Isotopes, Atomic Mass, and Ion Formation in Chemistry

Isotopes

Isotopes are atoms of the same element (same number of protons) but different numbers of neutrons.

Mass number

They have different mass numbers, but same chemical behavior.

Carbon-12

Carbon-12: 6 protons, 6 neutrons.

Carbon-13

Carbon-13: 6 protons, 7 neutrons.

Carbon-14

Carbon-14: 6 protons, 8 neutrons.

Atomic number

The number of protons = atomic number.

Relative Atomic Mass

Each element has isotopes, and the atomic mass on the periodic table is the weighted average of all its isotopes.

Average atomic mass formula

Average atomic mass = (mass1 × abundance1) + (mass2 × abundance2) + ... / 100.

Chlorine isotopes

Chlorine has two isotopes: Cl-35 (75.77%) and Cl-37 (24.23%).

Average atomic mass of Chlorine

Average atomic mass = 35.48 amu.

Ion Formation

Atoms become ions when they gain or lose electrons to get a stable electron configuration.

Cation

Cation: Loses electrons (Example: Na → Na⁺ + e⁻, Charge: Positive (+)).

Anion

Anion: Gains electrons (Example: Cl + e⁻ → Cl⁻, Charge: Negative (-)).

Metals

Metals → form cations (+).

Nonmetals

Nonmetals → form anions (-).

Classification of elements

You can classify elements based on their position on the periodic table.

Metals properties

Metals: Shiny, conduct heat/electricity, malleable, form cations.

Nonmetals properties

Nonmetals: Dull, poor conductors, form anions.

Metalloids properties

Metalloids: Properties of both (Ex: B, Si, As).

Main Group Elements

Main Group Elements: Groups 1-2 and 13-18.

Alkali metals

Group 1: Alkali metals → +1 ions.

Alkaline earth metals

Group 2: Alkaline earth metals → +2 ions.

Halogens

Group 17: Halogens → -1 ions.

Noble gases

Group 18: Noble gases → stable, no ions.

Transition Metals

Transition Metals: Groups 3-12 can form multiple charges (ex: Fe²⁺, Fe³⁺).

Predicting ions

Use group number to predict charge.

Alkali metals ion formed

Group 1: Alkali metals → +1.

Alkaline earth metals ion formed

Group 2: Alkaline earth metals → +2.

Boron group ion formed

Group 13: Boron group → +3.

Nitrogen group ion formed

Group 15: Nitrogen group → -3.

Oxygen group ion formed

Group 16: Oxygen group → -2.

Halogens ion formed

Group 17: Halogens → -1.

Noble gases ion formed

Group 18: Noble gases → 0 (no ions).

Examples of ions

Examples: Mg → Mg²⁺, O → O²⁻, Cl → Cl⁻, Al → Al³⁺.