Electrons are only found outside the nucleus. We can say that the electron has a negligible mass because it is ~2000 times less heavy than both the proton and neutron. The atom is neutral therefore the number of protons is equal to the number of electrons which in turn is equal to the atomic number of atom. The Atomic Number is the number of protons (equal to the number of electrons). The Mass Number is the number of protons + neutrons. Neutrons do not change the way that an element behaves chemically. Different number of protons and electrons makes a different element; one which
behaves in a different way chemically.
Elements are typically represented in the following way:
A
X
Z
Where A is the Mass Number, Z the Atomic Number and X the symbol.
E.g.
23
Na
11
This sodium atom contains 11 protons, 11 electrons and 23 – 11 = 12 neutrons.
Isotopes
Many elements have more than one type of atom. These atoms have the same number of protons but different numbers of neutrons. Atoms having different mass numbers but identical atomic numbers are called isotopes.
E.g. 35Cl and 37Cl
Atomic scale
Neutrons, protons (and electrons) have a definite mass, so atoms must have mass. It is convenient to define an atomic mass unit (written as amu or, simply, u) where 1u = 1.6605 x 10-24 g. Th e atomic mass unit is defined with relation to a standard scale. It was originally defined as being equal to the mass of one H atom, but now a more precise scale is used - the carbon 12 scale. On this scale one atom of carbon 12 is assigned a mass of exactly 12u. 1u is exactly 1/12 of the mass of one atom of carbon isotope 12.
