Overview
This lecture explains how electrons are arranged in atoms, why having a full outer shell is important for stability, and how atoms gain or lose electrons to form ions.
Electron Arrangement in Atoms
- Electrons are arranged in shells around the nucleus, filling from the innermost shell outward.
- The first shell holds up to 2 electrons; the second and third shells hold up to 8 electrons each.
- Electrons are added one at a time in a set order (top, right, bottom, left) until the shell is full.
Atomic Stability and Reactivity
- Atoms are stable when their outermost electron shell (valence shell) is full.
- Most atoms have incomplete outer shells and are thus unstable, tending to react to achieve a full shell.
- Atoms react by gaining, losing, or sharing electrons to become more stable.
Electron Structure Examples
- Sodium (atomic number 11): Electron arrangement is 2, 8, 1 (unstable, wants to lose 1 electron).
- Argon (atomic number 18): Electron arrangement is 2, 8, 8 (stable, full outer shell).
- Calcium (atomic number 20): Electron arrangement is 2, 8, 8, 2 (unstable, wants to lose 2 electrons).
- Fluorine (atomic number 9): Electron arrangement is 2, 7 (unstable, wants to gain 1 electron).
- Noble gases (Group 0) naturally have full outer shells and do not react easily.
Ions and Charges
- Atoms that lose electrons become positively charged ions (cations), e.g., calcium ion (Ca²⁺): electron structure is 2, 8, 8.
- Atoms that gain electrons become negatively charged ions (anions), e.g., fluoride ion (F⁻): electron structure is 2, 8.
- When drawing ions, use square brackets and indicate the charge in the top right.
Key Terms & Definitions
- Atomic number — the number of protons in an atom, which also equals its number of electrons.
- Electron shell — energy level surrounding the nucleus where electrons are found.
- Valence shell — the outermost shell of an atom.
- Ion — an atom or molecule with a net electric charge due to the loss or gain of electrons.
- Cation — a positively charged ion.
- Anion — a negatively charged ion.
- Noble gases — elements in Group 0 with full valence shells and low reactivity.
Action Items / Next Steps
- Practice drawing electron arrangements for the first 20 elements.
- Complete any assigned questions on electron structure and ion formation.