Short Answer

Energy levels in an atom are the fixed amounts of energy that electrons can have while orbiting around the nucleus. Electrons cannot have any random energy; they must stay in specific energy levels or shells. These levels are arranged in increasing order of energy, with the lowest level closest to the nucleus.

When an electron gains energy, it moves to a higher energy level, and when it loses energy, it falls back to a lower level by releasing energy as light. These fixed levels help explain atomic structure, spectra, and many properties of elements.

Detailed Explanation :

Energy levels in an atom

Energy levels in an atom refer to the specific and fixed energies that electrons are allowed to possess. Electrons in an atom do not move in random orbits, nor can they have just any amount of energy. Instead, they occupy particular regions around the nucleus where their energy remains constant. These regions are called energy levels, shells, or energy states.

The idea of energy levels was first introduced to solve the problem of atomic stability. According to classical physics, an electron moving around the nucleus should continuously lose energy and eventually fall into the nucleus. But atoms are stable, which means electrons must occupy fixed energy states where they do not radiate energy. Niels Bohr explained this by suggesting that electrons revolve around the nucleus in certain allowed orbits, each associated with a fixed energy.

Electrons can move from one energy level to another, but only by absorbing or releasing a specific amount of energy. This difference in energy is called a quantum of energy. When electrons move between levels, they release or absorb photons of specific frequencies. This process is responsible for the colourful lines seen in the spectra of elements.

Energy levels explain why each element emits or absorbs light of particular colours. They also help understand chemical bonding, atomic structure, and the behaviour of materials.

Structure and arrangement of energy levels

Energy levels are arranged in a layered structure around the nucleus. They are labelled as K, L, M, N, or by numbers n = 1, 2, 3, 4, and so on. The first level (n = 1) is closest to the nucleus and has the lowest energy. Higher levels have greater energy and are farther from the nucleus.

Each energy level can hold a limited number of electrons. The formula 2n² shows the maximum number of electrons that can fit in a level. For example:

• n = 1 → 2 electrons
• n = 2 → 8 electrons
• n = 3 → 18 electrons

This arrangement helps explain the periodic table, chemical reactivity, and valency of elements.

Electrons do not always stay in the highest possible level. They fill the lowest energy levels first because systems in nature tend to move toward minimum energy. When all lower levels are filled, electrons move to higher ones.

Electron transitions between energy levels

Electrons can move from one energy level to another if they gain or lose energy. This movement is called a transition.

When an electron absorbs energy:

• It jumps to a higher energy level.
• This state is called the excited state.
• Electrons in excited states are unstable.

When an electron loses energy:

• It falls back to a lower energy level.
• It releases energy in the form of light (photon).
• The colour of light depends on the amount of energy released.

These transitions produce emission spectra, which are unique for each element. Similarly, when electrons absorb specific wavelengths of light and jump to higher levels, absorption spectra are formed.

This is why every element has its own “spectral fingerprint.”

Importance of energy levels

Energy levels help explain many properties and behaviours of atoms:

1. Stability of atoms:
   Electrons in fixed energy levels do not fall into the nucleus, making atoms stable.
2. Spectra of elements:
   Energy levels explain why elements emit or absorb light at specific wavelengths.
3. Chemical bonding:
   The arrangement of electrons, especially in the outermost energy level, determines how atoms bond with each other.
4. Periodic table structure:
   The periodic table is arranged according to how electrons fill energy levels.
5. Behaviour of solids:
   Energy levels in atoms lead to energy bands in solids, which explain why materials are conductors, insulators, or semiconductors.

Quantum mechanical understanding

Modern quantum mechanics gives a more accurate picture of energy levels. Instead of fixed circular orbits, electrons exist in orbitals, which are regions where there is a high probability of finding an electron. Each orbital has a specific energy and shape. These orbitals are grouped into sublevels known as s, p, d, and f.

Quantum mechanics explains that electrons are described by wave functions, and their energies come from solutions of Schrödinger’s equation. Although this description is more complex, the basic idea remains the same: electrons have fixed energies and occupy specific levels.

Relation between energy levels and light emission

The light emitted by atoms in lamps, stars, and flames is due to electrons jumping between energy levels. For example:

• Sodium produces yellow light.
• Hydrogen shows different coloured lines like red, blue, and violet.
• Neon produces red-orange light.

These colours appear because the energy difference between levels determines the frequency of the emitted photons.

This principle is used in lasers, fluorescent lights, street lamps, and even fireworks.

Conclusion

Energy levels in an atom are the fixed energies that electrons can have. Electrons occupy specific levels and can move between them only by absorbing or releasing energy. These levels explain the stability of atoms, their spectra, and their chemical behaviour. The concept of energy levels is essential for understanding atomic structure, quantum mechanics, and many modern technologies.