Question

What questions do you need to ask about a substance in order to determine whether it is (1) an atomic element, (2) a molecular element, (3) a molecular compound, or (4) an ionic compound? Write a detailed set of instructions describing how to determine the classification of a substance based on the answers to your questions.

Short answer

To categorize a substance, first determine if it consists of atoms, molecules, or ions. Atomic elements have single atoms; molecular elements have multiple identical atoms bonded together; molecular compounds have molecules with more than one type of atom; and ionic compounds have positive and negative ions.

Step-by-step solution

Determine the Composition

Ask what types of particles compose the substance. If the substance is composed of only one type of atom, proceed to Step 2. If it is composed of molecules, proceed to Step 4. If it is composed of ions, it is an ionic compound.

Check for Identical Atoms

If the substance is composed of only one type of atom, ask whether it is a single atom or a group of the same atoms bonded together. If it is a single atom, it is an atomic element. If it consists of multiple identical atoms bonded together, it is a molecular element.

Conclude for Atomic or Molecular Element

If the substance consists of single, separate atoms, classify it as an atomic element. If it consists of two or more identical atoms bonded together, classify it as a molecular element.

Determine Molecule Types

If the substance is composed of molecules, determine the types of atoms that make up these molecules. If there is more than one type of atom in the molecule, it is a molecular compound. If all atoms in the molecule are the same, refer back to Step 3.

Define Ionic Compound

If the substance is composed of a positive and a negative ion, classify it as an ionic compound. Typically, ionic compounds form from metals (which form positive ions) and non-metals (which form negative ions).

Key concepts

Atomic Element

An atomic element is a pure substance that consists of a single type of atom. Atoms are the smallest units of matter that retain all of the chemical properties of an element. Atomic elements can't be broken down into a simpler substance by chemical means. An example of an atomic element is helium (He), where each atom exists independently and not bonded to other atoms.

When identifying atomic elements, ask if the substance consists of only one kind of atom with no bonds connecting them. This characterizes an atomic element. These elements can be found on the periodic table and are the fundamental building blocks of chemistry. Most atomic elements are gases, like noble gases, under normal conditions. They display unique individual properties that differentiate them from molecular elements and compounds.

Molecular Element

Molecular elements, unlike atomic elements, are composed of two or more atoms of the same element that are chemically bonded together. These elements exist as molecules, which are distinct groups of atoms bonded together as a single unit. A familiar example of a molecular element is oxygen (O₂), which naturally exists as diatomic molecules.

To classify a substance as a molecular element, it should be confirmed that the substance is made up of molecules composed of the same type of atom. For instance, if a substance contains only nitrogen and the molecules are formed by nitrogen atoms bonding together (N₂), it is considered a molecular element. These are more common in nature than atomic elements and can be solids, liquids, or gases.

Molecular Compound

A molecular compound consists of molecules that are formed by two or more different types of atoms combining through covalent bonds. These compounds are typically the result of nonmetallic elements reacting with each other. For example, water (H₂O) is a simple molecular compound consisting of hydrogen and oxygen atoms.

To determine if a substance is a molecular compound, we need to inspect its molecular composition. If the substance is made up of molecules comprising different types of atoms, it is indeed a molecular compound. These compounds generally have low melting and boiling points compared to ionic compounds, and they can exhibit a wide range of physical and chemical properties based on the elements involved and how they are bonded.

Ionic Compound

Ionic compounds are composed of positive and negative ions that are held together by ionic bonds, which are electrostatic forces of attraction between oppositely charged ions. They typically result from the reaction between metals, which form positive ions (cations), and nonmetals, which form negative ions (anions). A common example is sodium chloride (NaCl), or table salt.

To classify a substance as an ionic compound, it must exhibit ionic bonding between its constituent ions, often revealed by the substance's high melting and boiling points due to the strong ionic bonds. Ionic compounds generally form a crystalline lattice structure, in which the ions are systematically arranged in a repeating pattern, adding to their distinct properties such as brittleness and electrical conductivity when molten or dissolved in water.