뒤로Study Notes: Atomic Structure, Isotopes, and Atomic Mass Calculations
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Atoms, Molecules, and Ions
Atomic Structure and Isotopes
Atoms are the fundamental building blocks of matter, consisting of protons, neutrons, and electrons. Isotopes are atoms of the same element that have different numbers of neutrons, resulting in different mass numbers.
Atomic Number (Z): The number of protons in the nucleus of an atom. It defines the element.
Mass Number (A): The total number of protons and neutrons in an atom's nucleus.
Isotopes: Atoms of the same element (same Z) with different numbers of neutrons (different A).
Example: Carbon-12 and Carbon-14 are isotopes of carbon. Both have 6 protons, but Carbon-12 has 6 neutrons, while Carbon-14 has 8 neutrons.
Symbols for Isotopes
Isotopes are represented using the following notation:
General Format: AZX, where X is the chemical symbol, A is the mass number, and Z is the atomic number.
Example: 2311Na represents a sodium atom with 11 protons and 12 neutrons.
Calculating Atomic Mass
The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes. This calculation takes into account both the mass and the relative abundance of each isotope.
Formula:
Example: If an element has two isotopes: 70% with mass 10.0 amu and 30% with mass 11.0 amu, the atomic mass is:
Tabular Data: Isotopic Abundances and Atomic Masses
The following table summarizes isotopic data for several elements, showing the number of protons, neutrons, and electrons, as well as the atomic mass calculation based on isotopic abundances.
Isotope | Number of Protons | Number of Neutrons | Number of Electrons | Isotopic Mass (amu) | Fractional Abundance |
|---|---|---|---|---|---|
10B | 5 | 5 | 5 | 10.0129 | 0.199 |
11B | 5 | 6 | 5 | 11.0093 | 0.801 |
23Na | 11 | 12 | 11 | 22.9898 | 1.000 |
24Mg | 12 | 12 | 12 | 23.9850 | 0.7899 |
25Mg | 12 | 13 | 12 | 24.9858 | 0.1000 |
26Mg | 12 | 14 | 12 | 25.9826 | 0.1101 |
Purpose: This table allows calculation of the average atomic mass for each element by multiplying each isotope's mass by its fractional abundance and summing the results.
Worked Example: Calculating the Atomic Mass of Boron
Given: Boron has two naturally occurring isotopes: 10B (19.9% abundance, mass = 10.0129 amu) and 11B (80.1% abundance, mass = 11.0093 amu).
Calculation:
Conclusion: The atomic mass of boron is approximately 10.81 amu.
Summary Table: Isotope Properties
The table above can be used to compare the properties of isotopes, such as the number of protons, neutrons, and electrons, as well as their contributions to the atomic mass of the element.
