur at different energy frequencies. Thus, the correct choice is (c).

Key Concepts

Energy Levels

Atoms consist of a central nucleus surrounded by electrons. These electrons occupy different regions around the nucleus called energy levels or shells. Each energy level can hold a specific number of electrons and is associated with a certain amount of energy.
As you move further from the nucleus, the energy levels increase in energy, allowing electrons more room to exist away from the pull of the nucleus. In simple terms, an atom's energy level can be visualized as a ladder where each rung represents a different amount of energy. Electrons reside on these rungs and can sometimes "jump" from one rung to another when they gain or lose energy.

• The innermost energy level (closest to the nucleus) is the lowest in energy.
• Higher energy levels are further from the nucleus and can harbor more electrons.

Understanding energy levels is key to comprehending how atoms emit light, leading us to the phenomenon of electronic transitions.

Electronic Transitions

Electronic transitions occur when electrons jump between energy levels in an atom. These jumps can either be upwards, absorbing energy, or downwards, releasing energy in the form of light or other forms of electromagnetic radiation.
When an electron absorbs energy, it moves to a higher energy level, a process known as excitation. Conversely, when it releases energy, it falls to a lower energy level, often emitting light, a process known as relaxation. The energy difference between the levels determines the wavelength of light emitted or absorbed. This is why different elements show different colors or specific wavelengths of light.

• Excitation: Electrons absorb energy and move to a higher energy level.
• Relaxation: Electrons release energy and move to a lower energy level.
• The amount of energy absorbed or emitted corresponds to specific wavelengths of light.

These transitions are the source of the unique emission or absorption lines observed in the line spectra of elements.

Line Spectra

Line spectra are signatures of light emitted by electrons as they transition between different energy levels within an atom. Each element has its own unique line spectrum, which acts like a "fingerprint" that distinguishes one element from another.
The specific pattern of lines in a spectrum is due to the unique set of possible electronic transitions available in each element. Since each kind of atom has a different arrangement of energy levels, the wavelengths of light emitted during electronic transitions vary between elements.

• Emission spectra consist of bright lines against a dark background, showing wavelengths of light emitted by electrons.
• Absorption spectra consist of dark lines on a colored background, representing wavelengths of light absorbed by electrons.
• Line spectra are used in spectroscopy to identify elements in stars and other celestial bodies.

Thus, line spectra provide valuable information about the atomic structure and energy transitions of elements, allowing scientists to study the composition of substances and celestial objects accurately.