Atomic Number and Atomic Mass

Atomic Number

The chemical elements can be distinguished by the number of protrons in thier nucleus. Different sorts of atoms cannot share the same number of protons.
For example)
  1. Be has 4 protons in its nucleus, which also means an atom containing 4 protons must be Be.
  2. C has 6 protons in its nucleus, so an atom having 6 protons must be C.

Definition: The Atomic Number of an atom is the number of protons in the nucleus.
Atomic number = the # of protons
One proton has +1 charge. A nucleus is made of neutron s and protons, because neutrons do not have any charges and protons have + charges, nucleus has + charges as well.
An neutral atom has no overall charge, so the # of + charges and the # of - charges should be equal. Since an electron has a -1 charge, therefor:
the # of protons = the # of electrons.
In addition,
atomic number = the # of protons = the + charges in nucleus = the # of electrons = the - charges surrounding the necleus. ( This is only for neutral atoms.)
For example)
  1. The atomic number of F is 9 .
  2. There are 9 protons in one atom.
  3. An F atom has 9 positive charges in its necleus.
  4. There are 9 electrons in an F atom.
  5. There are 9 negative charges surrounding its nucleus.
The picture below shows that Ca has 20 protons and 20 electrons, then check with the periodic table its atomic number is also 20.
external image Ca.gif external image periodic-table.gif picture#2
If electrons are added in or taken away from a netral atom, a new partical with charges is produced called ion.

  • Adding an electron ( negative charge ) produces a negative ion.
  • Taking an electron away (negative charge) produces a positive ion
For an ion, the atomic number, the # of protons and the + charges in its nucleus do not change, but the # of electrons and the - charges do change depending on how many electrons are added in or taken away.
Such as:
Atomic symbol
Atomic number
the number of protons
+ charges
the number of electrons
- charges

Atomic Mass
The mass of an atom is calculated by adding the number of protons and the number of neutrons together. The electrons are too tiny and light to be considered as part of the mass of an atom.
The Atomic Mass of an atom is the sum of the # of protons and neutrons.

atomic number = # of protons
atomic mass = # of protons + # of neutrons
So: number of neutrons = atomic mass - atomic number

Look at the picture below, it shows both atomic mass and atomic number with the atomic symbol,

external image isotopes.jpgpicture#3

Accrounding to the picture, though the same kind of atoms may have different atomic mass
  1. of protons = 6
atomic mass = 12
#of neutrons = 12 - 6 = 6
The picture below also shows the same thing:
external image NM2_6Ani.gifpicture#4

  1. An element's or isotope's atomic number tells how many protons are in its atoms.
  2. An element's or isotope's mass number tells how many protons and neutrons in its atoms.