Atomic structure

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Atomic theory

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An Atom is a small part of an element that takes part in chemical reactions. It is made up of three subatomic structures called Protons, Neutrons, and Electrons.

Subatomic structures:

    Particle:               Charge:              Mass:            
    Proton                     +1                1 *u  
    Neutron                     0                1 *u
    Electron                   -1              1 / 1836*u
    Positron                   +1.             1 / 1836*u

The Nucleus, in the center of the atom, consists of protons and neutrons. Orbiting around the nucleus are the electrons.

Model of a Lithium atom
Model of a Lithium atom

Atomic Structure

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The nucleus, in the center of an atom, consists of protons and neutrons. Orbiting around the nucleus are the electrons. Each unique element has an Atomic Number equal to the number of protons it contains. There are 94 naturally occurring elements (1-94) and others which have been artificially created (95+..) Each element has an Atomic Weight for the most commonly found isotope. Atomic Weight = number of protons + number of neutrons; see: The Periodic table. In a stable uncharged atom the number of electrons will equal the number of protons. If the number of electrons is changed the atom will become ionized and gain either a positive (fewer electrons) or negative (greater electrons) charge.


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The same element can exist in different forms, each form having the same atomic number, but different mass numbers. These forms are called isotopes. Isotopes that cannot decay during a defined period are called stable isotopes. And isotopes that can decay during a defined period are called unstable (or radioactive) isotopes. For example :

Atomic Structure of isotopes of Hydrogen[1]
        1               2                        3   
        1H             1H                       1H                                                     
        Protium.        Deuterium.        Tritium.  

Tritium is an unstable isotope of hydrogen.

In accordance with the Bohr Model, electrons are considered to move around the nucleus in fixed shells (orbits), at various energy levels. These levels may be designated K L M N shells (or 1 2 3 4 orbits). The first level contains only 2 electrons. The second level can hold 6 electrons. The number of maximum electrons that can occupy one shell is defined by the formula of 2nî or 2n square, where n is the number of the shell.

When the electrons are excited, they can transfer between the shells. As we move away from the nucleus, the energy levels increase. Electrons have their own energy and the energy increases with increasing the n value or orbit.

Excited state of atom

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The configuration of electrons occupying the least amount of space (in the Bohr model) is called the ground state. But when electrons are excited (by getting electricity, heat), they jump to a higher level.

This condition of the atom is called an excited state. When the electrons return to the ground state they give off energy.

Levels and sublevels

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All energy levels contain sublevels known as s p d f.

S sub level can contain 2 electrons. (One pair.)

P can contain 6 electrons. (Three pairs.)

D can contain 10 electrons. (Five pairs.)

F can contain 14 electrons. (Seven pairs.)

The first shell has only the sublevel s and therefore has only 2 electrons The second shell has both the sublevel s & p and can hold 8 electrons The third shell has the sublevel s,p & d and can hold 18 electrons The fourth and the last shell has the sublevel s,p,d & f and can hold 32 electrons.


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  • The regions around a nucleus where the probability of finding an electron of a particular energy level is highest are called orbitals.
  • An Orbital can hold only two electrons. They are spinning opposite ways. They are called orbital pairs. 'n' represents principal quantum number or simply the number of the shell. So nth shell contains n^2 number of orbitals hence 2*n^2 number of electrons.
  • The shape of orbital depends on the sublevel.


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  • Valence electrons revolve in valence shells.
  • Electrons that can enter into a reaction are only at the last level.
  • These electrons are called valence electrons. The maximum number

of valence electrons is 8. The Valence determines how many electrons the atom has to borrow or lend. All parts of atoms except the last orbit are called a kernel.

  • The electron dot formula represents the valence electrons.
Examples:         . .
                 . N .   The dotted formula of the nitrogen.

Ionization energy

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Ionization energy is the amount of energy needed to remove or add an electron from a gaseous atom or ion. Ionization energy depends on the number of protons in the nucleus and the shielding (screening) of the inner electrons.

See also

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  1. "The Three Isotopes of Hydrogen - Video & Lesson Transcript |". Retrieved 2018-09-18.