Gen Chem (Molecules and Ions)

Molecules and Ions

Monatomic Gases

Only the six noble gases in Group 8A of the periodic table exist in nature as single atoms. For this reason, they are called ______.

Molecules

an aggregate of at least two atoms in a definite arrangement, held together by chemical forces (chemical bonds). It may contain: 1. Same element atoms (e.g., H₂) 2. Different element atoms in a fixed ratio, following the law of definite proportions.

Diatomic Molecules

Molecules with two atoms. Can be same elements and can also be different elements: Examples: HCl, CO

Polyatomic Molecules

Molecules with more than two atoms. Can be: Same element: O₃ (ozone) – 3 oxygen atoms; Different elements: H₂O (water), NH₃ (ammonia)

Ions

Atoms become ___ when they gain or lose electrons. Metals, like sodium (Na), lose 1 or 2 electrons easily, so they become positive. Nonmetals, like chlorine (Cl), gain electrons, so they become negative. This way, both atoms get a stable electron arrangement, and ionic bonds can form.

Monatomic Ions

An ion that consists of only one atom.

Naming Cations

The name of the cation is the same as the element’s name.

Naming Anions

For anions, the name of the element is modified by the suffix -ide.

Polyatomic Ion

Consists of two or more atoms that are covalently bonded and have an overall charge (positive or negative).

Chemical Formulas - Composition

uses element symbols to show the composition of molecules and ionic compounds. They indicate: What elements are present; The ratios in which atoms are combined.

Molecular Formula

Shows the exact number of atoms of each element in the smallest unit of a substance. Subscripts indicate the number of atoms. No subscript = only 1 atom (e.g., O in H₂O). Oxygen (O₂) and ozone (O₃) are allotropes — different forms of the same element.

Empirical Formula

Shows the elements present and the simplest whole-number ratio of their atoms. It does not show the actual number of atoms in a molecule.

Formula of Ionic Compounds

is made up of positive ions (cations) and negative ions (anions) that are held together. The formula is written in its empirical form, meaning the smallest whole number ratio of cations to anions that makes the compound electrically neutral.

Binary Compounds

The first element keeps its name. The second element gets the suffix "-ide". Greek prefixes are used to indicate the number of atoms of each element in the molecule.

Molecules or Ions

Most matter is composed of this formed by atoms.

Choose This
Just Memorize the IONS

Common Polyatomic Ions

Allotropes

different forms of the same element

Subscripts

indicate the number of atoms

Cation

loss of electrons

Anion

gain of electrons

ammonium

NH₄⁺

carbonate

CO₃²⁻

hydroxide

OH⁻

nitrate

NO₃⁻

nitrite

NO₂⁻

phosphate

PO₄³⁻

sulfate

SO₄²⁻

sulfite

SO₃²⁻