The structure of the Atom

Atom is considered the smallest particles of matter, according to Dalton's Atomic Theory,  but it was found by some scientists that atom has still subatomic particles called the electron, proton and neutron.    

Electron was the first to be discovered by J.J Thomson, a British scientist, while he was studying the cathode ray tube.   He found out that this particle is negatively charged.   In 1908 to 1917 R.A. Milikan, an American physicist, found the charge of the electron to be -1.6022 x 10^-19 C.  From these data he also calculated the mass of the electron to be 9.10 x 10^-28 g.   J.J. Thomson proposed the plum pudding model of the atom.  He pictured the small electrons to be embedded  in a uniform and positive charge sphere.

After the discovery of proton, in 1896 a French scientist Henri Becquerel (1852-1908) while studying a uranium mineral , he found out that it spontaneously emits high energy radiation.  This spontaneous emission of radiation is called radioactivity.  Consequently, any element that spontaneously emits radiation is said to be radioactive.

Further study about radioactivity, a British scientist Ernest Rutherford revealed three types of radiation: alpha (𝛂), beta (𝛃) and gamma (𝛄).  Rutherford showed that 𝛂 and 𝛃 rays are consist of fast-moving particles which were called the 𝛂 and 𝛃 particles.  𝛃 particles are high-speed electrons and can be considered the radioactive equivalent  of cathode rays.  They are attracted to a positive plate.  The 𝛂 particles  are much more massive than the 𝛃 particles  and have a positive charge.  They are therefore attracted to negative plate.  𝛃 particles have a charge of -1 while the 𝛂 particles has +2 charge.  Rutherford showed that 𝛂 particles combine with electrons to forms atoms of helium.  He concluded that an 𝛂 particle consist of the positively charged core of the helium atom. He further concluded that 𝛄 radiation is high-energy radiation similar to x-rays; it does not consist of particles and carries no charge and not affected by electrical charge.

The Discovery of Nucleus of an Atom

Plum Pudding Model of Thomson was accepted for how many years but it was proved wrong by Rutherford.  In 1910, Rutherford together with his team use 𝛂 particles to probe the structure of the atom.   They carry out series of experiments using very thin foils of gold and other metals as targets for alpha particles from a radioactive source.  Below is the diagram of their gold foil experiment:

The result of the experiment were the following:

1.  Majority of particles penetrated the foil either undeflected or with only a slight deflection.
2. They also noticed that every now and then an 𝛂 particle was scattered (or deflected) at a large angle.
3.  Some instances, 𝛂 particle actually bounced back in the direction from which it had come.

From the result of the experiment Rutherford devised a new model of the atomic structure of an atom, he concluded that:

1.  Most of the atom is an empty space, since most of the 𝛂 particles pass through the gold foil with little deflection or no deflection at all.
2.  That the atom's positive charges are all concentrated in the nucleus, a dense central core of the atom.  Whenever an alpha particle came close to the nucleus in the scattering experiment, it experienced a large repulsive force and therefore large deflection.  An when an 𝛂 particle travelling directly towards the nucleus, it would experience a strong repulsion that would completely reverse the direction of the moving particle.

The positive charge found in the nucleus of the atom is called proton.  They found out that the charge of the proton has the same but opposite in magnitude of that of electron and has a mass of 1.67262 x 10^-24 g, about 1840 times heavier than that of the opposite charge electron.  The model is shown below:

At this point of investigation, scientists perceived that the mass of the atom is concentrated in the nucleus of an atom, but the nucleus only occupies 1/10¹³ of the volume of an atom.  A typical atomic radius is about 100 picometer (pm), whereas the radius of an atomic nucleus is only about 5 x 10^-3 pm.  To understand how small the atomic nucleus is, imagine the size of sports stadium, if that is the size of the atom; the size of the nucleus is comparable as that of a marble.


The neutron

Rutherford's atomic model of  an atom left one problem unsolved.  They found out that the mass of the total electrons and protons inside the atom did not equalize with the total mass of the atom, they concluded that there is still subatomic particle inside the nucleus that is missing.  The problem was solved when an English physicist James Chadwick, bombarded a thin sheet of beryllium  with 𝛂 particles, a very high energy radiation similar to 𝛄 rays was emitted by the metal.  His experiment showed that the rays consist of electrically neutral particles having a mass slightly greater than that of protons.  Chadwick named this particles as neutron.

Although there were new findings of some subatomic particles of atom.  The three fundamental particles of atom:  the electron, proton and neutrons are still very important in the study of chemistry.