S1.2 The Nuclear Atom
1.2.1: Structure of The Atom
The atom consists of a nucleus (containing protons and neutrons) surrounded by electrons in orbitals. These observations were deduced from Ernest Rutherford’s experiment in 1911 where: Rutherford shot positive alpha particles through a gold foil and made three important observations:
1. Most alpha particles passed through → Atom are mostly empty space.
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2. Some were deflected → Nucleus is positively charged.
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3. A few bounced back → Nucleus is very small, dense, and contains most of the atom’s mass.
Atomic Number & The Nuclear Symbol
X
A
Z
Charge
X: Element
A: Mass Number: Number of Protons & Neutrons
Z: Atomic Number: Number of Protons
Charge: Number of gained/lost electrons.
Example: Magnesium Ion
Mg
24
12
-2
1.2.2: Isotopes
Isotopes are atoms of the same element that have the same number of protons (same atomic number) but a different number of neutrons, leading to different mass numbers.
Properties:
- Chemical properties: Similar (same number of electrons).
- Physical properties: Different (e.g., boiling/melting points, density).
Relative Atomic Mass ( Aáµ£ )
Since elements exist as mixtures of isotopes, we calculate their atomic mass as a weighted average of the isotopes based on their natural abundance.
Relative Atomic Mass = (Isotope1 x % Abundance)+(Isotope1 x % Abundance) ...
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100​
Example:
For example, chlorine has two main isotopes: Cl-35 and Cl-37, with abundances of about 75% and 25%, respectively. Its relative atomic mass is
Relative Atomic Mass = (35 x 75%) +(37 x 25% )
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100​
= 35.5
1.2.3: Mass Spectrometry
A mass spectrometer is the tool used to detect the relative abundance of isotopes
Stages:
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#1-Vaporization: a sample is heated at low pressure to vaporize.
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​#2-Ionization: high-energy electrons bombard the sample, knocking off electrons to form positive ions.
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#3-Acceleration: Ions are accelerated by an electric field.
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#4-Deflection: Ions are deflected by a magnetic field depending on their mass-to-charge ratio (m/z).
#5-Detection: Ions are detected. Lighter ions or those with higher charge are deflected more. A mass spectrum is generated by a computer.​​​
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