It is difficult to answer that why electron has negative charge just like it is difficult to answer other naturally existing things.
It was J.J. Thomson in 1897 who discovered sub atomic particles in an atom and named it as electron. J.J Thomson performed cathode rays experiment to find the charge of an electron. Rays were emitted at the cathode in a discharge tube, the negatively charged particles which were repelled are attracted to the anode so the rays must be negatively charged.
After the discovery of the electron, Rutherford thought that there must be positive charge particles within the atom to balance the negative electrons. Rutherford's discovery positive charged particles and named it as proton by performing gold foil experiment.
For more details about the experiment follow the link provided.
( some important points are given below. you may understand how electrons and protons got their name )( read carefully otherwise nothing will be understood)
- electron is the first identified particle in an atom
- the electron is a fundamental particle discovered by J.J Thomson during the study of conduction of electricity through gases taken in a gas discharged tubes.
- it is now known that some invisible rays are formed at cathode of tube and when trays strike glass tubes they emit greenish light.
- since these rays are formed at the cathode they are known as cathode rays.
- the particles present in cathode rays are called electrons.
- you may know that gas will get ionized when electricity is passed through it at low pressure.
- these electrons and net possitive particles will flow to the oppositely charged electrodes.
- like this those that flow to +ve electrode are -ve rays and those that flow to -ve electrode are +ve rays.
- the particles of +ve rays are protons and particles of -ve rays are callesd electrons.
- atoms donot have charge bcoz +vely charged protons and negatively charged electrons are in same number.
- so to maintain a balance of charge of atom opposite charges have to be there .
- thats why one is -ve and one is +ve
- if there were only electrons in an atom then we have to say that atom is negativly charged and vise versa.