Question

why the energy of an electron in an orbit is taken as negative?

Answer 1

the potential energy of an electron is negative inside the atom and hence we learn that energy of electron is negative

hope you understand

Answer 2

It is because when an electron is at an infinite distance from the nucleus, there the kinetic energy of that electron cannot be calculated. So, it is taken to be zero. When that electron comes close to the nucleus, the stability increases due to loss of energy of the electron. And thus the energy of electron becomes less negative. That's why the electron's energy is taken as negative.