Conserve atom inventories when substances form new arrangements

Learning goal: Distinguish elements, compounds and molecules, use a simple balanced model to conserve each type of atom, and explain why a new substance can have different properties without gaining new atoms.

An element contains one type of atom. A compound contains two or more different elements chemically combined in a characteristic composition. A molecule is a group of atoms bonded together as a discrete unit. A hydrogen molecule, H₂, contains two atoms of one element, so being a molecule does not automatically make something a compound. A water molecule, H₂O, contains two hydrogen atoms and one oxygen atom and is a molecule of a compound.

Not every compound consists of separate molecules like water. Some form extended structures, such as the repeating arrangement in solid sodium chloride. For this lesson, use water and hydrogen as explicit molecular examples rather than drawing every compound as an isolated three-circle group. A model key must say whether a symbol represents an atom, a molecule or a larger amount of material.

The small numbers in formulas describe composition. H₂O specifies two hydrogen atoms and one oxygen atom in each water molecule. A number placed before the whole formula counts how many represented molecules: 2H₂O has four hydrogen atoms and two oxygen atoms altogether. Changing a front coefficient preserves the substance's composition; changing a subscript can name a different substance. H₂O₂ is not simply another way to write two water molecules.

Consider the model equation 2H₂ + O₂ → 2H₂O. On the left are two hydrogen molecules and one oxygen molecule; on the right are two water molecules. Count atoms before and after: four H and two O on each side. Count molecules: three before and two after. The atom inventory is conserved even though the number of molecules changes. The diagram makes this distinction visible using identical labeled atom symbols in different groupings.

This is a paper or digital representation, not an instruction to combine hydrogen and oxygen gases. Real reactions require appropriate conditions and can release substantial energy. For the model, move counters representing atoms and change their connections without adding, deleting or changing any counter's element label. A line between circles represents a bond schematically, not a rigid miniature stick or a drawing of subatomic particles.

The properties of a compound can differ greatly from those of its constituent elements. Hydrogen and oxygen are gases under ordinary room conditions, whereas water is a liquid under those conditions. Water does not retain the same observable behavior as a bag containing separate hydrogen and oxygen gases. New composition and arrangement matter to properties, while the elements' atom identities remain conserved when substances interact and form new substances. Every water molecule contains the same atoms in the same arrangement, while a water molecule and a hydrogen molecule differ in both. For pure water, these properties are the same for a drop as for a bucketful when the conditions match: compare density at the same temperature and pressure, and melting and boiling points at the same pressure.

Energy must also be considered. Separating bonded atoms requires energy input; formation of new bonds releases energy. The overall change depends on both parts, not on a claim that breaking bonds alone releases all the energy. In a simplified accounting example, if separating the initial connections requires 6 model units and forming the final connections releases 10, the net transfer outward is 4 units. These invented amounts illustrate an energy account, not measured energies for the water equation.

A closed system prevents matter from entering or leaving during the process being studied. It can still exchange energy with its surroundings. An isolated system would exchange neither matter nor energy; that stronger condition is difficult to realize perfectly. A sealed container can therefore warm or cool without becoming open to matter. State the boundary and what may cross it before using a conservation claim.

For ordinary changes in a closed system, total mass remains the same because the number of atoms of each type stays the same. The volume need not remain the same: spacing, state and arrangement can change. Gas formation can increase occupied volume without creating matter. In an open system, a measured container may gain or lose mass because material crosses the boundary; conservation applies when those transfers are included in the wider account.

Measurements rarely give perfectly identical displayed numbers. Instrument resolution, spilled material, evaporation and inconsistent boundaries can affect results. Investigate a mismatch rather than declaring atoms destroyed or silently changing a recorded value. A model explains a conservation principle; an experiment tests whether its measured system and uncertainty support that explanation.

Worked coefficient check: H₂ + O₂ → H₂O is not atom-balanced: two oxygen atoms appear before but only one after. Use 2H₂ + O₂ → 2H₂O, giving four H and two O on both sides. Do not change water to H₂O₂ just to make a count fit; that changes the represented product.

Worked mass boundary: A closed container and all contents read 150.0 g before and 150.0 g after an internal rearrangement. The match supports conservation within the instrument's resolution. If an opened version later reads 148.8 g, investigate the possible 1.2 g leaving as gas or other material rather than treating it as vanished atoms.

Supported modeling: Build the equation with two labeled H–H pairs and one O–O pair. Rearrange into two H–O–H groups. Record atom counts and molecule counts separately and explain the difference. Repeat using twice as many starting groups.

Independent performance: Create an illustrated conservation argument for one supplied simple balanced equation. Use a key, before-and-after counts of every element and a clearly drawn system boundary. Add a separate invented energy-in/energy-out account with units labeled as model units. Explain why changing a formula subscript is different from changing the number of molecules and identify two limitations of the counter model.