Unit 3 Lesson 3 Electrons and Chemical Bonding

Unit 3 Lesson 3 Electrons and Chemical Bonding Bound to Change How do atoms join together? • A huge variety of substances are possible because atoms join together by forming chemical bonds. • A chemical bond is an interaction that holds atoms or ions together. • A group of atoms that are held together by chemical bonds is called a molecule. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding What happens to atoms during chemical changes? • Chemical changes change the identity of substances. • Chemical changes do not create or destroy atoms. In a chemical change, atoms are rearranged to make new substances with different properties. • In order for atoms to be rearranged, chemical bonds have to be formed or broken. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding A Model Atom What do different atom models show? • The electron cloud model of an atom shows how electrons are found in a region around the nucleus. • This model helps to show the general locations of the different parts of the atom. • However, it does not show the number of electrons. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding What do different atom models show? • Where are the nucleus and the electrons in the electron cloud model of a lithium atom? Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding What do different atom models show? • The Bohr model of an atom shows the number of electrons in an atom. • The electrons are shown as dots placed in rings around the nucleus, with each ring representing an energy level. • Showing the electrons in energy levels helps predict how and why atoms form chemical bonds. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding What do different atom models show? • How are electrons distributed in the Bohr model of an oxygen atom? Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding What do different atom models show? • In the space-filling model of a molecule, atoms are represented as solid spheres. • The spheres do not show the parts that make up the atoms. • However, this model clearly shows how the atoms are connected to each other. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding What are valence electrons? • Bohr models show electrons in energy levels, each of which can hold a specific number of electrons. • The energy level furthest from the nucleus is called the outermost energy level. • Electrons found in the outermost energy level of an atom are called valence electrons. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding Fill It Up! How does the periodic table show the number of valence electrons? • Elements in the same group, or vertical column, have the same number of valence electrons. • You can use the group number to find the number of valence electrons in Groups 1, 2, and 13– 18. • There is no simple rule to find the number of valence electrons in Groups 3– 12. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding How does the periodic table show the number of valence electrons? • Determine the number of valence electrons in atoms of carbon (C), nitrogen (N), and fluorine (F). Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding Why do atoms form bonds? • An atom tends to form bonds if its outermost energy level is not full. • Atoms that have fewer than eight valence electrons, except for helium, do not have a full outermost energy level. • These atoms gain, lose, or share valence electrons to form bonds. Copyright © Houghton Mifflin Harcourt Publishing Company

Unit 3 Lesson 3 Electrons and Chemical Bonding Why do atoms form bonds? • Forming bonds allows atoms to fill their outermost energy level. • An ion is a charged particle that forms when an atom loses or gains an electron. • The ions formed by chlorine and sodium both have eight valence electrons. Bonds hold the negative and positive ions together, producing sodium chloride. Copyright © Houghton Mifflin Harcourt Publishing Company