Science · Grade 8 Science
Chapter 1: Atoms and the Particle Model
About a hundred kinds, and everything else is combinations.
Sodium is a soft metal that fizzes and can burst into flame in water. Chlorine is a choking, poisonous green gas. Joined together, they are the salt on your fries.
How can two dangerous elements make something safe to eat?
Make your guess to turn the page.
Still to come: 10 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Students expect a compound to be a blend of its ingredients, a little of each, because that is how mixtures and recipes behave. Iron and sulfur make the difference measurable: the magnet finds the iron in the mixture and almost none in the compound. Ask: if iron sulfide were a mixture, how much iron should the magnet find? The idea to reach is that bonding makes a new substance, so the properties of a compound tell you nothing about its elements.
Atoms and molecules — An atom is a single particle of an element; a molecule is two or more atoms bonded together. Oxygen gas is molecules of two oxygen atoms, and water is molecules of two hydrogen atoms and one oxygen atom. Sodium chloride and iron sulfide are not made of separate molecules but of a repeating arrangement of bonded atoms, which is why the book says "atoms bonded" rather than "molecules".
About the numbers — Iron and sulfur combine in the mass ratio 56 to 32, so 7 g of iron reacts with 4 g of sulfur. Heating must be done by a teacher in a fume hood, because some sulfur burns to sulfur dioxide, which is why the product weighs a little under 11 g. About 90 elements occur naturally, of 118 known. The trial results are a realistic illustration, not a published study.