1. States of Matter. The Atom and the Periodic Table
1.5. Atomic Number, Mass Number, and Atomic Mass
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Atomic number
Each element has its own number: an atom number.
- The number of protons in the nucleus decides which element it is.
- The atom number = the number of protons.
Examples
| Element | Atomic number | Number of p+ |
| Hydrogen (H) | 1 | 1 |
| Helium (He) | 2 | 2 |
| Carbon (C) | 6 | 6 |
| Chlorine (Cl) | 17 | 17 |
Mass number
Every element (except hydrogen) also contains neutrons.
- Mass number = number of protons + number of neutrons
- \(A = Z + N\)
Two definitions
- Nuclide: An atomic species with a certain number of p+ and n.
- Isotopes: Different nuclides of the same element
The universe's most simple nuclide.
Some isotopes and their mass numbers
| Isotope | Nuclide | Atomic nr. | p+ | n | mass number |
| Hydrogen |
\({\sf ^1_1\text{H}}\)
|
1 | 1 | 0 | 1 |
| Deuterium | \({\sf ^2_1\text{H}}\) | 1 | 1 | 1 | 2 |
| Tritium |
\({\sf ^3_1\text{H}}\)
|
1 | 1 | 2 | 3 |
| Helium |
\({\sf ^4_2\text{He}}\)
|
2 | 2 | 2 | 4 |
| Carbon-12 |
\({\sf ^{12}_{\text{ }\text{ }6}\text{C}}\)
|
6 | 6 | 6 | 12 |
| Carbon-13 |
\({\sf ^{13}_{\text{ }\text{ }6}\text{C}}\)
|
6 | 6 | 7 | 13 |
| Carbon-14 |
\({\sf ^{14}_{\text{ }\text{ }6}\text{C}}\)
|
6 | 6 | 8 | 14 |
| Chlorine-35 |
\({\sf ^{35}_{17}\text{Cl}}\)
|
17 | 17 | 18 | 35 |
| Chlorine-37 |
\({\sf ^{37}_{17}\text{Cl}}\)
|
17 | 17 | 20 | 37 |
Atomic mass
A single carbon-12 atom weighs 1.994✕10–23 g
- Cumbersome figure!
The unified atomic mass unit ”u” (sometimes "amu") has been defined:
- A single carbon-12 atom weighs exactly 12u:
- \(m_{^{12}_{\text{ }\text{ }6}\text{C}}\equiv 12\text{u}\)
Also note: 1u = 1Da
How much do the building blocks of the atom weigh?
- 1 proton weighs approx. 1 u
- 1 neutron weighs approx. 1 u
- 1 electron weighs approx. 1/1800 u
The atomic mass of chlorine
- The atomic mass of chlorine: 35.453u ≈ 35.5u
- There are two chlorine isotopes: chlorine-35 (75 %) and chlorine-37 (25 %)
- The average atomic mass of chlorine:
\[0.75 \times 35\text{u} + 0.25 \times 37\text{u} = 35.5\text{u}\]
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Contents
- 1. States of Matter. The Atom and the Periodic Table
- 1.1. Matter. States of Matter
- 1.2. Elements and Chemical Compounds. Pure Substances and Mixtures
- 1.3. The Birth of Chemistry
- 1.4. Atomic Theory. The Atomic Model
- 1.5. Atomic Number, Mass Number, and Atomic Mass
- 1.6. Electron Configurations
- 1.7. Beyond Bohr's Atomic Model
- 1.8. Redox Reactions
- 1.9. The Structure of the Periodic Table
- 1.10. The Noble Gases
- 1.11. The Alkali Metals and the Halogens
- 1.12. The Alkaline Earth Metals and the Oxygen Group
- 1.13. A Few of the Elements in Group 13, 14, and 15
- 2. Chemical Calculations
- 2.1. Physical Quantity, Magnitude, and Units
- 2.2. Atomic Mass, Molecular Mass, and Unit Mass
- 2.3. Amount of Substance, Molar Mass, and Mass
- 2.4. Stoichiometry. The Mass is Preserved
- 2.5. Water of Crystallization
- 2.6. Calculating the Formula of a Chemical Compound
- 2.7. From Empirical to Molecular Formulas
- 2.8. Equivalent Amounts of Substance and Masses
- 2.9. Gases and Pressure
- 2.10. Concentrations
- 2.11. Dilutions
- 2.12. Yield
- 2.13. Limiting Reactants
- 3. Chemical Bonding
- 3.1. How Ionic Compounds are Formed
- 3.2. Precipitations
- 3.3. Names and Formulas of Ionic Compounds
- 3.4. Ionic Bonds
- 3.5. Properties of Ionic Compounds
- 3.6. Metal Bonding
- 3.7. Covalent Bonding
- 3.8. Polar Covalent Bonding
- 3.9. Dipoles. Polar and non-polar Molecules
- 3.10. The VSEPR Theory
- 3.11. Hydrogen Bonding. The Peculiar Water
- 3.12. Equals Solves Equal
- 3.13. Solubility of Gases in Water
- 3.14. Solubility of Salts in Water
- 4. Thermochemistry
- 5. Chemical Equilibrium
- 5.1. Reaction Rates
- 5.2. The Law of Mass Action
- 5.3. Calculations on Chemical Equilibrium
- 5.4. Heterogenous Equilibria. Solubility Product
- 5.5. Is the System at Equilibrium? The Reaction Quotient Q
- 5.6. Changing the Concentrations in a System in Equilibrium.
- 5.7. Diluting or Compressing Systems in Equilibrium, or Changing the Temperature
- 6. Acids and bases
- 7. Oxidation and Reduction
- 8. Electrochemistry
- 9. Organic Chemistry
- 9.1. Alkanes
- 9.2. Chain Isomers. Nomenclature
- 9.3. Haloalkanes
- 9.4. Nucleophilic Substitution
- 9.5. Alkenes
- 9.6. Electrophilic Addition. Markovnikov’s Rule
- 9.7. Elimination
- 9.8. Alkynes
- 9.9. Arenes and Aromatic Compounds
- 9.10. Alcohols
- 9.11. Oxidation of Alcohols
- 9.12. Aldehydes and Ketones
- 9.13. Thiols and Disulfides
- 9.14. Ethers
- 9.15. Amines
- 9.16. Nitro Compounds and Organic Nitrates
- 9.17. Carboxylic Acids
- 9.18. More on Carboxylic Acids
- 9.19. Stereoisomerism
- 9.20. Esters
- 9.21. Lipids
- 9.22. Mono-, Oligo-, and Polysaccharides
- 9.23. Amino Acids
- 9.24. Nucleotides
- 10. Biochemistry
- 10.1. Proteins
- 10.2. Enzymes
- 10.3. Catabolic Processes
- 10.4. Carrier Molecules
- 10.5. Glycolysis
- 10.6. Beta-oxidation
- 10.7. The Citric Acid Cycle
- 10.8. The Metabolism of Amino Acids
- 10.9. The Electron Transport Chain
- 10.10. Anabolic Processes
- 10.11. Gluconeogenesis and Fatty Acid Synthesis
- 10.12. DNA: Structure and Function
- 11. Analytical chemistry