Atomic Structure and the Periodic Table

Matter, Atoms, and Elements

• Matter Definition: Matter is defined as any substance that has mass and occupies space.
• Phases of Matter: Matter exists in three distinct phases or forms:
  • Solid
  • Liquid
  • Gas
• Composition of Matter: Matter is composed of elements.
  • Atom: The smallest particle that exhibits the chemical properties of an element.
  • Naturally Occurring Elements: There are $92$ naturally occurring elements.
  • Elements in Living Organisms: $25$ elements occur in living organisms.
  • Abundant Biological Elements: Only $12$ elements occur in living organisms in greater than trace amounts.

Components of an Atom

• An atom is composed of three subatomic particles:
  1. Neutrons ($n^0$):
     • Mass: $1\,\text{atomic mass unit (amu)}$
     • Charge: No charge ($0$)
     • Location: Found inside the nucleus
  2. Protons ($p^+$):
     • Mass: $1\,\text{atomic mass unit (amu)}$
     • Charge: Positive $1$ ($+1$)
     • Location: Found inside the nucleus
  3. Electrons ($e^-$):
     • Mass: $\frac{1}{1800}\text{th}$ of the mass of a proton or neutron
     • Charge: Negative $1$ ($-1$)
     • Location: Located at varying distances from the nucleus in regions called orbitals (also referred to as energy levels or shells)

Periodic Table Structure

• Atomic Number:
  • Defined as the number of protons in an atom of a given element.
  • Located above the chemical symbol and name.
  • Elements are arranged sequentially by atomic number from left to right within rows.
• Element / Chemical Symbol:
  • A unique designated symbol for each element.
  • Usually identified by a single capital first letter, or the first letter combined with an additional lowercase letter (for example, $\text{C}$ represents carbon).
• Average Atomic Mass:
  • Represents the combined mass of both protons and neutrons.
  • Shown below the element's symbol on the periodic table (for example, hydrogen has an average atomic mass of $1.008$\,amu).

Determining Subatomic Particle Counts

• Protons:
  • $\text{Number of protons} = \text{Atomic number}$
• Neutrons:
  • $\text{Number of neutrons} = \text{Atomic mass} - \text{Atomic number}$
• Electrons:
  • $\text{Number of electrons} = \text{Number of protons} = \text{Atomic number}$ (in a neutral atom)