Matter and Pure Substances: Elements, Compounds, and Mixtures

Flashcards covering key concepts from the lecture notes on matter, elements, compounds, mixtures, and periodic table.

What is an element?

A pure substance consisting of a single type of atom (or, for some elements, a diatomic or polyatomic molecule) and represented by a symbol on the periodic table.

What is a compound?

A pure substance formed when two or more elements are chemically bonded in a fixed ratio; described by a chemical formula.

What does a chemical formula describe for molecular vs ionic compounds?

For molecular compounds, the formula shows the number of atoms in a molecule; for ionic compounds, it shows the ratio of ions in a neutral formula unit.

What is a molecular compound?

A compound whose atoms are held together by covalent bonds and whose formula describes a molecule (e.g., H2O, C2H6O).

What is an ionic compound?

A compound consisting of positively and negatively charged ions in a fixed ratio forming a neutral unit (e.g., NaCl).

What is a network compound?

A substance with a giant covalent network where the formula does not describe a single molecule (e.g., silicon dioxide, SiO2).

Give an example of a diatomic element.

Nitrogen exists as N2 (two nitrogen atoms per molecule); other examples include O2.

What is white gold?

An alloy of gold with metals like palladium or nickel that appears silvery; a homogeneous mixture distributed at the atomic level.

What is a homogeneous mixture?

A uniform composition throughout at the atomic level that does not spontaneously separate (e.g., air, tap water).

What is a heterogeneous mixture?

A non-uniform mixture with visually or microscopically distinct regions of different composition (e.g., granite).

What is the composition of air?

About 78% nitrogen, 21% oxygen, and roughly 1% other gases (including argon and carbon dioxide).

What is dry ice and what warning accompanies it?

Dry ice is solid CO2 that sublimates; avoid putting it in drinks and be cautious of asphyxiation in enclosed spaces due to CO2 displacing oxygen.

What does Ca(OH)2 illustrate about subscripts in chemical formulas?

The subscript 2 in Ca(OH)2 indicates there are two OH groups in the charge-neutral unit.

How are element symbols standardized and presented on the periodic table?

Element symbols are internationally agreed, usually one- or two-letter abbreviations; some three-letter placeholders were used historically; the periodic table is organized by atomic number and properties.

What is ethanol's formula and what does the index describe?

Ethanol has the formula C2H6O; the subscripts describe the number of atoms of each element in one molecule.