Elements & Isotopes

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.

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

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