Isotopes | Chemistry for Grade 11 (IGCSE) PDF Download

Defining Isotopes

• Isotopes are variations of an element with the same number of protons but different numbers of neutrons.
• The symbol for an isotope consists of the chemical symbol followed by a dash and the mass number.
• For example, C-14 (carbon-14) is an isotope of carbon with 6 protons, 6 electrons, and 8 neutrons (14 - 6) = 8 neutrons
• It can also be written as ¹⁴C or ¹⁴₆C

Why Isotopes Share Properties

• Isotopes of the same element exhibit similar chemical behaviors.
• This similarity arises from having identical numbers of electrons in their outer shells, leading to the same electronic configuration, which fundamentally influences an atom's chemistry.
• The distinguishing factor between isotopes lies in the varying neutron count within the nucleus, impacting mass exclusively.
• Differences in mass contribute to distinct physical attributes like density, boiling point, and melting point.
• Despite different masses, isotopes appear indistinguishable externally; for instance, distinguishing C-14 from C-12 is visually impossible.
• Deuterium oxide, known as 'heavy' water due to its higher relative atomic mass of 20 compared to 18 for regular water, mirrors the appearance, taste, and feel of normal water. However, it's hazardous for consumption as it disrupts cellular biochemical reactions.

Calculating Relative Atomic Mass

Relative Atomic Mass

• The symbol for the relative atomic mass is denoted as Ar.
• In the Periodic Table, you can find the relative atomic mass for each element alongside the atomic number.
• While atoms are too tiny to be weighed accurately, scientists needed a method to compare their masses.
• Carbon-12 serves as the standard atom with a fixed mass of 12 units, against which other atom masses are compared.
• Relative atomic mass (Ar) is defined as the average mass of an element's isotopes compared to 1/12th of the mass of a carbon-12 atom.
• The relative atomic mass of Carbon is 12.
  • The relative atomic mass of Magnesium is 24, indicating it is twice as heavy as Carbon.
  • The relative atomic mass of Hydrogen is 1, meaning it has one-twelfth the mass of a Carbon-12 atom.
• The relative atomic mass of an element can be determined using the mass number and the relative abundances of its isotopes. This calculation can be done with the following formula:
  
• The top line of the equation can be extended to include the number of different isotopes of a particular element present.

Is mass number and relative atomic mass the same thing?

• Look at the Periodic Table during your exam to observe lithium's relative atomic mass of 7.
• The relative atomic mass differs from the mass number as it is a rounded figure that accounts for isotopes.
• It represents the average mass of all isotopes for a specific element.
• For simplicity, relative atomic masses are commonly rounded to the nearest whole number.

The relative atomic mass (RAM) of lithium is 6.94 when rounded to two decimal places. However, when rounded to the nearest whole number, the RAM is 7, which aligns with the mass number of this specific isotope of lithium.