Chemistry Electronic Structure and Periodic Trends

Energy level (n)

n is a positive integer that represents the energy level where an electron can be found. As n increases, both energy and distance of electrons from the nucleus increase

Subshells (s, p, d, f)

Each energy level can contain several subshells labeled s, p, d, f. These determine the shape and energy of orbitals.

Orbital

A region around a nucleus where an electron is likely to be found. Each orbital holds up to two electrons.

s orbital

Spherical in shape with perfect radial symmetry; every value of n has one s orbital holding 2 electrons.

p orbital

Exists for n ≥ 2; 3 orbitals (px, py, pz) shaped like dumbbells with nodes in between lobes. Holds 6 electrons total.

d orbital

Exists for n ≥ 3; has 5 degenerate orbitals, 4 shaped like clovers, 1 shaped like a dumbbell with a ring. Holds 10 electrons total.

f orbital

Exists for n ≥ 4; has 7 degenerate orbitals and holds 14 electrons. Shapes not needed for this course.

Blocks of the Periodic Table

s block: 2 left columns + He; p block: right 6 columns; d block: middle 10 columns; f block: bottom 2 rows.

Electron spin

A property of electrons causing magnetic dipole. Spin must be opposite to share an orbital.

Pauli Exclusion Principle

No two electrons can have the same four quantum numbers. Only electrons with opposite spin can share an orbital.

Electrons repel each other, opposite spins attract magnetically

Aufbau Principle

Electrons fill orbitals from lowest to highest energy: ns → np → nd → nf. For example, 4s fills before 3d.

Hund’s Rule

Electrons fill empty degenerate orbitals singly with the same spin before pairing. Electrons fill spin up first.

Quantum numbers

The four quantum numbers are n (energy level), l (subshell), ml (orbital), ms (spin). They form an “address” for electrons.

Principal quantum number (n)

Specifies energy level; n = 1, 2, 3,…

Angular momentum quantum number (l)

Specifies subshell/shape; l = 0 to n−1. s: l=0, p: l=1, d: l=2, f: l=3

Magnetic quantum number (ml)

Specifies orbital orientation; values from −l to +l including 0

Spin quantum number (ms)

Specifies electron spin; ms = +½ (spin up) or −½ (spin down)

Exception: Copper (Cu)

Expected: 4s² 3d⁹, Actual: 4s¹ 3d¹⁰ to get full d subshell

Exception: Chromium (Cr)

Expected: 4s² 3d⁴, Actual: 4s¹ 3d⁵ to get half-filled d subshell

Valence electrons

Electrons in outermost shell (largest n). Example: Boron has 3 valence electrons (n = 2)

Noble gas configuration

Abbreviated notation using previous noble gas + remaining configuration. Example: Cd = [Kr]5s² 4d¹⁰

Coulomb’s Law

•the closer an electron is to the nucleus, the stronger the force of attraction between the two objects is

•It takes energy to raise an electron against this attraction.Force between two charges increases with more protons (Z) and decreases with more distance or shielding. Explains periodic trends

Shielding

Core electrons repel valence electrons, weakening nuclear attraction. Increases down a group, stays constant across a period.

Effective nuclear charge (Zeff)

The net positive charge experienced by valence electrons. Increases across a period, decreases down a group.

Ionization energy

Energy required to remove an electron from its attraction too the nucleus. Increases across a period and up a group.

Electron affinity

Attraction of electron to nucleus, energy released when an electron is added. Increases across a period and decreases down a group.

Atomic radius

Size of an atom. Increases left and down the periodic table. Decreases with increasing Zeff.

Metallic character

Tendency to lose electrons. Increases left and down the periodic table.

Periodic table regions

Metals, nonmetals, and metalloids. Metalloids lie in a zigzag between metals and nonmetals in the p-block.

Isoelectronic series

Ions with same number of electrons. Ionic radius changes based on nuclear charge (Z).

Who was the 1st person to propose a model of the atom that had electrons confined to discrete energy levels around the nucleus?

Niels Bohr

Are electrons lower(more stable) or higher energy when they are close to the nucleus?

Lower, energy levels with a small n imply lower energy

How can n be represented in numbers?

Only in whole number integers greater than zero

Are electrons further from the nucleus low or high energy?

Higher energy, moving away from the attraction takes lots of energy. They are less stable than electrons closer to the nucleus

Sublevel

Named after the class of orbital shape they contain(spdf). Orbitals can only hold 2 electrons, but a sublevel can have one or many orbitals. Each orbital shape has a different energy from the other shapes.

S subshell

•Every energy level (value of n) contains an s subshell.

•This subshell only has one s orbital that can hold two electrons.

•The s orbital is a sphere. It has perfect radial symmetry.

P subshell

Only energy levels of n > 1 contain p subshells

•contains three p orbitals.

•Each orbital is the same shape and has the same energy.

•Orbitals that have the same energy are called degenerate orbitals.

•Orbitals in the same subshell are always degenerate.

•Notice the p orbitals are NOT degenerate with the s orbital in the same energy level

P orbitals

•dumbbell shape.

•The dumbbell can be aligned on the x, y, or z axis.

•The two areas where electrons are present are called lobes.

•The plus/minus symbols are NOT charges. They represent the phase of the electron wave in each node.

•The area between the two lobes is called a node. No electrons may be found there.

D subshell

•Only energy levels of n > 2 contain d subshells

•A d subshell contains five d orbitals.

•Four orbitals are the same shape (fifth is a weird exception).

•Orbitals in the same subshell are always degenerate.

•Notice that the d subshell is higher in energy than the p and s subshells.

D orbitals

•Four of the d orbitals have a cloverleaf shape.

•The fifth d_z2 orbital looks like a p orbital with a donut around the center.

•The cloverleaf can be aligned on the xy, xz, and yz planes.

•It can also be rotated between the planes or take on a fifth shape that sits directly on the z-axis.

F subshell

•Only energy levels of n > 3 contain f subshells.

•A f subshell contains seven f orbitals.

•Orbitals have complex geometries.

•Notice that the f subshell is higher in energy than the s, p, and d subshells.