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.

 

Materials and Bonding

Elements

Elements are the smallest substance a material is composed of, it cannot be broken down by physical techniques. There are 92 naturally occurring elements  found and there are some artificial ones as well. Elements are composed of atoms.

Atoms are the smallest part of a substance taking place in a chemical reaction. These atoms are further composed of three subatomic particles; neutron, proton, and electron.

The atoms may join to form larger molecules. Molecules are composed of 2 or more atoms bonded together. The number of atoms are stated by the atomic symbol of the element such as H2, O2, N2 etc.

Elements are written as a subscript of their name such as Hydrogen as H and Potassium as K. These are the symbols of their respective element. The symbols of metals are written in separate symbols even though the atoms of the metals do not exist as separate atoms.

Some atoms have a different symbol than their names such as sodium has the atomic symbol of Na, and potassium has the atomic symbol of K. This is because the atomic symbols are given after the Italian names of the elements.

Compounds and Mixtures

Compounds are pure substances made up of atoms of different elements. The compounds are not mixtures of elements, the compounds and mixtures have different properties.

Compounds cannot be separated by physical techniques while mixtures can. Compounds are also made by chemical processes compared to mixtures. Compounds are written as the combination of the atomic symbol of the elements such as Sodium Chloride has the atomic symbol NaCl. Mixtures are not given atomic symbols.

For example; if you mix iron filings with sulfur powder, this forms a mixture. Now this can be separated easily if you place a magnet over the mixture. The iron filings will separate from the sulfur powder. If the mixture is heated, it will soon turn red and then turn grey and form a compound called Iron Sulfide.

Forming Bonds

Bonds are formed between atoms to form compounds. Many atoms do not have full electronic configuration. To fill the shells with electrons the atoms tend to form bonds. This can be seen in the elements in Group VIII which are mostly gases. These gases are mainly nonreactive.

This is why they were first known as inert gases. But now many compounds have been found out of these gases and are now known as noble gases. This is because these gases have a full electronic configuration.

Due to having full electronic configuration these gases did not form any compound naturally as no bonding was needed. Although there is still no compound for helium and neon.

Bonding only involves the valence electrons in the outermost shell of the atom of an element while the outer/core electrons are not involved in bonding.

Ionic Bonding

Some elements gain a full electronic configuration by losing electrons and gaining electrons. The ionic bond is formed between an atom which wants to lose an electron and the atom which wants to gain them. The atom which loses the electrons gains a positive charge due to increased number of protons than electrons which produces a cation.

The atom that gains an electron has a negative charge because of increased number of electrons than protons. This forms an anion. These ions form a bond between themselves. This is the result of the electrostatic forces between the cation and the anion. This is why the ionic bonding is also known as electrovalent bonding.

Covalent Bonding

Covalent bonding is the sharing of electrons. Some elements form compounds this way mainly because the atoms can not produce sufficient energy for ionic bonding. This why these atoms form covalent bonds. These bonds are represented in words as Cl-Cl, O=O, N≡N etc. These bonds are a result of strong electrostatic forces between the valence electrons and the positively charge nuclei of the atom.

Formulae of Compounds

These bonds depend upon the number of electrons needed for a full electronic configuration. These bonds are also dependent upon the combining power of the atoms. The combining power is the amount of atoms an atom of an element can bond with. Such as magnesium has the combining power of 2 and will combine with 2 chlorine atoms to form magnesium chloride.

These bonds are mainly formed from between non metals or metals and nonmetals. Compounds cannot be formed between every element. In case a compounds has a metal in it, its atomic symbol will always be written first and the ending of the nonmetal will be written as “ide” such as oxygen will be written as oxide and chloride will be written as chloride.

Properties of Ionic Bonding

 Ionic compounds form a crystal lattice. This means that its structure is fixed in a definite shape. The ions have strong bonds between them which is why ionic compounds tend to have a very high melting and boiling point. The crystal lattice does not allow conductivity as a solid because the ions are not free to move. If the ionic compounds are melted or mixed in a solvent the ions can now move freely and therefore conduct electricity. The ionic compounds are also soluble in water but this does not mean that the soluble compounds are ionic compounds as many such compounds are not soluble in water.

Properties of covalent compounds

Covalent compounds mainly exist as small molecules of compounds. The covalent bonding has intra-molecular forces between them but these tend to have weaker inter-molecular bonds. As a consequence of this these bonds tend to have low boiling point and melting point. Covalent compounds do not have ions and therefore does not conduct electricity.

Many covalent compounds have larger macro-molecules which have large covalent bonds between them. Macromolecules have high melting and boiling points. Some examples are; Silicon dioxide, Silicon Carbide etc.

Diamond and Graphite

Diamond and graphite are allotropes of carbon. Allotropes are different structural forms composed of the same element in the same physical state. Diamonds are clear crystals which have covalent bonds between carbon atoms. Each carbon atom is bonded with 4 other carbon atom. Since every atom is set in bonding, there is no valence electron available to conduct electricity, thus it is an insulator. Other than that the diamond has a high refractive index causing the light to refract and brighten up.

Graphite is also composed of carbon. Each carbon atoms are bonded with 3 other carbon atoms. The atoms form a hexagon. There are valence electrons available in graphite causing it to be a semi conductor. Both diamond and graphite have a very high melting point but other wise each have different properties. Graphite has weak intermolecular bonds thus it can be used as a lubricant whether a solid or mixed in a solvent. It is also used in pencil which is not composed of lead at all. Graphite is comparatively softer material than diamond.

Metallic Bonding

Metallic bonds has a lattice of cations with a sea of electrons between them. The electrostatic forces between the valence electrons and the cations forms a very strong bond between the atoms. The valence electrons also result in the metal being a very good conductor.

Properties of metallic bonds

Metals have valence electrons between their bonds making it a good conductor of heat and electricity. The heat energy agitates the electrons which collide and transfer the heat energy to other ions. Metals are also malleable. The layers of ions tend to fall over each other as the metal is pulled apart.