Unit 2 – Atoms, Molecules and Ions

Actinide

Metals in the bottom row of the periodic table (inner transition metals)

example) Uranium

Alkali Metal

Elements in group 1 of the periodic table, very reactive

example) Sodium (Na)

Alkaline Earth Metal

Elements in group 2 of the periodic table, less reactive than alkali metals

example) Calcium (Ca)

Alpha Particle (α particle)

A particle with 2 protons and 2 neutrons, like a helium nucleus

example) Released in radioactive decay

Anion

Negatively charged atom or molecule with more electrons than protons

example) Cl⁻

Atomic Mass

Average mass of atoms of an element, measured in amu

example) Carbon ≈ 12.01 amu

Atomic Mass Unit (amu or Dalton, Da)

Unit of mass equal to 1/12 the mass of a carbon-12 atom

example) 1 amu ≈ mass of 1 proton or neutron

Atomic Number (Z)

Number of protons in the nucleus of an atom, defines the element

example) Oxygen = 8

Binary Acid

Acid made of hydrogen and one other element, releases H⁺ in water

example) HCl

Binary Compound

Compound with two different elements example) NaCl

Cation

Positively charged atom or molecule with fewer electrons than protons

example) Na⁺

Chalcogen

Elements in group 16

example) Oxygen (O), Sulfur (S)

Chemical Symbol

One- to three-letter abbreviation for an element

example) H for hydrogen, Fe for iron

Covalent Bond

Bond where atoms share electrons

example) The O–H bonds in water

Covalent Compound (Molecular Compound)

Compound made of atoms held together by covalent bonds

2 or more non metal

example) CO₂

Dalton (Da)

Alternative name for the atomic mass unit

example) 1 Da = 1 amu

Dalton’s Atomic Theory

Early theory saying matter is made of atoms, atoms of one element are the same, and compounds form in whole-number ratios

example) Explains why water is always H₂O

Electron

Negatively charged particle outside the nucleus, very small mass

example) Found orbiting around the nucleus

Empirical Formula

Simplest whole-number ratio of atoms in a compound

example) H₂O₂ → HO

Fundamental Unit of Charge

The charge of one electron (e = 1.602 × 10⁻¹⁹ C)

Group

Vertical column in the periodic table

Halogen

Elements in group 17, very reactive

example) Fluorine (F)

Hydrate

Compound with water molecules attached inside its crystals

example) CuSO₄·5H₂O

Inert Gas (Noble Gas)

Elements in group 18, very unreactive

example) Neon (Ne)

Inner Transition Metal

Elements in the two bottom rows of the periodic table, includes lanthanides and actinides

example) Uranium or Cerium

Ion

Atom or molecule with a positive or negative charge from losing or gaining electrons

example) Na⁺, Cl⁻

Ionic Bond

Attraction between oppositely charged ions example) Na⁺ and Cl⁻ in NaCl

Ionic Compound

Compound made of cations and anions in fixed ratios

Opposite sides of the periodic table (metal+nonmetals)

example) NaCl

Isomers

Compounds with the same chemical formula but different structures

example) Butane and isobutane

Isotopes

Atoms of the same element with different numbers of neutrons

example) Carbon-12 and Carbon-14

Lanthanide

Metals in the top row of the bottom section of the periodic table (inner transition)

example) Cerium (Ce)

Law of Constant Composition (Law of Definite Proportions)

A pure compound always has the same elements in the same ratios

example) Water is always H₂O

Main-Group Element (Representative Element)

Elements in groups 1, 2, and 13–18 example) Sodium (Na), Oxygen (O)

Mass Number

Total number of protons and neutrons in the nucleus

example) Carbon-14 has 14

Metal

Shiny, bendable, good conductor of heat and electricity

example) Copper (Cu)

Metalloid

Element with some properties of metals and some of nonmetals

example) Silicon (Si)

Molecular Compound (Covalent Compound)

Compound of atoms joined by covalent bonds

example) H₂O

Molecular Formula

Shows the exact number of atoms of each element in a molecule

example) C₆H₁₂O₆

Neutron

Neutral particle in the nucleus of an atom

Noble Gas (Inert Gas)

Elements in group 18, unreactive

example) Helium (He)

Oxyacid

Acid with hydrogen, oxygen, and another element

example) H₂SO₄

Oxyanion

Negatively charged ion made of oxygen and another

element example) NO₃⁻

Period (Series)

Horizontal row in the periodic table example) Period 2 has Li, Be, B, C, N, O, F, Ne

Periodic Law

Properties of elements repeat in a pattern based on atomic number

example) Explains periodic trends like reactivity

Pnictogen

Elements in group 15 example) Nitrogen (N), Phosphorus (P)

Polyatomic Ion

Ion made of more than one atom

example) SO₄²⁻

Proton

Positively charged particle in the nucleus

example) Hydrogen atom has 1 proton

Structural Formula

Shows which atoms are in a molecule and how they are connected example) H–O–H for water

Transition Metal

Elements in groups 3–12

example) Iron (Fe)

Unified Atomic Mass Unit (u)

Another name for the atomic mass unit (amu)

example) 1 u = 1 amu