Chapter 2 active reading guide the chemical context of life this chapter covers the basics that you may have learned in your chemistry class. whether your teacher goes over this chapter, or assigns it for you to review on your own, the questions that follow should you focus on the most important points. section 1 1. define and give an example of the following terms: mr. croft term definition matter: element: compound: 2. what four elements make up 96% of all living matter? 3. what is the difference between an essential element and a trace element? section 2 4. sketch a model of an atom of helium, showing the electrons, protons, neutrons, and atomic nucleus.

1. Element is a simple substance that can't be converted or broken down into simpler substances. Ex: Oxygen (O). The compound is a substance formed when two or more elements are bonded. Ex: sodium chloride (NaCl).

2. Carbon, hydrogen, oxygen, and nitrogen.

3. Trace elements are only required in only minutes quantities, such as iron. Essential elements are required in high quantities, such as oxygen.

4. See image

Explanation:

2) These four elements are essential for life, they form DNA, RNA, proteins and other substances.

4) Helium is on group 8 and the first period of the periodic table and it has atomic number 2, so it has 2 protons and 2 electrons. The mass of helium is 4, so it also has 2 neutrons (mass = neutrons + protons). Only the first shell has electrons ( shell K). In the figure, the circle is the shell, in yellow, is represented the electrons, in green the neutrons and red the protons.

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.