Overview
This lecture introduces the structure of atoms, focusing on subatomic particles, the organization of electrons, and how atomic structure relates to chemical properties.
Structure of the Atom
- Atoms are the smallest units of matter with the properties of an element.
- The orbital model is a simplified way to visualize atoms, with electrons orbiting a nucleus.
- The real structure of an atom is an electron cloud, not defined orbits.
Subatomic Particles
- Three main subatomic particles: protons, neutrons, and electrons.
- Protons have a positive charge and are found in the nucleus; they determine atomic identity (atomic number).
- Neutrons are neutral and also found in the nucleus; they add mass but do not change the element's identity.
- Electrons are negatively charged, orbit the nucleus, and have negligible mass; they determine chemical properties.
Atomic Number and Atomic Mass
- Atomic number = number of protons and defines the element (e.g., H=1, C=6, O=8).
- Atomic mass = protons + neutrons; usually not a whole number because it is an average due to isotopes.
- Isotopes have the same number of protons but different numbers of neutrons; they may be stable or radioactive.
Electron Arrangement
- Electrons occupy energy shells around the nucleus, each with a specific capacity.
- First shell holds up to 2 electrons; subsequent shells can hold up to 8 (octet rule).
- The outermost shell (valence shell) determines chemical properties.
- Atoms in the same group on the periodic table have the same number of valence electrons.
Valence Electrons and Chemical Properties
- Only the number of electrons in the valence shell affects chemical properties, not the total number of electrons.
- Elements with the same number of valence electrons (like H, Li, Na) have similar chemical behavior.
- Full valence shells (e.g., noble gases: He, Ne, Ar) are associated with chemical stability.
Electron Configuration Examples
- For carbon (atomic number 6): 2 electrons in first shell, 4 in the second, spread across four orbitals.
- For oxygen (atomic number 8): 2 electrons in first shell, 6 in second; two orbitals fully paired, two unpaired.
- The distribution of electrons among orbitals influences stability and reactivity.
Key Terms & Definitions
- Atom — Smallest unit of an element maintaining chemical properties.
- Proton — Positively charged particle in atom's nucleus; defines element.
- Neutron — Neutral particle in nucleus; adds mass.
- Electron — Negatively charged particle orbiting nucleus; determines chemical properties.
- Atomic Number — Number of protons in an atom.
- Atomic Mass — Combined number of protons and neutrons (average due to isotopes).
- Isotope — Atoms of the same element with different neutron counts.
- Valence Shell — Outermost electron shell, determines reactivity.
- Octet Rule — Tendency for atoms to have eight electrons in their valence shell.
Action Items / Next Steps
- Review the structures of atoms on the periodic table.
- Prepare for the next module on molecular basics.