Chemistry · Structure Of Atom · NEET
Planck's constant h = 6.626 x 10^-34 J s (joule second). In NEET numericals you can use 6.626 x 10^-34 J s. Sometimes it is rounded to 6.63 x 10^-34 J s. Always keep the power of ten (10^-34) correct - that is where most students lose marks.
The unit is J s (joule multiplied by second), NOT J/s. Here is why: E = hv, so h = E/v. Energy is in joules (J) and frequency is in per second (s^-1 or 1/s). So h = J / (1/s) = J x s = J s. If you ever write J/s (watt), your answer is wrong. J s is also the unit of angular momentum, which is why h appears in Bohr's mvr = nh/2pi.
It means energy is not smooth - it comes in tiny fixed packets called quanta. h is the size-link of one packet. For light of frequency v, one quantum carries energy E = hv. A bigger h would mean bigger energy jumps. Because h is extremely small (10^-34), the packets are tiny, so in daily life energy looks continuous. In atoms, this tiny h controls electron energy levels.
It is small because energy packets of light are extremely tiny compared to everyday energy. One packet (photon) of visible light carries only about 10^-19 J. Since E = hv and v is around 10^15 per second, h must be near 10^-34 to give such a small energy. The tiny size of h is the reason quantum effects are only seen for atoms and light, not for cricket balls.
h = 4.136 x 10^-15 eV s. You get this by dividing 6.626 x 10^-34 J s by 1.6 x 10^-19 J/eV. NEET usually gives J s, but eV s is handy in photoelectric effect problems where energies are already in electron volts. Do not mix the two in one calculation.
No. h is Planck's constant = 6.626 x 10^-34 J s. h-bar (written as h with a slash) equals h/(2pi) = 1.054 x 10^-34 J s. h-bar appears in angular momentum and advanced physics. For NEET Chemistry, Bohr's rule mvr = nh/2pi uses plain h divided by 2pi, so you rarely need h-bar by name, but know they differ by 2pi.
Max Planck introduced it in 1900 to explain black-body radiation - the way hot objects (like a heated iron rod) glow different colours. Classical physics failed to explain this. Planck said energy is emitted or absorbed only in whole packets E = hv, and h is the constant that made his equation match experiments. This was the birth of quantum theory.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
h = 6.626 x 10^-34 J s in SI units. In CGS it is 6.626 x 10^-27 erg s.
It is a scalar - just a fixed number with units J s. It has magnitude but no direction.
[h] = [M L^2 T^-1]. This is the same as the dimensions of angular momentum, which is why h shows up in mvr = nh/2pi.
No. Planck's constant is a universal constant - it is the same everywhere in the universe and does not depend on temperature, material, or the light source.
In E = hv (photon energy), E = hc/lambda, the photoelectric equation, Bohr's angular momentum mvr = nh/2pi, and de Broglie's lambda = h/mv. It is one of the most-used constants in the chapter.
