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Atomic Structure and Electron Shells

Sep 9, 2025

Overview

This lecture introduces the chemical level of organization by focusing on atomic structure, the properties of subatomic particles, and the importance of electron shells in determining atom reactivity.

The Chemical Level of Organization

  • The chemical level consists of atoms, which are the smallest units of matter.
  • Atoms combine to form molecules, making up all living cells and organisms.
  • Understanding atomic structure is essential for comprehending molecular interactions in physiology.
  • Matter is anything that takes up space and has mass.

Atomic Structure

  • An atom is composed of three subatomic particles: protons, neutrons, and electrons.
  • Protons are positively charged, located in the nucleus, and have a mass of about 1 atomic mass unit.
  • Neutrons are neutral (no charge), located in the nucleus, and have about the same mass as protons.
  • Electrons are negatively charged, have very little mass, and orbit the nucleus in shells.
  • The nucleus contains both protons and neutrons, contributing most of the atom’s mass.
  • Atoms are usually neutral, with equal numbers of protons and electrons.

Electron Shells and Reactivity

  • Electrons occupy regions called shells or energy levels around the nucleus.
  • The first shell can hold up to 2 electrons (duet rule); the second shell can hold up to 8 electrons (octet rule).
  • Third and higher shells can hold more electrons, but are stable with 8 electrons (octet rule).
  • Atoms are reactive if their outer shell is not full and will seek to gain, lose, or share electrons to achieve stability.
  • Atoms with full outer shells (like helium and neon) are non-reactive.

Examples of Elements and Shell Filling

  • Hydrogen: 1 proton, 1 electron (first shell not full, reactive).
  • Helium: 2 protons, 2 electrons (first shell full, non-reactive).
  • Carbon: 6 protons, 6 electrons (four electrons in the second shell, can form four bonds).
  • Neon: 10 protons, 10 electrons (eight electrons in the second shell, full and non-reactive).

Key Terms & Definitions

  • Atom — The smallest unit of matter with unique properties.
  • Proton — Positively charged subatomic particle in the nucleus.
  • Neutron — Neutral subatomic particle in the nucleus.
  • Electron — Negatively charged subatomic particle orbiting the nucleus.
  • Nucleus — The atom’s central core containing protons and neutrons.
  • Shell — Energy level or region around the nucleus where electrons are found.
  • Duet Rule — First shell is full with two electrons.
  • Octet Rule — Second and higher shells are stable with eight electrons.

Action Items / Next Steps

  • Review the structure of atoms and subatomic particles.
  • Prepare for the upcoming lecture on elements and the periodic table.
  • Focus on understanding how electron shells influence atom bonding and reactivity.