What is an Atom ? It's Structure, Components and Properties.

What is an Atom ? It's Structure, Components and Properties.

To understand What is Atom ? First we need to know what is "Matter". Matter is anything that can be touched physically. Everything in the universe (except energy) is made of matter. An Atom is the smallest unit of Matter. Hence Everything in the universe is made of atoms.

In relation to Physics & Chemistry An Atom is defined as the smallest unit of an element, having all the characteristics of that element. Thus Atoms can join together to form molecules, which in turn form most of the objects around us.

Till the end of the 19th century, atoms were considered to be the ultimate particles of Matter, But Modern Researches have shown that an atom is not the ultimate particle of an element and it is composed of some fundamental particles. These fundamental particles of an atom are also known as "subatomic particles". These are electrons, protons and neutrons. An atom consists of two parts :- Nucleus which is present in the centre consisting of protons and neutrons and the extra nuclear portion containing electrons. The protons and neutrons present in the nucleus of an atom are collectively called as "nucleons".

1. Nucleus :- It is the central part of an atom consisting of protons and neutrons.It is positively charged.It's size(diameter) is about 10^-15 m (10^-15 m is typical for the smaller nuclei; larger ones go up to about 10 times that.) Density of nucleus is 1017 kg/m³ . Stability of Nucleus depends upon 3 factors. 

They are :-

a) Mesons :- Mesons were first predicted by a Japanese scientist Yukawa in 1935 to explain strong forces which binds together nuclear particles i.e protons and neutrons in the nucleus. The mesons may be positively charged or negatively charged or neutral. There is interconversion of proton in to neutron and vice-versa through the exchange of mesons. For eg. A proton may change into neutron by loosing a positive meson or by accepting a negative meson. Similarly a neutron may change into proton by loosing a negative meson or by accepting a positive meson.

b) Mass defect :- It is defined as the difference between the total calculated mass of nucleons and the actual observed mass of nucleons.

c) Binding Energy :- Mass defect is converted into energy which binds the nuclear particles i.e protons and neutrons in the nucleus of an atom this energy is called Binding Energy.

2.Protons :- Protons are present in the nucleus of an atom and are positively charged. E . Goldstein in 1886 discovered proton. Protons are denoted by the symbol " p ". The mass of a proton is about 1.673 x 10^-27 kg or 1.00783 amu (atomic mass unit ). It has a unit positive charge on it which is approximately equal to 1.602 x 10^-19 C .

3.Neutrons :- Neutrons are present in the nucleus of an atom and are electrically neutral. James Chadwick in 1932 discovered neutron. Neutrons are denoted by the symbol " n ". The mass of a neutron is about 1.675 x 10^-27 kg or 1.000866 amu( atomic mass unit ) . It has no charge on it.

4.Electrons :- Electrons are present outside the nucleus of an atom in the extra nuclear region revolving round the nucleus and are negatively charged. J.J. Thomson in 1896 discovered electron. Electrons are denoted by the symbol " e- ". The mass of an electron is about 9.1 x 10^{- 31} kg or 0.000545 amu. It has a unit negative charge on it which is approximately equal to -1.602 x 10^-19 c .

Important stuff regarding subatomic particles :-

1. The number of protons and electrons in an atom are always equal.

2.The number of neutrons are not necessarily equal to the number of electrons.

3. Stability of nucleus also depends upon neutron to proton ( n/p ) ratio.

Characteristics of Subatomic Particles :-

1. Atomic Number :- Atomic Number is defined as the total number of protons or electrons present in an atom. It is denoted by the symbol "Z". Chemical properties of elements depend totally on Atomic Number. Atoms of the same element have same Atomic Number.

2.Mass Number :- It is defined as the total number of protons and neutrons present in the nucleus of an atom. It is denoted by the symbol " A ". It represents the total mass of the atom. Chemical properties of elements are not dependent on mass number.Atoms of the same element can have different mass number.Thus A = p + n . An Element is represented as
 ^Z Element _A .

3. Neutron Number :- It is defined as the total number of neutrons present in the nucleus of an atom. It is denoted by the symbol " N ". Hence we can say that A = Z + N .

4. Isotopes :- Atoms of the same element having same Atomic Number( number of protons ) but different Mass Number( number of nucleons ) are known as Isotopes. Soddy in 1913 discovered Isotopes. Isotopes have different physical nature but same chemical nature. Isotopes of an element have same electronic configuration and are placed at the same position in periodic table. Eg., 8O16 , 8O17 , 8O18 .

5. Isobars :- Atoms of the different elements having same Mass Number( number of nucleons ) but different Atomic Number( number of protons ) are known as Isobars. Isobars have different physical and chemical properties. Isobars of an element have different electronic configuration and are placed at different positions in the periodic table. Eg. ¹⁸Ar₄₀ , ¹⁹K₄₀ .

6.Isotones :- Atoms of the elements having same number of neutrons but different number of nucleons are known as Isotones. Isotones of an element have different physical and chemical properties and also different electronic configuration and are placed at different positions in the periodic table. Eg, ¹⁵P₃₁ and ¹⁶Si₃₂ .

7. Isoster's :- Molecules having same number of atoms and same number of electrons are known as Isoster's. Eg, SO₂ and N₂O .

8. Isodiaphers :- Atoms of elements having different Mass Number( number of nucleons ) and different Atomic Number( number of protons ) are known as Isodiaphers. Eg. ¹⁹K₃₉ and ⁹F₁₉ .