1. Atoms, Element, Compounds, & Chemical Reactions
Section Objectives:
Relate the structure of an atom to the identity of elements.
Relate the formation of covalent and ionic chemical bonds to the stability of atoms

2. Atoms: The Building Blocks of Elements
An atom is the smallest particle of an element that has the characteristics of that element.
Atoms are the basic building blocks of all matter.
An atom is made up of protons ( + charge) and neutrons (no charge) located in a central nucleus
The nucleus is surrounded by electrons ( - charge)

3. The Structure of an atom
The region of space surrounding the nucleus contains extremely small, negatively charged particles called electrons (e-)
Because opposites attract, the negatively charged electrons are held in the electron cloud by the positively charged nucleus.
Nucleus
This region of space is referred to as an electron cloud.
Electron energy levels

4. Elements
An element is a substance that can’t be broken down into simpler chemical substances.
Everything – whether it is a rock, frog, or flower – is made of substances called elements

5. Natural elements in living things
Carbon, hydrogen, oxygen, and nitrogen make up more than 96 percent of the mass of a human body
Of the naturally occurring elements on Earth, only about 25 are essential to living organisms.

6. Isotopes of an Element
Atoms of the same element always have the same number of protons but may contain different numbers of neutrons.
Atoms of the same element that have different numbers of neutrons are called isotopes of that element

7. Radioactive isotopes can be used to help
Radioactive isotopes can be useful tracers for studying biological processes
PET scanners use radioactive isotopes to create anatomical images

8. Compounds
A compound is a substance that is composed of atoms of two or more different elements that are chemically combined
Table salt (NaCl) is a compound composed of the elements sodium and chlorine.

9. Electron arrangement determines the chemical properties of an atom
Electrons are arranged in shells
The outermost shell determines the chemical properties of an atom
In most atoms, a full outer shell holds eight electrons
Electrons occur in energy levels called electron shells
Information about the distribution of electrons is found in the periodic table of the elements
Atoms whose shells are not full tend to interact with other atoms and gain, lose, or share electrons

10. Electron arrangement determines the chemical properties of an atom
Atoms want to fill their outer electron shells
To accomplish this, the atom can share, donate, or receive electrons
This results in attractions between atoms called chemical bonds
Chemical bonds cause atoms to stay close together. Recall that a compound is a substance consisting of two or more different elements combined in a fixed ratio. Chemical bonds are responsible for holding atoms together.
Student Misconceptions
1. The dangers posed by certain chemicals in our food and broader environment often misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why natural does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals.
Copyright © 2009 Pearson Education, Inc.

11. How covalent bonds form
Atoms combine with other atoms only when the resulting compound is more stable than the individual atoms.
For many elements, an atom becomes stable when its outermost energy level is full.
Sharing electrons with other atoms is one way for elements to become stable.

12. How covalent bonds form
Two hydrogen atoms can combine with each other by sharing their electrons.
Each atom becomes stable by sharing its electron with the other atom.
Hydrogen molecule

13. How covalent bonds form
Water molecule
A covalent bond holds the two hydrogen atoms together.
A molecule is a group of atoms held together by covalent bonds.
It has no overall charge.

14. How ionic bonds form
An atom (or group of atoms) that gains or loses electrons has an electrical charge and is called an ion. An ion is a charged particle made of atoms.
The attractive force between two ions of opposite charge is known as an ionic bond.

15. Chemical Reactions
Chemical reactions occur when bonds are formed or broken, causing substances to recombine into different substances.
Ionic bond + +
Chlorine atom
Sodium+ Ion
Chlorine― ion
Sodium atom

16. Chemical Reactions
All of the chemical reactions that occur within an organism are referred to as that organism’s metabolism.