Chemistry · Structure Of Atom · NEET
Only the RATIO e/mₑ, not e or mₑ by itself. In his cathode ray tube experiment, the deflection of the beam depends on charge and mass together. From the deflection he could only get the value of charge divided by mass. He got e/mₑ = 1.758820 × 10¹¹ C/kg. To find the charge e alone, you need Millikan's oil drop experiment. Only after Millikan gave e could the mass mₑ be worked out by dividing.
e/mₑ = 1.758820 × 10¹¹ coulomb per kilogram (C kg⁻¹). NEET usually rounds it to 1.76 × 10¹¹ C/kg. The unit is C/kg because it is charge (coulomb) divided by mass (kilogram). Remember the unit — a wrong unit is a common trap.
Because the electron's mass is extremely tiny (about 9.1 × 10⁻³¹ kg) while its charge is 1.6 × 10⁻¹⁹ C. When you divide a small charge by an even smaller mass, you get a very large number. A large e/m means the electron is deflected a lot by electric and magnetic fields — that is why cathode rays bend easily.
NCERT lists three: (i) the magnitude of the charge — more charge means more deflection; (ii) the mass — a lighter particle bends more; (iii) the strength of the electric or magnetic field — a stronger field bends the beam more. By balancing the electric field and the magnetic field so the beam goes straight, he calculated e/mₑ.
The electron has the LARGEST e/m of any particle because it is the lightest. A proton is about 1836 times heavier, so its e/m is about 1836 times smaller (roughly 9.58 × 10⁷ C/kg). Same-size charge, but much bigger mass means much smaller e/m. In NEET, 'highest e/m' almost always points to the electron.
No. Thomson found the same e/mₑ no matter which gas filled the tube or which metal the electrodes were made of. This constant value proved that electrons are the same in all atoms — a basic building block of every atom. This is an important NCERT conclusion often asked in NEET.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
e/mₑ = 1.758820 × 10¹¹ C kg⁻¹ (often written 1.76 × 10¹¹ C/kg). It was measured by J.J. Thomson in 1897.
J.J. Thomson, a British physicist, in 1897, using a cathode ray discharge tube with electric and magnetic fields applied perpendicular to the electron path.
Coulomb per kilogram (C/kg or C kg⁻¹), because it is the electric charge divided by the mass.
His experiment gave only e/mₑ. To get the mass, you also need the charge e, which came from Millikan's oil drop experiment. Then mₑ = e ÷ (e/mₑ).
The electron, because it has the smallest mass. Protons and other heavier particles have much lower e/m values.
