Why the energy of electron in an atom is negative?

What do you understand by Dual nature of matter?
Or
What is de-Broglie relationship?

How will you correlate Bohr’s concept of hydrogen atom with de-Broglie concept?
Or
Show that the circumference of Bohr orbit for the hydrogen atom is an integral multiple of de-Broglie wavelength associated with electron revolving around the orbit.

185.

What will be the wavelength of a ball of mass 0.1 kg moving with a velocity of 10 ms^-1? 

If the velocity of the electron in Bohr's first orbit is 2.19 x 10⁶ ms^-1, calculate the de Broglie wavelength associated with it.

The velocity associated with a proton moving in a potential difference of 1000V is 4.37 x 10⁵ ms^-1. If the hockey ball of mass 0.1 kg is moving with this velocity, calculate the wavelength associated with this velocity. 

Dual behaviour of matter proposed by de Broglie led to the discovery of electron microscope often used for the highly magnified images of biological molecules and other type of material. If the velocity of the leectron in this microscope is 1.6 X 10⁶ ms^-1, calculate de Broglie wavelength associated with this electron.