Atomic Number:
An atom is named after the number of protons in it's nucleus. If the nucleus has 1 proton, it is hydrogen; if it has 2 protons, it is helium; if it has 3, it is lithium etc. The number of protons in the nucleus of an atom is called the atomic number. It has the symbol Z.
Mass Number:
Not all atoms of the same element have equal numbers of neutrons; this may vary slightly. The sum of the number of protons and neutrons in the nucleus of an atom is called its mass number. It is represented by the symbol A.
Isotopes:
The nucleus of an atom can thus be completely described by its mass number and its atomic number.
Atoms with the same atomic number but with different mass numbers (i.e. different numbers of neutrons) are called isotopes.
Atoms with the same atomic number but with different mass numbers (i.e. different numbers of neutrons) are called isotopes.
Examples:
Magnesium (atomic number 12) has 3 naturally occurring isotopes:
Mg-12 (Mr=24): 12 protons, 12 neutrons
Mg-12 (Mr=25): 12 protons, 13 neutrons
Mg-12 (Mr=26): 12 protons, 14 neutrons
Magnesium (atomic number 12) has 3 naturally occurring isotopes:
Mg-12 (Mr=24): 12 protons, 12 neutrons
Mg-12 (Mr=25): 12 protons, 13 neutrons
Mg-12 (Mr=26): 12 protons, 14 neutrons
Ions:
In a neutral atom, the number of protons and electrons are the same. However, many elements do not exist as neutral atoms, but they exist as ions. Ions are species in which the proton and electron numbers are not the same, and hence have an overall positive or negative charge. The number of electrons in a species can be deduced from its charge.
Examples:
Mg-12 (2+): 12p, 12n, 10e
Mg-12 (+): 12p, 12n, 11e
Mg-12 (no charge): 12p, 12n, 12e
Mg-12 (-): 12p, 12n, 13e
Hence:
Ions with a positive charge are called CATIONS.
Ions with a negative charge are called ANIONS.
Mg-12 (2+): 12p, 12n, 10e
Mg-12 (+): 12p, 12n, 11e
Mg-12 (no charge): 12p, 12n, 12e
Mg-12 (-): 12p, 12n, 13e
Hence:
Ions with a positive charge are called CATIONS.
Ions with a negative charge are called ANIONS.