What is atomic number? How atomic number is responsible for the chemical properties of any element? How valency is associated with electron distribution?

What is atomic number?

Number of protons present in an atom of an element is called atomic number of the element.

Atoms are formed with 3 types of particles. The names of the particles are electron, proton and neutron. Protons are positively charged, electrons are negatively charged, protons are neutral. Protons and neutrons stay together in the center. The cloud of protons and neutrons in the center is called nucleus. Electrons stay in one or more orbits surrounding the nucleus. Atom of different element has different number of protons in its nucleus. if number of protons are changed, then it is definitely different element.

How atomic number is responsible for the chemical properties of any element?

Number of protons in an atom is called its atomic number. Number of electrons in the orbit/orbits in an atom is same as number of protons in the nucleus of that atom. Chemical property of an element is completely dependent on the number of electron in the outer orbit in an atom of that molecule. Electrons of an atom is distributed in the orbits according to the formula 2n² where n is the number of orbit. Maximum number of electrons can stay in an orbit according to the formula 2n².

If n=1 (First orbit) then maximum number of electron can stay in the first orbit is 2X1²=2

If n=2 (Second orbit) then maximum number of electron can stay in the second orbit is 2X2²=8

If n=3 (Third orbit) then maximum number of electron can stay in the third orbit is 2X3²=18

So maximum number of electrons in any orbit can be calculated as shown above. It can be calculated with the value of n=4, n=5 like that for higher number of orbit.

Although electrons will be distributed in different orbits according to the 2n² formula, but another important criteria of electron distribution is that there can not be more than 8 electrons in the outer orbit of an atom.

There are some exceptions (Chromium, Copper etc.) Except the exceptions, electrons of the atom of almost all the elements are distributed according to the 2n² formula and maximum number of electron in the outer orbit can not be more than 8. This is the basic concept and it must be clear first.

There may be one or more number of orbit in a atom but it should be remembered that outer orbit and more specifically number of electron in the outer orbit of an atom is the most important criteria for the chemical property of an element. If number of electrons is 1 or 2 or 3 in the outer orbit then the atom will give those electron to other atom which has 7 or 6 or 5 electrons in the outer orbit and make some compound with ionic bond. If there are 4 electron in the outer orbit of an atom (Carbon) then it will share electrons with other atom and create compound with covalent bond. So how atom(s) of an element react with other element is completely dependent on the number of electrons in the outer orbit in the atom of that element.

What is valency? How valency is associated with electron distribution?

The combining power of an element is called it’s valency. It is measured by the number of hydrogen atoms it can combine  or displace with. Valency of element completely depends on the number of electrons in the outer orbit. If valency is known for any elements then it is possible to tell the formula of the compound made by those two elements. If anybody understand how to find number of electron in the outer orbit, then it is easy to determine the valency of that element. There are some exceptions also and there are some different way to find the valency and multiple valency also but it can be understood further. The basic concept should understand first as shown below.

If number of electron in the outer orbit is 1 then valency=1

If number of electron in the outer orbit is 2 then valency=2

(Exception only Helium. As it has only 2 electrons, So first orbit is completely filled with maximum number of electrons possible to be accommodated in the first orbit. So it is stable and can not react with any other element. It is a inert gas. Valency of Helium is zero)

If number of electron in the outer orbit is 3 then valency=3

If number of electron in the outer orbit is 4 then valency=4

If number of electron in the outer orbit is 5 then valency=3

If number of electron in the outer orbit is 6 then valency=2

If number of electron in the outer orbit is 7 then valency=1

If number of electron in the outer orbit is 8 then valency=0 those are inter gases

Example: Atomic number of Sodium is 11. So number of proton in an atom of Sodium is 11 and number of electrons in an atom of Sodium is also 11. Electron configuration of Sodium is 2, 8, 1. Number of electron in the outer orbit is 1. So valency of sodium is 1.

When two elements reacts with each other then the chemical formula of the formed compound can be written easily. Just interchange the valency of the elements with each other and write it as the number of atoms. As for example when Sodium and oxygen reacts with each other Valency of sodium (1) will be the number of atom of Oxygen and valency of Oxygen (2) will be number of atom of the Sodium. So the formula of final product will be Na₂O.

List of atomic number, symbol and electron distribution of the first 30 elements

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