Question

How atomic masses of atoms are determined accurately by modern techniques?​

Answer 1

Answer:

A moment's thought shows you can't weigh an atom on a traditional scale.

Instead, physicists for over a century have used an instrument called a mass spectrometer. Invented in 1912 by physicist J.J. Thomson and improved incrementally, it works like this: First, physicists "ionize" a gas of atoms by firing a beam of particles at the gas, which either adds electrons to the atoms in it or knocks a few of their electrons off, depending on the type of particle beam used. This gives the atoms — now known as "ions" — a net negative or positive electric charge.

Next, the ions are sent through a tube in which they're subjected to electric and magnetic fields. Both of these fields exert a force on the ions, and the strengths of the two forces are proportional to the ions' charge (neutral atoms don't feel the forces). The electric force causes the ions to change speed, while the magnetic force bends their path.

The ions are then collected by "Faraday cups" at the end of the tube, generating a current in wires attached to the cups. By measuring where and when the stream of ions hits the Faraday cups, the physicists can determine how much they must have accelerated, and in what direction, as a result of the electric and magnetic forces. Lastly, by way of Newton's second law of motion, F=ma, rearranged as m=F/a, the physicists divide the total force acting on the ions by their resulting acceleration to determine the ions' mass.

The mass of the electron has also been determined using a mass spectrometer — in that case, electrons were simply sent through the instrument themselves. That measurement enables physicists to determine the mass of an atom when it has the correct number of electrons, rather than a dearth or surplus of them.

Using a mass spectrometer, physicists have determined the mass of a hydrogen atom to be 1.660538921(73) × 10-27 kilograms, where the parenthetical digits are not known with complete certainty. That's accurate enough for most purposes.