Defining Terminology
Atomic Number
- The atomic number (or proton number) is the number of protons in the nucleus of an atom
- The symbol for this number is Z
- It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table
- The proton number is unique to each element, so no two elements have the same number of protons
- Electrons can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction
Mass Number
- The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom
- The symbol for this number is A
- The mass number minus the proton number gives you the number of neutrons of an atom
- Note that protons and neutrons can collectively be called nucleons
- The atomic number and mass number for every element is on the Periodic Table
Diagram showing the notation used on the Periodic Table
Isotopes
- Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
- The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number
- So, C-14 is the isotope of carbon which contains 6 protons and 6 electrons, but the 14 signifies that it has 8 neutrons (14 - 6 = 8)
- It can also be written as 14C
- Isotopes display the same chemical characteristics
- This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry
- The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only
The Atomic Structure and Symbols of the Three Isotopes of Hydrogen
Ions
- An ion is an atom or group of atoms that has an electrical charge, either positive and negative
- Atoms have an equal number of protons and electrons and so do not have an overall charge
- Atoms with incomplete outer electron shells are unstable
- By either gaining or losing electrons, atoms can obtain full outer electron shells and become stable
- When this happens, atoms have an unequal number of protons and electrons and so have an overall charge.
- This is how atoms become ions
- An atom that loses electrons has more protons than electrons and so has a positive overall charge
- This is called a positive ion or cation
- An atom that gains electrons has more electrons than protons and so has a negative overall charge
- This is called a negative ion or anion