Isotopes- CIE iGCSE Chemistry Notes - New Syllabus
Isotopes for iGCSE Chemistry Notes
Core Syllabus
- Define isotopes as different atoms of the same element that have the same number of protons but different numbers of neutrons
- Interpret and use symbols for atoms, e.g. ${}^{12}\text{C}$, and ions, e.g. ${}^{35}_{17}\text{Cl}^{-}$
Supplement Syllabus
- State that isotopes of the same element have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration
- Calculate the relative atomic mass of an element from the relative masses and abundances of its isotopes
Isotopes and their Notations
Isotopes
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.
- Every atom of an element has the same number of protons (this defines the element).
- However, atoms of the same element can have different numbers of neutrons.
- Since neutrons affect the mass of the atom, isotopes of an element have different mass numbers.
Examples:
- Carbon-12: 6 protons, 6 neutrons – mass number = 12
- Carbon-14: 6 protons, 8 neutrons – mass number = 14
- These are both isotopes of carbon (same number of protons, different number of neutrons).
Interpreting Atom and Ion Symbols
Atoms and ions are often written using symbols that include the atomic number (proton number) and mass number (nucleon number), such as:
\( ^{12}_6\text{C} \) or \( ^{35}_{17}\text{Cl}^- \)
In this notation:
- The top number (superscript) is the mass number (A) – total number of protons and neutrons.
- The bottom number (subscript) is the proton number (Z) – number of protons in the nucleus.
- The element symbol (e.g. C, Cl) identifies the type of atom.
- If there’s a charge (e.g. Cl⁻), it indicates the atom is an ion (Cl⁻ has gained one electron).
Key Point: All isotopes of the same element have the same proton number but different mass numbers due to differing neutrons.
Example
Interpret the information in the atomic and ionic symbols:
- \( ^{24}_{12}\text{Mg} \)
- \( ^{35}_{17}\text{Cl}^- \)
▶️Answer/Explanation
1. \( ^{24}_{12}\text{Mg} \):
- Proton number = 12 → 12 protons
- Mass number = 24 → 24 – 12 = 12 neutrons
- Neutral atom → 12 electrons
2. \( ^{35}_{17}\text{Cl}^- \):
- Proton number = 17 → 17 protons
- Mass number = 35 → 35 – 17 = 18 neutrons
- Negative charge (Cl⁻) → gained 1 electron → 18 electrons
Chemical Properties of Isotopes
Isotopes of the same element have the same chemical properties because:
- They have the same number of protons.
- They have the same number of electrons.
- Chemical properties are determined by the electronic configuration of an atom, especially the number of electrons in the outer shell.
- Since isotopes have the same number of electrons and identical electron arrangements, they undergo the same types of chemical reactions.
The only difference between isotopes is in their number of neutrons, which affects their mass and physical properties (e.g. density or rate of diffusion), but not their chemical reactivity.
Physical Properties of Isotopes
Although isotopes of the same element have the same chemical properties, they may differ in their physical properties due to the difference in mass. Key physical properties that may differ:
- Mass: Heavier isotopes have more neutrons, so their atomic mass is greater.
- Density: Heavier isotopes are generally more dense than lighter ones.
- Melting and Boiling Points: There may be slight variations in the melting and boiling points between isotopes, especially for lighter elements (e.g. hydrogen isotopes).
- Rate of Diffusion: Lighter isotopes diffuse faster than heavier ones (according to Graham’s law of diffusion).
- Rate of Evaporation: Lighter isotopes evaporate slightly faster than heavier ones.
Note: These differences are more noticeable in elements with very light atoms (e.g. hydrogen) and become less significant in heavier elements.
Example
Chlorine exists as two isotopes: \( ^{35}_{17}\text{Cl} \) and \( ^{37}_{17}\text{Cl} \). Explain why these two isotopes of chlorine have the same chemical properties, even though they have different mass numbers.
▶️Answer/Explanation
Both isotopes have the same number of protons (17) and electrons (17), meaning they have the same electronic configuration (2,8,7). Since chemical properties depend on the arrangement of electrons, and not the number of neutrons, the two isotopes react chemically in the same way.
Relative Atomic Mass from Isotopes
The relative atomic mass (\( A_r \)) of an element is the average mass of the naturally occurring isotopes of the element, taking into account their relative abundances.
Formula:
\( A_r = \frac{(m_1 \times \%a_1) + (m_2 \times \%a_2) + \dots}{100} \)
Where:
\( m_1, m_2, \dots \) are the mass numbers of the isotopes
\( \%a_1, \%a_2, \dots \) are their respective percentage abundances
Relative atomic mass is not a whole number because it reflects an average of all isotopes.
Example
Calculate the relative atomic mass of chlorine, given the following isotopes and abundances:
- \( ^{35}\text{Cl} \): 75%
- \( ^{37}\text{Cl} \): 25%
▶️Answer/Explanation
Apply the formula:
\( A_r = \frac{(35 \times 75) + (37 \times 25)}{100} = \frac{2625 + 925}{100} = \frac{3550}{100} = 35.5 \)
So, the relative atomic mass of chlorine is \( A_r = 35.5 \).
Example
The element boron has two isotopes: \( ^{10}\text{B} \) (20%) and \( ^{11}\text{B} \) (80%). Calculate its relative atomic mass.
▶️Answer/Explanation
\( A_r = \frac{(10 \times 20) + (11 \times 80)}{100} = \frac{200 + 880}{100} = \frac{1080}{100} = 10.8 \)
The relative atomic mass of boron is \( A_r = 10.8 \).