Mass Number
Specification Point 1.7:
Recall the meaning of the term mass number of an atom.
- 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 nucleon 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.
Atomic Number
Specification Point 1.8:
Describe atoms of a given element as having the same number of protons in the nucleus and that this number is unique to that element.
- 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 and no two elements have the same number of protons.
- Electrons come and go during chemical processes but the proton number doesn’t change.
Diagram showing the notation used on the Periodic Table
Isotopes
Specification Point 1.9:
Describe isotopes as different atoms of the same element containing the same number of protons but different numbers of neutrons in their nuclei.
- Isotopes are different 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, 6 electrons and 14 – 6 = 8 neutrons.
- Since isotopes of the same element have the same number of electrons, their chemical properties are identical.
The atomic structure and symbols of the three isotopes of hydrogen
Author: Morgan
Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.