Primary School (K-2)
Different properties are suited to different purposes.
A great variety of objects can be built up from a small set of pieces.
Different kinds of matter exist and many of them can be either solid or liquid, depending on temperature. Matter can be described and classified by its observable properties.
Elementary School (3-5)
Matter of any type can be subdivided into particles that are too small to see, but even then the matter still exists and can be detected by other means. A model showing that gases are made from matter particles that are too small to see and are moving freely around in space can explain many observations, including the inflation and shape of a balloon and the effects of air on larger particles or objects.
The amount (weight) of matter is conserved when it changes form, even in transitions in which it seems to vanish.
Measurements of a variety of properties can be used to identify materials. (Boundary: At this grade level, mass and weight are not distinguished, and no attempt is made to define the unseen particles or explain the atomic-scale mechanism of evaporation and condensation.)
Middle School (6-8)
Substances are made from different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms.
Each pure substance has characteristic physical and chemical properties (for any bulk quantity under given conditions) that can be used to identify it.
Gases and liquids are made of molecules or inert atoms that are moving about relative to each other.
In a liquid, the molecules are constantly in contact with others; in a gas, they are widely spaced except when they happen to collide. In a solid, atoms are closely spaced and may vibrate in position but do not change relative locations.
Solids may be formed from molecules, or they may be extended structures with repeating subunits (e.g., crystals).
The changes of state that occur with variations in temperature or pressure can be described and predicted using these models of matter.
High School (9-12)
Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons.
The periodic table orders elements horizontally by the number of protons in the atom’s nucleus and places those with similar chemical properties in columns. The repeating patterns of this table reflect patterns of outer electron states.
The structure and interactions of matter at the bulk scale are determined by electrical forces within and between atoms.
Stable forms of matter are those in which the electric and magnetic field energy is minimized. A stable molecule has less energy than the same set of atoms separated; one must provide
at least this energy in order to take the molecule apart.