CHE217 LECTURE 1-2

Atomic Structure & Interatomic Bonding

DEMOCRITUS (400 BC)

Proposed “atomos” as the smallest, unbreakable part of matter

DALTON (1805)

Formulated an evidence-based theory stating all elements are made of unique atoms that combine in whole number ratios

THOMSON (1904)

Discovered the electron via cathode ray experiments; proposed the "plum pudding" model.

RUTHERFORD (1911)

Discovered the dense, positive nucleus using the gold foil experiment.

BOHR (1913)

Theorized that electrons move at fixed energies in discrete paths called orbits.

MODERN ELECTRON CLOUD MODEL

Describes electron positions through probability distributions (orbitals) rather than discrete paths.

PROTONS

+1.602×10^-19 C

ELECTRONS

1.602×10^-19 C

ATOMIC NUMBER (Z)

Total number of protons

ATOMIC MASS (A)

Sum of the masses of protons and neutrons

PRINCIPAL QUANTUM NUMBER (n)

Relates to the distance of an electron from the nucleus and the energy state/size of the shell

AZIMUTHAL QUANTUM NUMBER (I)

Defines the shape of the electron subshell/orbital (s, p, d, f)

MAGNETIC QUANTUM NUMBER (m_s)

Direction of electron spin, either +1/2 or -1/2

PAULI’S EXCLUSION PRINCIPLE

Each electron state can hold no more than 2 electrons; no two electrons will have the same set of quantum numbers

HUND’S RULE

The lowest energy is attained by maximizing the number of electrons with the same electron spin; orbitals are filled singly before pairing

STABLE ELECTRON CONFIGURATION

Occurs when the valence electron shell is completely filled (max 8 in outermost shell, e.g., Ne, Ar, Kr)

CARBON ALLOTROPES - DIAMOND

It is the hardest material and a poor conductor

CARBON ALLOTROPES - GRAPHITE

Soft with a greasy feel and a good conductor

Carbon Allotropes (Diamond vs. Graphite)

Differences are caused by a specific type of interatomic bonding in graphite that is absent in diamond

IONIC BONDING

Electron transfer between a metal (donates) and a nonmetal (accepts), requiring a large electronegativity difference (predominant in ceramics like NaCl, MgO)

COVALENT BONDING

Electrons are shared between nonmetals with similar electronegativities

METALLIC BONDING

Atoms in metals are closely packed; loosely bounded valence electrons form a non-directional "sea" of electrons around ion cores, causing conductivity and ductility

VAN DER WAALS (SECONDARY BONDING)

Weak forces arising from interactions between induced or permanent dipoles

HYDROGEN BONDING

A strong type of secondary bond involving hydrogen and highly electronegative atoms like F or O

GECKO ADHESIVES

Rely on weak van der Waals forces between microscopic hairs and surfaces, leading to "self-cleaning" properties and inspiring synthetic adhesives like GeckSkin