[Chemistry Class Notes] on Element Definitions Pdf for Exam

Element definition in chemistry is “Any pure substance that is made entirely of one type of atom and cannot be broken by chemical means”. It has a set number of protons in it and changing the number of protons of an element changes the type of element entirely. The number of protons in the nuclei is unique for each element and is known as its atomic number. There are around 118 known elements to date. Since elements are made up of atoms, let us first see some basic terms that you will need to know to understand elements.

• Proton: It is a subatomic (contained inside the atom) particle that has a positive charge which is equal in size to an electron and has a mass that is 1836 times the mass of an electron.

• Electron: This is a negatively charged subatomic particle which is the most stable and lightest subatomic particle known. It carries a negative charge of 1.60217 x 10-19 coulomb (which is the basic unit of electric charge)

• Neutron: It is a neutral subatomic particle that is present in all atomic nucleus except hydrogen atoms. It has absolutely zero electric charges and mass is the same as that of a proton.

• Atoms: This is the smallest unit of matter, it consists of protons, electrons, and neutrons. All solids, liquids, gas, and plasma (a special type of state of matter) is made from neutral or ionized atoms. These atoms are very small (around 100 picometers). All atoms are composed of a nucleus and one or more electrons which is bound to the atom’s nucleus. The nucleus of the atom is made up of protons and neutrons.

• Atomic Number: (Z) This is the number of protons that are present in the nucleus of an atom. Each element has a unique atomic number that helps us to also identify them when we discover such elements.

• Mass Number: (A) This number represents the number of protons and neutrons present in the nucleus of an atom. When the same element has a different mass number (the number of neutrons differs), it is called an isotope.

• Periodic Table: The periodic table of elements have the various chemical elements arranged according to the similarity in properties. There are seven rows (with metals on the left and nonmetals on the right) and 18 groups in the periodic table. The Russian chemist Dmitri Mendeleev published the first periodic table in 1869 which was recognized widely, but these consisted of only the elements that were discovered at the time. The modern periodic table includes all the elements discovered to date and is arranged mainly based on atomic number.

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How Were Elements Discovered?

Elements like carbon, sulfur, iron, tin, lead, copper, mercury, silver, and gold were already known to humans. They made things like weapons, shelter, fuel, etc with these elements. Gradually elements like arsenic, antimony, bismuth, and zinc also came to be known. In the span of the next 200 years, about 65 more elements were discovered. These included very essential elements like oxygen, silicon, iodine, calcium, magnesium, etc. 

Geochemical Classification of Elements

Elements can be classified into various categories. This is used in the geochemical analysis. The classification developed by V.M. Goldschmidt that depended on the affinity of elements to form different compounds was one of the most important classification schemes relevant to this aspect of chemistry. The elements distribute themselves in the earth’s major geochemical reservoirs such as core, mantle, crust, hydrosphere, and atmosphere. They were classified into four groups:

• Lithophiles: (rock-loving) These consist mainly of silicate minerals which include cations that commonly form oxides. They are difficult to reduce and have a lot of compounds that are formed from the reaction with oxygen. Most of these elements are found in rock structures, mainly the crust and mantle of the earth. The elements that fall into this category are: Ca, Mg, Mn, Ti, Na, K, U, Th, Si, and Fe.

• Chalcophile: These elements have a low affinity for oxygen and form better bonds with sulfur to form sulfide type minerals and highly insoluble sulfides. Elements: Cu, Pb, Zn, Cd, Mo, Hg, Sb, Sn, Tl, Te, As, etc.

• Siderophile: (iron-loving) These elements are found in the metallic core of the earth in the form of alloys with Fe. Elements exhibiting metallic behavior include the noble metals (Pt, Pd, Ir, Ru, Rh, and Os) as well as W, Ni, and Co fall under this category.

• Atmophile: (gas-loving) These elements readily form volatile (evaporates easily) compounds at exceptionally low temperatures. Elements are H, C, N, and noble gases like He, Ne, Ar, Kr, Xe, Rn.

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