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Chemistry LibreTexts

2: Atoms and Molecules

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    • 2.1: Chemical Symbols and Formulas
      In this chapter, we will learn the meaning of the symbols used in chemistry to represent elements and compounds, and the information these symbols convey microscopically and macroscopically.
    • 2.2: Atomic Structure
      Chemistry is based on the modern atomic theory, which states that all matter is composed of atoms. Atoms themselves are composed of protons, neutrons, and electrons. Each element has its own atomic number, which is equal to the number of protons in its nucleus. Isotopes of an element contain different numbers of neutrons. Elements are represented by an atomic symbol. The periodic table is a chart that organizes all the elements.
    • 2.3: Isotopes - When the Number of Neutrons Varies
      All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Many isotopes occur naturally.
    • 2.4: Masses of Atoms and Molecules
      The atomic mass unit (u) is a unit that describes the masses of individual atoms and molecules. The atomic mass is the weighted average of the masses of all isotopes of an element. The molecular mass is the sum of the masses of the atoms in a molecule.
    • 2.5: Avogadro's Number
      In this section, we will learn how using Avogadro's number, we can translate the microscopic information contained in chemical formulas into macroscopic quantities that we can actually measure in the laboratory. It's like magic!
    • 2.6: The Mole
      The mole is a key unit in chemistry. The molar mass of a substance, in grams, is numerically equal to one atom's or molecule's mass in atomic mass units.
    • 2.7: Percentage Composition
      In this section, we will discuss the meaning of percentage composition in chemistry

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