Isotopes
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge AS Level Physics 9702
What are Isotopes? Not all atoms of the same element are identical. Isotopes are atoms of the same element that have different numbers of neutrons. Isotopes Atoms of the same element that have the same number of protons but different numbers of neutrons.
They have the same atomic number but different mass numbers. Key Features of Isotopes Same element: All isotopes of an element have the same number of protons (same atomic number) Different neutrons: Isotopes have different numbers of neutrons Different mass numbers: Because they have different numbers of neutrons, isotopes have different mass numbers Same chemical properties: Isotopes have identical chemical behavior because they have the same number of electrons Different physical properties: Isotopes may have slightly different physical properties (e.g., density, melting point) Remember The number of protons determines the element.
Changing the number of neutrons creates isotopes of the same element, not a different element. Examples of Isotopes Carbon Isotopes Carbon has three naturally occurring isotopes: Carbon-12 ( 12 ₆C): 6 protons, 6 neutrons, 6 electrons Carbon-13 ( 13 ₆C): 6 protons, 7 neutrons, 6 electrons Carbon-14 ( 14 ₆C): 6 protons, 8 neutrons, 6 electrons (radioactive) All three are carbon because they all have 6 protons.
They differ only in the number of neutrons. Chlorine Isotopes Chlorine exists naturally as two main isotopes: Chlorine-35 ( 35 ₁₇Cl): 17 protons, 18 neutrons (75% abundance) Chlorine-37 ( 37 ₁₇Cl): 17 protons, 20 neutrons (25% abundance) Both have 17 protons (so both are chlorine), but different numbers of neutrons.
Hydrogen Isotopes Hydrogen has three isotopes with special names: Protium ( 1 ₁H): 1 proton, 0 neutrons (99.98% abundance) - ordinary hydrogen Deuterium ( 2 ₁H or D): 1 proton, 1 neutron (0.02% abundance) - heavy hydrogen Tritium ( 3 ₁H or T): 1 proton, 2 neutrons (trace amounts, radioactive) Isotope Notation Isotopes are written using standard notation: Mass number ₍Atomic number₎ Element Symbol or simply: Element name - Mass number Examples: 12 ₆C or Carbon-12 Isotope Protons Neutrons Electrons Mass Number 12 ₆C 6 6 6 12 13 ₆C 6 7 6 13 14 ₆C 6 8 6 14 Why Do Isotopes Have the Same Chemical Properties?
Chemical properties depend on the number and arrangement of electrons All isotopes of an element have the same number of electrons Therefore, isotopes have the same electronic configuration This means they react in the same way chemically Key Concept Neutrons don't affect chemical behavior because they're in the nucleus, not involved in bonding.
Electrons (which are the same for all isotopes of an element) determine chemical properties. Physical Property Differences While isotopes have the same chemical properties, they may have slightly different physical properties: Mass/Density: Heavier isotopes have greater mass and density Rate of diffusion: Lighter isotopes diffuse faster Melting/boiling points: May be slightly different Radioactivity: Some isotopes are radioactive, others are stable Relative Atomic Mass and Isotopes Most elements exist as mixtures of isotopes.
The relative atomic mass is the weighted average of all isotopes. Relative Atomic Mass (Ar) The weighted average mass of all isotopes of an element, taking into account their relative abundances. Calculating Relative Atomic Mass Ar = Σ (isotope mass × % abundance) / 100 Example: Chlorine Chlorine exists as two isotopes: Cl-35: mass = 35, abundance = 75% Cl-37: mass = 37, abundance = 25% Calculation: Ar = (35 × 75) + (37 × 25) / 100 Ar = (2625 + 925) / 100 Ar = 3550 / 100 = 35.5 This is why chlorine's relative atomic mass is 35.5, not a whole number!
Important Relative atomic masses are not whole numbers because they're averages of different isotopes. Elements with only one stable isotope (like fluorine) have relative atomic masses very close to whole numbers.
Radioactive Isotopes Some isotopes are unstable and undergo radioactive decay. Radioisotope An unstable isotope that spontaneously breaks down, emitting radiation in the process. Examples of Radioactive Isotopes Carbon-14: Used in radiocarbon dating of ancient objects Uranium-235 & Uranium-238: Used in nuclear power and weapons Iodine-131: Used to treat thyroid conditions Cobalt-60: Used to sterilize medical equipment Tritium (H-3): Used in glow-in-the-dark watches Uses of Isotopes In Medicine Diagnosis: Radioactive tracers to detect diseases Treatment: Radiotherapy to kill cancer cells Sterilization: Gamma radiation to sterilize medical instruments In Industry Thickness control: Monitoring paper or metal sheet thickness Leak detection: Finding leaks in pipelines Carbon dating: Determining age of archaeological specimens In Research Tracers: Following chemical reactions Dating: Determining age of rocks and fossils Biochemistry: Studying metabolic pathways Mass Spectrometry Mass spectrometry is a technique used to identify and measure the relative abundance of isotopes in a sample.
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