Thomson's model of the Atom

According to Thomson's Model, an atom consists of a sphere of positive charge with negatively charged electrons embedded in it (just like seeds in a watermelon). These positive and negative charges are equal in magnitude. Therefore, the atom is electrically neutral.

Rutherford's model of the atom

Rutherford's Nuclear Model of the atom can be described as:

• An atom consists of a positively charged, dense, and very small region called the nucleus. Almost the entire mass of the atom is concentrated in the nucleus, which contains all the protons and the neutrons.
• The electrons revolve round the nucleus in circular paths, called orbits, at very high speed.
• The electrostatic forces of attraction between the positively charged nucleus and negatively charged electrons hold the atom together.
• The number of protons and electrons in an atom is equal. Therefore, the atom is electrically neutral.
• A major part of the atom is empty space.

Bohr's model of the atom (Modern Atomic Theory)

 The modern concept of the atom, give by Neils Bohr, can be described as:

• An atom is made up of electrons, protons, and neutrons. Electrons have negative charge, protons have positive charge, and neutrons have no charge. The atom is electrically neutral as the number of electrons is equal to the number of protons.
• The protons and neutrons are located in the nucleus of the atom. The nucleus is positively charged due to the presence of protons.
• The electrons revolve around the nucleus in circular paths called energy levels or shells. The energy levels are counted from the centre outwards and are represented either by the numbers 1,2,3,4,5 and 6 or by the letters K, L, M, N, O and P.
• There is a maximum number of electrons which each energy level can hold. For example K the innermost shell, can hold a maximum of 2 electrons, L shell can hold 8 electrons, M shell can hold 18 electrons and N shell can hold 32 electrons.
• Each shell is associated with a certain amount of energy. The shell nearest to the nucleus has minimum energy and the shell farthest from the nucleus has maximum energy.
• As long as an electron keeps revolving in a particular shell, there is no change in its energy. The change in energy of an electron takes place when it moves from one shell to another.

Arrangement of Electrons in the Atom

The arrangement of electrons in the various energy levels of an atom of an element is called the electronic configuration of the element. The maximum number of electrons which can be accommodated in any energy level of the atom is given by 2n², (where n is the number of that energy level). The outermost shell cannot accommodate more than 8 electrons, even if it has the capacity to accommodate more electrons. Electrons do not occupy a new shell unless all the inner shells are completely filled with electrons. Thus, the electronic configuration of magnesium (with atomic number 12) can be written as 2, 8 and 2.

Valence electrons

The electrons present in the outermost shell of an atom are known as valence electrons because they determine the valency (combining capacity) of the atom. Only the valence electrons are involved in chemical reactions. For example, a magnesium atom has 2 valence electrons.

Noble gases (helium, neon, argon, krypton, xenon, and radon) do not react with other elements to form compounds. The electron arrangements in their atoms are very stable and do not allow the outermost electrons to take part in chemical reactions. Thus, they are also known as inert gases. All the noble gases have completely filled outermost shells, which is a highly stable state. That is why they can exist in the free state as individual atoms. Neon gas is used in advertising signs (called neon signs). It glows red when electricity is passed through it. Argon gas is filled in light bulbs to prevent tungsten filament from reacting.

The atoms combine with one another to achieve the inert gas electronic configuration and become more stable. This can be done by:

• Losing one or more electrons to another atom
• Gaining one or more electrons from another atom
• Sharing one or more electrons with another atom

Valency of Elements

Valency of  an element is defined as the capacity of its atoms to form chemical bonds. The valency of an element is either equal to the number of valence electrons in its atom or equal to the number of electrons required to complete eight electrons in the valence shell.

Valency of metal = Number of valence electrons in its atom

Valency of a non-metal = Number of valence electrons in its atom

There are two types of valency:

• Electrovalency: In the formation of an electrovalent compound (or ionic compound), the number of electrons lost or gained by one atom of an element to achieve the nearest inert gas electronic configuration is known as its electrovalency.
• Covalency: In the formulation of a covalent compound (or molecular compound) the number of electrons shared by one atom of an element to achieve the nearest inert gas electronic configuration in known as its covalency.