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2. Atomic mass - Wikipedia

   en.wikipedia.org/wiki/Atomic_mass

   The atomic mass (ma or m) is the mass of an atom. Although the SI unit of mass is the kilogram (symbol: kg), atomic mass is often expressed in the non-SI unit dalton (symbol: Da) – equivalently, unified atomic mass unit (u). 1 Da is defined as 1⁄12 of the mass of a free carbon-12 atom at rest in its ground state. [1]

3. List of physical constants - Wikipedia

   en.wikipedia.org/wiki/List_of_physical_constants

   For example, the atomic mass constant is exactly known when expressed using the dalton (its value is exactly 1 Da), but the kilogram is not exactly known when using these units, the opposite of when expressing the same quantities using the kilogram.

4. Periodic table - Wikipedia

   en.wikipedia.org/wiki/Periodic_table

   The periodic table, also known as the periodic table of the elements, is an ordered arrangement of the chemical elements into rows (" periods ") and columns (" groups "). It is an icon of chemistry and is widely used in physics and other sciences. It is a depiction of the periodic law, which states that when the elements are arranged in order ...

5. List of chemical elements - Wikipedia

   en.wikipedia.org/wiki/List_of_chemical_elements

   List of chemical elements - Wikipedia. List of chemical elements. 118 chemical elements have been identified and named officially by IUPAC. A chemical element, often simply called an element, is a type of atom which has a specific number of protons in its atomic nucleus (i.e., a specific atomic number, or Z). [ 1 ]

6. Chemical element - Wikipedia

   en.wikipedia.org/wiki/Chemical_element

   However, the relative atomic mass of each isotope is quite close to its mass number (always within 1%). The only isotope whose atomic mass is exactly a natural number is 12 C, which has a mass of 12 Da; because the dalton is defined as 1/12 of the mass of a free neutral carbon-12 atom in the ground state.

7. Atom - Wikipedia

   en.wikipedia.org/wiki/Atom

   Hydrogen-1 (the lightest isotope of hydrogen which is also the nuclide with the lowest mass) has an atomic weight of 1.007825 Da. [62] The value of this number is called the atomic mass. A given atom has an atomic mass approximately equal (within 1%) to its mass number times the atomic mass unit (for example the mass of a nitrogen-14 is roughly ...

8. Mass number - Wikipedia

   en.wikipedia.org/wiki/Mass_number

   For 12 C, the isotopic mass is exactly 12, since the atomic mass unit is defined as 1/12 of the mass of 12 C. For other isotopes, the isotopic mass is usually within 0.1 u of the mass number. For example, 35 Cl (17 protons and 18 neutrons) has a mass number of 35 and an isotopic mass of 34.96885. [7]

9. Helium - Wikipedia

   en.wikipedia.org/wiki/Helium

   Helium. hexagonal close-packed (hcp) Helium (from Greek: ἥλιος, romanized: helios, lit. 'sun') is a chemical element; it has symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table.