[ssssss]

Why the Energy of electron in Bohrs orbit is taken as negative?
{-2.175x10^-18z²/n² in joules}

[AWK]

This is convention.

[movies]

It's because the zero-point energy is set for infinite charge separation, right?  That's the convention.

[Mitch]

Right, so when you bring them into an orbital energy is given; which is by convention negative.

[ssssss]

Yeah you are right but i am answering in more expressive manner for that convention of negative sign.

As you know an electron is most unstable when infinite distance from Nucleus because there is no Coloumbic forces acting on it.Now since it is most unstable we regard its energy to be higher then its position at any other distance from the Nucleus[ie at any other orbit other than infinity] so as we got the energy of electron Zero[by the same bohrs formulae] we have to chose the negative Values of other energies which are lower than it.Hence to make the Mathematical Model of Bohrs atom we took the negative sign.

[movies]

I think the point is that the zero point of the energy diagram is set arbitrarily.  They could have just as easily selected the zero of energy to be at the lowest energy point of the energy curve, so that any excited state was at a positive energy and there could be no negative energy values.

The point of selecting infinite separation is that it avoids any energetic interactions with other particles, therefore it is really the energy of a lone electron and a lone nucleus.

It makes sense to have some meaningful zero-point for the energy because it allows for comparrison of the energy as the electron approaches very close to the nucleus (energy starts to go up fast, and is eventually higher than the zero-point energy).  I think that this can also explain how electrons can reach excited states that are high enough in energy to allow for loss of the electron; all you would need, theoretically, is a positive energy.

[ssssss]

Yeah since Energy of electron in the orbit is taken as negative we take the energy of Ionisation as Positive.