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 6.1 & 6.2 pg Atoms, Element, Compounds, & Chemical Reactions

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)

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 Electron energy levels This region of space is referred to as an electron cloud.

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

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.

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

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

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.

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

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 Copyright © 2009 Pearson Education, Inc.

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.

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

How covalent bonds form 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. Water molecule

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.

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

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