◆ According to the classical Bohr model, the atom is viewed as having a planetarytype structure with electrons orbiting at various distances around the central nucleus.
◆ The nucleus of an atom consists of protons and neutrons. The protons have a

maximum number of electrons that can exist in shell 2 is

The maximum number of electrons that can exist in shell 3 is

The maximum number of electrons that can exist in shell 4 is

Valence Electrons

Electrons that are in orbits farther from the nucleus have higher energy and are less tightly

bound to the atom than those closer to the nucleus. This is because the force of attraction

between the positively charged nucleus and the negatively charged electron decreases with

increasing distance from the nucleus. Electrons with the highest energy exist in the outermost shell of an atom and are relatively loosely bound to the atom. This outermost shell is

known as the valence shell and electrons in this shell are called valence electrons. These

valence electrons contribute to chemical reactions and bonding within the structure of a

material and determine its electrical properties. When a valence electron gains sufficient

energy from an external source, it can break free from its atom. This is the basis for conduction in materials.


When an atom absorbs energy from a heat source or from light, for example, the energies

of the electrons are raised. The valence electrons possess more energy and are more

loosely bound to the atom than inner electrons, so they can easily jump to higher energy

shells when external energy is absorbed by the atom.

If a valence electron acquires a sufficient amount of energy, called ionization energy, it

can actually escape from the outer shell and the atom’s influence. The departure of a valence

electron leaves a previously neutral atom with an excess of positive charge (more protons

than electrons). The process of losing a valence electron is known as ionization, and the

resulting positively charged atom is called a positive ion. For example, the chemical symbol

for hydrogen is H. When a neutral hydrogen atom loses its valence electron and becomes a

positive ion, it is designated H. The escaped valence electron is called a free electron.

The reverse process can occur in certain atoms when a free electron collides with the atom

and is captured, releasing energy. The atom that has acquired the extra electron is called a

negative ion. The ionization process is not restricted to single atoms. In many chemical reactions, a group of atoms that are bonded together can lose or acquire one or more electrons.

For some nonmetallic materials such as chlorine, a free electron can be captured by the

neutral atom, forming a negative ion. In the case of chlorine, the ion is more stable than the

neutral atom because it has a filled outer shell. The chlorine ion is designated as

The Quantum Model

Although the Bohr model of an atom is widely used because of its simplicity and ease of

visualization, it is not a complete model. The quantum model, a more recent model, is considered to be more accurate. The quantum model is a statistical model and very difficult to

understand or visualize. Like the Bohr model, the quantum model has a nucleus of protons

and neutrons surrounded by electrons. Unlike the Bohr model, the electrons in the quantum model do not exist in precise circular orbits as particles. Two important theories underlie the quantum model: the wave-particle duality and the uncertainty principle.

◆ Wave-particle duality.

Just as light can be both a wave and a particle (photon),

electrons are thought to exhibit a dual characteristic. The velocity of an orbiting electron is considered to be its wavelength, which interferes with neig

Charles Muthura

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