Atoms and Molecules

DALTON’S SYMBOLS OF ELEMENTS:

• Dalton’s was the first scientist to use the symbols to represent elements in a short way.

• Berzilius suggested that the symbols of elements be made from one or two letters of the name of the element.
• The first letter of symbol is always written as a capital letter and the second letter as a small letter.

Symbols Derived from English Names of the Elements

Symbols Derived from Latin Names of the Elements

ATOMIC MASS OF AN ELEMENT

• • Matter is made up of atoms that have certain amount of mass.
  • Atomic mass is the mass of an atomic particle and is commonly expressed as per the international agreement in terms of a unified atomic mass unit (amu).

• Carbonatom has been assigned an atomic mass of exactly 12 atomic mass units.

Or

• The atomic masses of all other elements are determined by comparing the mass of their atom with the mass of a
  ${\mathrm{carbon}}^{–12} \mathrm{atom}$
• The atomic masses of elements vary from 008 amu for hydrogen up to 250 amu for elementswhich have a very high atomic number.
• The mass of an atom is the sum of mass of protons and neutrons which is almost equal to the atomic mass.
• The small change is due to the binding energy mass loss.

• The atomic weight of an atom is a dimensionless number when it is divided by unified atomic weight or Daltons giving the relative isotropic mass.
• The addition of average atomic mass of each atom in the molecule gives the mass of molecules.

Atomic masses of a few elements

HOW DO ATOMS EXIST?

·         Atoms of most of the elements are chemically very reactive and so they do not exist in Free State

 H   However, atoms of few elements called noble gases like helium, neon, argon and krypton, etc. are chemically unreactive and exists in the free state.