Unit 4 Chemistry: Electrons and Periodic Trends

Electromagnetic radiation

Energy that travels through space as waves, like visible light, X-rays, or radio waves.

Frequency (ν)

How many waves pass a point in one second, measured in hertz (Hz).

Wavelength (λ)

The distance between two wave peaks, usually measured in meters or nanometers.

Amplitude

The height of a wave, which shows how strong or intense it is.

Speed of light (c)

How fast light travels in space, which is about 299,792,458 meters per second.

Planck's constant (h)

A tiny number (6.626 × 10^-34 joule-seconds) used to connect the energy of a photon with its frequency.

Photoelectric effect

When light hits certain materials and makes them release electrons.

Photon

A tiny particle of light that carries energy.

Quantum

A small packet of energy that electrons can absorb or release.

Light's wave-like and particle-like behavior

Light acts like both waves (e.g., it bends) and particles (e.g., it knocks electrons loose).

Atomic emission spectra

The unique colors of light an element gives off when its electrons drop to lower energy levels.

Heisenberg uncertainty principle

It's impossible to know exactly where an electron is and how fast it's moving at the same time.

Emission

When electrons release energy as light while moving to a lower energy level.

Absorption

When electrons take in energy and move to a higher energy level.

Electron cloud

The area around an atom's nucleus where electrons are most likely found.

Quantum number

Numbers that describe where an electron is likely to be and its energy level.

Energy level

The 'shells' around an atom where electrons are located.

Electronic configuration

The way electrons are arranged in an atom.

Aufbau Principle

Electrons fill the lowest energy levels first.

Aufbau Diagram

A chart showing the order that energy levels and orbitals are filled.

Pauli Exclusion Principle

No two electrons in the same atom can have the exact same position and energy.

Hund's Rule

Electrons fill empty orbitals of the same energy one at a time before doubling up.

Noble gas configuration

A shorthand way to write electron arrangements using the nearest noble gas as a starting point.

Orbitals

Regions where electrons are likely to be, such as s, p, d, and f orbitals.

Sublevels

Parts of energy levels that include different orbitals (e.g., s, p, d, f).

Ions

Atoms that have gained or lost electrons, giving them a positive or negative charge.

Octet Rule

Atoms want to have 8 electrons in their outer shell to be stable.

Valence electrons

Electrons in the outermost shell that determine how an element reacts.

Dimitri Mendeleev

The scientist who first organized elements in a periodic table by atomic mass.

Henry Moseley

Improved the periodic table by organizing it by atomic number instead of mass.

Glenn Seaborg

Discovered many new elements and helped place the actinide series on the periodic table.

Periodic law

Elements show repeating patterns in properties when arranged by increasing atomic number.

Period

A horizontal row on the periodic table.

Group

A vertical column on the periodic table where elements have similar properties.

10 groups on the periodic table

Includes alkali metals, alkaline earth metals, transition metals, halogens, noble gases, and others.

Atomic radius

The size of an atom. It gets smaller as you move across a period and larger as you move down a group.

Ionic radius

The size of an ion. Positive ions are smaller, and negative ions are larger compared to their neutral atoms.

Electronegativity

How strongly an atom pulls on electrons in a bond.

First vs. second ionization energy

The energy needed to remove the first electron (first ionization) is less than the energy needed to remove the second electron (second ionization).

Electron affinity

The energy change when an atom gains an electron.

Nuclear charge

The total positive charge of an atom's nucleus, equal to the number of protons.

Coulombic attraction

The force between positively charged protons and negatively charged electrons.

Shielding

Inner electrons block the pull of the nucleus on outer electrons.