1. Name and describe the three subatomic particles in an atom.
2. Determine the number of protons, neutrons, and electrons in an atom or ion.
3. Define isotope and atomic mass.
proton neutron atomic mass unit atomic number ion mass number isotope atomic mass
Atoms are made of even smaller subatomic particles called protons, neutrons, and electrons.
The atomic numbers indicated on the periodic table indicate the number of protons in the nucleus.
Whenever an atom gains or looses electrons it becomes an ion. You can find the net charge of the atom by subtracting the number of electrons from the atomic number.
Most elements in the first two rows of the periodic table have at least two known isotopes.
Example: hydrogen-1 hydrogen-2 hydrogen-3
(mass #) 37Cl1- (charge)
(atomic #) 17
The atomic mass of an atom is expressed in atomic mass units (amu). This unit is derived from the carbon atom and is measured to be 1/12 of the mass of the carbon-12 atom. Mass of an atom is found by adding the protons and neutrons of an atom. Since most atoms exist in isotopes with known ratios and neutrons do not have the exact weight as protons, the mass number expressed on the periodic table will usually not agree exactly with the amu number.
Determining the amu in atoms of multiple isotopes:
Look at the three common isotopes of silicon. Multiply the masses of the isotopes by their fractional abundances (percent found in nature) and add the products together.
Last modified: September 19, 2003