What Is an Element?
An element is a chemical atom with a fixed number of protons in its nuclei. It is a pure chemical substance, and we can not break it into more spartan substances. Proton is an element’s elucidate property that signifies an element’s atomic number. All atoms with the same atomic number are atoms of the same element.
The IUPAC (International Union of Pure and Applied Chemistry) has recognised 118 elements. Out of which, 94 occur naturally on earth, whereas 24 are synthesised in nuclear reactions. Hydrogen, helium, and lithium are some naturally occurring elements on the earth’s crust. In contrast, elements like promethium, astatine, neptunium, and plutonium are synthesised during nuclear reactions.
Helium
Helium is a chemical element with the symbol He and atomic number 2 in the periodic table. It is the second most abundant element in the universe and is present in all stars. Helium is colourless, odourless, inert, non-toxic, and the periodic table’s first element of group 18.
Helium has two comprehended stable isotopes – 2He3 and 2He4. 2He4 is utmostly found in the earth’s atmosphere, whereas 2He3 is significantly limited. This difference in quantity can be observed in the earth’s atmosphere, where the proportion of 2He3 atoms to 2He4 atoms is roughly 1:1000000.
Physical Properties of Helium
| Name of Element | Helium |
| Chemical Symbol | He |
| Boiling Point | 268.928 oC or 4.222 K |
| Melting Point | -272.2 oC or 0.95 K |
| Density | 0.145 gcm-3 at its melting point or 0.1786 gL-1 at STP |
| Critical Temperature | 5.195 K |
| Pressure | 0.227 MPa |
| Appearance (at STP) | Colourless gas |
| Triple Point (Temperature) | 2.177 K |
| Triple Point (Pressure) | 5.043 kPa |
Chemical Properties of Helium
| Name of Element | Helium |
| Electron Configuration | 1 s2 |
| First Ionisation Enthalpy | 2372.3 kJmol-1 |
| Second Ionisation Enthalpy | 5250.5 kJmol-1 |
| Enthalpy of Fusion | 0.0138 kJmol-1 |
| Van der Waals Radius | 140 picometers |
Promethium
Promethium is a chemical element with the symbol Pm and atomic number 61 in the periodic table. Chemically, promethium is a lanthanide and shows one stable oxidation state of +3. All its isotopes are radioactive and are widely used in laboratories.
Promethium does not emit gamma rays and has a minimum penetration depth and a prolonged half-life. It is commonly synthesised by bombarding uranium with thermal neutrons.
Physical Properties of Promethium
| Name of Element | Promethium |
| Chemical Symbol | Pm |
| Boiling Point | 3273 K, 3000 °C, 5432 °F |
| Melting Point | 1315 K, 1042 °C, 1908 °F |
| Density | 7.26 gcm-3 |
| Appearance at 20 °C | Solid |
| Relative Atomic Mass | 145 g |
| Atomic Number | 61 |
| Discovered by | Mariinsky in 1945 |
Chemical Properties of Promethium
| Name of Element | Promethium |
| Electron Configuration | [Xe], 4 f5, 6 s2 |
| Total Number of Isotopes | 18 |
| Stable Isotopes | 61Pm145, 61Pm147 |
| Oxidation State | +3 |
Promethium belongs to the cerium group of lanthanides. It is chemically comparable to the neighbouring elements, shows an oxidation state of +3 and forms pink or red-coloured compounds. However, the chemical introspection of promethium is incomplete due to its instability.
