The Discovery of Subatomic Particles

Salaam (May God Bless You). Elements are made of atoms which are made of three more subatomic particles which are known as proton, neutron and electron. The word atom is from a Greek word ‘atoms’ which means indivisible. It was proposed by the Greek philosopher, Democritus. He proposed that objects were made by atoms which could not be physically separated. His theory was based on reason and philosophy rather than on scientific experiments.

The Discovery of the Subatomic Particles

Then came John Dalton, an english chemist who in 1808 made several experiments along with other scientists, to give a description of the atoms. He suggested that atoms were small invisible and indivisible atoms which made up to be elements, atoms are neither created nor destroyed, atoms of same elements are identical and atoms of different elements are different in mass and size and that atoms combine to form compounds in small whole numbers. This was not entirely correct. He gave a supposed structure of the atom to be like a ball.

Then came J.J Thompson. He was experimenting with the cathode rays in 1897, he used a discharge tube which had a cathode and an anode, with two opposite charged electric plates. The discharge tube had inert gas in it which was at low pressure. The discharge tube had a high voltage of about 1500 V. He observed that the cathode rays travelled directly towards the anode and through a small hole travelled further on. It got repelled away by the negatively charged plate and attracted towards the positively charged plate. The cathode rays usually end up travelling straight without the plates. He had discovered the electrons. He then test with other gases at low pressures as well and gave following conclusions after a few more experiments; The cathode rays must be negatively charged as it got repelled by the negatively charged plate but attracted towards the positively charged plate.The cathode rays must carry a charge as they got deflected by both the magnetic and electric field. The particles present in the cathode rays must be present in all atoms. Thompson then provided another model of the atom, saying that atoms had electrons in them but since they are neutral the rest of the mass must be positive. He also identified the mass of the electrons to be the same as always.

The scientist from New Zealand, Ernest Rutherford along with his students Geiger and Marsden discovered the proton in 1910. He had set up a fluorescent sheet around a thin gold sheet and an alpha particle emitter. He bombarded the gold sheet with alpha particles and observed parts of the fluorescent sheet glow. The glowing points indicated the path taken by the alpha particles. He noticed that a lot of the alpha particles went straight through the gold sheet to reach the fluorescent sheet directly behind the gold sheet but some were at an angle away from the straight path and some were completely repelled backwards. He deduced that the alpha particles had passed through empty space which made up the nucleus of the atom and that there must be a subatomic particle with a positive charge which repelled the alpha particles. Those which were deflected backwards must have come in direct contact with the proton. He then proposed a new model for the atomic structure.

In 1932, Chadwick discovered the neutron and made the understanding of the atomic structure much more clearer. He bombarded alpha particles on to materials like beryllium and saw that it emitted rays like the gamma rays which were extremely penetrating but were not deflected in the magnetic and the electric field making this clear that these rays were neutral. He had discovered neutron and made many conclusions with this.

Then in 1913, Henry Moseley conducted experiments. He bombarded metals with cathode rays which produced X-rays. He observed that as the mass of the metals increased the wavelength of the X-rays increased as well and the square root of the frequency of the waves was half the atomic mass of the metals. This was the proton number of the elements.

 

Atomic Structure

Subatomic particles

Atoms were considered to be the smallest particle a matter is composed of but this is not true. Atoms are composed of three subatomic particles; protons, neutrons and electrons. Electrons are negatively charged subatomic particles which are found in the shells of the atom. The neutrons and protons are found in the nucleus of the atoms. Protons have a positive charge while neutrons carry no charge. Since atoms are not charged particles, that means that the number of protons and electrons are same.

The mass of protons and neutrons are greater than electrons. If the mass of proton is 1 then the mass of electron is taken 0. Even though these two subatomic particles are different in masses they carry a same charge.

Neutrons make up the mass of an atom along with protons (not electrons since they are outside the nucleus). The only exception is hydrogen in which no neutron is found.

Proton number and Nucleon number

The number of protons found in a nucleus of the atom of an element, then that number is called its proton number/atomic number. Its symbol is Z.

The total mass of an atom build up by neutrons and protons make up the Nucleon number/Mass Number of the element. It is given the symbol A. 

The neutron number is the total number of neutrons in an atom of the element and is given the symbol N.

The atomic number and mass number can be written as a subscript by the side of the symbol of an element.

Ions are formed when

the atoms gain or lose electrons. Losing an electron causes the positive charge of the proton to overpower the negative charge causing the formation of cation while gaining electron forms an anion which is negatively charged due to the same principle.

Electronic Configuration

The electrons are found in the shells of the atom. There are 4 shells in total. The equation given to calculate the amount of electrons each shell can take is; 2n^2. “n” means the number of shells. The 1st shell only holds 2 electrons, which forms a duplet. The 2nd shell holds 8 electrons forming an octect. The 3rd can hold 18 and the fourth can hold up to 32. If these limits are crossed then the electrons are to be placed in the next shell. However calcium and potassium are free of such pattern because as their 3rd shells exceed 8 electrons it is placed in another shell.

The electronic configuration, is basically telling the amount of electrons in each shell in an element. This is given in a table by stating the number of electrons present with “,” in between without any space.

Isotopes

An element might have a constant number of protons and electrons but this is not true for neutrons. These neutrons make different masses of the same element causing the element to divide into smaller groups of the same element called isotopes. One example of isotope is chlorine. It has two masses. One type is 35 which is more common, exactly 75.77%. While the other is 37 which is 25% found. This gives rise to the two isotopes; chlorine-35 and chlorine-37. This is exactly the way every element is written to express its isotope, such as, carbon-12, carbon-13 etc.

Carbon-12 Scale

To ensure the consistency of an isotope, the isotope is compared to an ideal isotope which is normally carbon-12. This isotope is chosen because it is found in all the organic chemicals but not all the isotopes can be compared to this mass scale. So to ensure accuracy the isotopic mass of any element is compared to 1/12th that of the mass of carbon 12.

Relative atomic mass

The relative atomic mass shows the abundance of a mass and the mass of an atom. This is showed in the following equation:

mass1 x abundance/100 + mass2 x abundance/100

For chlorine we will use this equation. Using this equation tells us that the relative atomic mass of chlorine is 35.5. The equation could be altered just to cope with more isotopes of the same element.

Relative molecular mass

The relative molecular mass of an atom is the sum of the relative atomic masses of the atoms that build up the molecule. Atoms combine to form larger molecules of an element. To find the relative molecular mass we can just add the relative atomic masses of the atoms that make up the molecule:

Cl2 = 35.5 x 35.5 = 71

The above equation is the example of finding out the relative molecular mass of the chlorine molecule.

Relative Formula Mass

Ionic compounds are made of multiple elements so we use relative formula mass to find the mass of such compounds. Such as if you find the relative formula mass of NaCl (Sodium Chloride), you will find that it is 58.5 as Na (sodium)= 23 and Cl (chlorine) = 35.5 and 35.5 + 23 = 58.5.

Radioactive isotopes

Many isotopes are stable and will remain in the environment for several years. But there are some unstable or radioactive isotopes which break down into other elements. One example is carbon-14 which converts into nitrogen-14.