Elements and their combinations account for all the varied types of matter in the world.

Some of the Grade 5 Physical Science standards cover very broad areas (such as the nature of elements and the organization of the periodic table) while others focus on discrete facts such as the properties of salts. Cumulatively, these standards teach that all the varied types of matter in the world consist of elements and their combinations.

Practically everything that we experience is a mixture of elements, compounds, or sometimes both. It is rare to naturally encounter a pure element or pure compound. Even rain water that has just fallen from the sky is a mixture since it contains some carbon dioxide gas dissolved in it. Salt water in the ocean is a mixture that contains a variety of salts as well as carbon dioxide and oxygen dissolved in the water. Similarly, air is a mixture that contains the elements oxygen and nitrogen as well as the compounds water and carbon dioxide.

It generally requires practice, feedback, reflection, and discussion to develop the ability to confidently distinguish atoms, molecules, elements, compounds, and mixtures. Just memorizing definitions can readily lead to confusion. Practicing applying these words to real world things such as water is essential. A Table such as the one below illustrates some of the issues.

Substance	Element/Compound/Mixture	Smallest Piece
Gold	Element	A gold atom
Helium	Element	A helium atom; the smallest piece of helium gas is a helium atom
Water	Compound	A molecule of water consisting of two hydrogen atoms and one oxygen atom combined
Oxygen	Element	An atom of oxygen; the smallest piece of oxygen gas is an oxygen molecule consisting of two oxygen atoms combined
Nitrogen	Element	An atom of nitrogen; the smallest piece of nitrogen gas is a nitrogen molecule consisting of two nitrogen atoms combined
Air	Mixture	Different molecules mixed together
Sugar (glucose)	Compound	A molecule of sugar consisting of 6 carbon, 6 oxygen and 12 hydrogen atoms combined
Carbon dioxide	Compound	A molecule consisting of one carbon atom and two oxygen atoms combined
Soda	Mixture	Different molecules mixed together

Note that some gases such as nitrogen, hydrogen, and oxygen can cause confusion because they exist in nature as two atoms combined with each other. The smallest piece of oxygen is an oxygen atom. However, oxygen atoms are so reactive that an isolated oxygen atom will always combine with another one or with another element. The smallest piece of oxygen gas is therefore a molecule of oxygen consisting of two oxygen atoms combined with each other.

Also note that helium is a non-reactive element. A helium atom does not readily combine with another helium atom or an atom of any other element. Unlike oxygen and nitrogen, helium gas consists of helium atoms.

Note that oxygen gas is not a compound. Even though it is made of two atoms combined, they are two of the same atoms. A compound is formed when atoms of at least two different elements combine with each other. See the CONTENT TIP for Standard 1f for an explanation of the differences between a compound and a mixture.

The order in which the standards were written does not imply that they are supposed to be taught in that sequence. As in all the strands, these standards can be taught in many ways and in many sequences. The concept map below provides one way to organize these standards. The wording of some of the standards has been slightly changed for space reasons and to emphasize a particular conceptual flow.

[A different concept map for these standards is on page 49 of the book "Making Connections" available from the California Science Teachers Association (CSTA).]

Note that in the concept map above, a combination of Standards 1a and 1b plays a central organizing role. Atoms combine to form molecules, and these molecules have different properties than those of the reacting atoms. We build to this concept by examining metals and non-metals, and comparing their properties. These observations lead to an understanding of the periodic table, which, in turn, deepens the knowledge of the nature of metals and non-metals (hence the double arrow).

The central molecular standard provides the foundation for understanding compounds, mixtures, chemical reactions, and the physical states of matter (solid, liquid, gas). The properties of salt provide an excellent example of how a compound has vastly different properties than its constituent atoms. Safe, crystalline table salt is composed of a green, toxic gas (chlorine) and a metal (sodium) that bursts into flame when it encounters water.

This general principle that the whole has properties that are qualitatively different than those of the parts is what makes it possible to have such a diverse and complex world that consists of just a few elements. Most of the beings and things we encounter consist primarily of about a dozen different elements that combine with each other in myriad ways to produce millions of different kinds of things. The Grade 5 Life Sciences standards also illustrate this major principle that parts come together to form systems (such as the circulatory system) that have properties that are qualitatively different than those of the parts.