hydrogen and lithium
Elemental hydrogen and helium have atomic masses less than twice their atomic numbers.
The atomic mass of an isotope is the sum of protons, neutrons and electrons masses.
Johann Dobereiner observed that in his triads of elements, the atomic mass of the middle element was approximately the average of the atomic masses of the other two elements in the group. This observation suggested a relationship between the properties of elements and their atomic masses, laying foundational ideas for the development of the periodic table. His work highlighted an early attempt to categorize elements based on their similarities, influencing later chemists in their understanding of element relationships.
A German chemist Dobernier observed relationship between atomic masses of several groups of three elements called traids.In these groups the central or middle element had atomic mass average to the other two elements.
Atoms have atomic weight not molar mass. Any atom-gram has 6,022 141 29(27)×1023 (this is the number of Avogadro) atoms.
An element is made up of only one type of atom. Atoms are only different from each other due to their atomic number - which is the number of protons. Different elements may have different relative atomic masses, but it is the differing number of protons in each atom that determines which element it is.
An 'atom' is the smallest unit that ANY element can be divided up into. An 'atom' of silver is the smallest piece of silver that there can be, similarly an 'atom' of oxygen is the smallest bit of oxygen that can be. Thus all elements are made of 'atoms'.
No two elements may have the same atomic number. But two elements may have same atomic mass. Hence atomic number is better than atomic mass.
Cobalt and Nickel are the two elements from the first 20 elements of the periodic table that are not arranged in order of increasing atomic mass. This is due to the placement of cobalt with a slightly lower atomic mass compared to nickel, which occurs due to the meticulous arrangement of elements based on their chemical properties and not just atomic masses.
Strontium is the smallest atom among magnesium, calcium, barium, and strontium. This is because as you move down a group in the periodic table, atomic size increases due to the addition of more electron shells. Strontium is located lowest in the group 2 elements, so it has the smallest atomic size among the four elements listed.
Atomic is when two or more elements bond together. This is called atomic bonding.
Atomic mass and Atomic number