Periodic Table Trends and Ion Formation
Mass Number
- Mass number is the sum of protons and neutrons in an atom.
- Calculated as: Protons + Neutrons.
Atomic Size Trends
- Atomic radius increases down a group (adding new electron shells).
- Atomic radius decreases across a period (left to right) (increasing nuclear charge pulls electrons closer, no new shells).
Ionization Energy
- Definition: Minimum energy required to remove an electron from a neutral atom ($$A \rightarrow A^+ + e^-$).
- Decreases down a group (electrons further from nucleus, less attraction).
- Increases across a period (left to right) (electrons held tighter by increased nuclear charge).
- Lowest in the bottom-left (e.g., metals); highest in the top-right (e.g., nonmetals/noble gases) of the periodic table.
- Definition: Ability of an element to easily lose its valence electrons.
- Increases down a group.
- Decreases across a period (left to right).
- Correlates with larger atomic size and low ionization energy.
- Highest in the bottom-left; lowest in the top-right of the periodic table.
Ions: Cations and Anions
- Ion: An atom with an overall electrical charge (not neutral).
- Cation: Positively charged ion, formed by losing one or more electrons.
- Anion: Negatively charged ion, formed by gaining one or more electrons.
- The number of protons (atomic number) in the nucleus never changes during ion formation.
Octet Rule
- Definition: Atoms tend to achieve a stable electron configuration, usually 8 valence electrons (an octet), like noble gases.
- Achieved by: losing, gaining, or sharing electrons.
- Metals typically lose electrons to form cations.
- Nonmetals typically gain electrons to form anions.
- Noble gases (Group 8A) already have an octet and therefore do not readily form ions.
- Carbon typically shares electrons rather than forming ions.