Periodic trends refer to patterns on the periodic table which describe changes in chemical properties.
Using these trends, properties can be predicted down a group or across a period.
Overview of the Periodic Table
Elements are arranged by increasing atomic number, representing the number of protons.
Each element's symbol is displayed with its atomic number on top and relative atomic mass below.
Periods: Horizontal rows on the table.
Groups: Vertical columns on the table.
Groups
Characteristics:
Elements in a group have the same number of outermost shell electrons.
They exhibit similar chemical and physical properties.
Groups numbered from 1 to 18.
Group Highlights:
Group 1 (Alkali Metals): 1 valence electron, soft, highly reactive, low melting points, form soluble salts.
Group 2 (Alkaline Earth Metals): 2 valence electrons, harder, higher melting points, less reactive.
Groups 3-12 (Transition Metals): D block, unique properties due to D orbitals.
Groups 13-18: Mostly non-metals; Group 17 (Halogens) are reactive due to 7 valence electrons; Group 18 (Noble Gases) have a full octet and are unreactive.
Lanthanides and Actinides: Found below the main table, contain F orbitals.
Periods
Elements in a period have the same number of electron shells.
Blocks:
S block: Groups 1 and 2, highest energy electrons in S orbitals.
D block: Transition metals, highest energy electrons in D orbitals.
P block: Elements following the D block, highest energy electrons in P orbitals.
Important Periodic Trends
Atomic Radius
Trend: Decreases across a period and increases down a group.
Reason: Increased nuclear charge pulls electrons closer, reducing size across a period. More shells increase size down a group.
Ionization Energy
Definition: Energy needed to remove an electron from an atom or ion.
Molar Ionization Energy: Energy to remove a mole of electrons.
Trend: Increases across a period (stronger nuclear attraction) and decreases down a group (further electrons from nucleus).
Electron Affinity
Definition: Energy change when a mole of electrons is added to a mole of gaseous atoms.
Trend: More negative across a period (smaller atoms, stronger attraction) and less negative down a group (larger atoms, weaker attraction).
Electronegativity
Definition: An atom's ability in a bond to attract electrons.
Scale: Measured using the Pauling scale (0 to 4).
Trend: Increases across a period (stronger nuclear attraction) and decreases down a group (increased distance from nucleus).
Summary
Groups and periods organize elements by valence electrons and electron shells.
Atomic size decreases across a period and increases down a group.
These changes in size influence ionization energy, electronegativity, and electron affinity.