[Kartiky14]

When finding the size of an electron,
the textbook states that its approximately equal to e²/r(How did they land here?), where r is the radius of the electron. And then directly it states that if the electron is considered to be electromagnetic in nature then its energy = mc²(How is that?And dont just say its the mass-energy relation by Einstein.I want to know why we state electromagnetic nature in here? ). And then it relates the e²/r (Curly equals sign (Whats that?) mc²(And finally what does this mean) 
??

[amber]

mispost

[Jorriss]

So, this is a classical result and is not really valid.

The idea is that you imagine an electron in its rest frame so its energy is E=mc^2. If the electron is considered a sphere then it's classical electrostatic energy is E=ke^2/r (upto a constant, depending on some factors).

Equate the two assuming that it gets its mass from its electrostatic energy. Solve for r.

That's the gist though.

I'm not sure if I answered your question though... it was hard to follow.

[juanrga]

When finding the size of an electron,
the textbook states that its approximately equal to e²/r(How did they land here?), where r is the radius of the electron. And then directly it states that if the electron is considered to be electromagnetic in nature then its energy = mc²(How is that?And dont just say its the mass-energy relation by Einstein.I want to know why we state electromagnetic nature in here? ). And then it relates the e²/r (Curly equals sign (Whats that?) mc²(And finally what does this mean) 
??

Electrons are pointlike particles. They do not have radius r, although sometimes you can read about a so-named classical electron radius, that is a misapplication of classical electromagnetic theory to a quantum particle. That 'radius' does not represent the size of the electron.