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Chapter 4 Section 2 A Guided Tour of the Periodic Table Objectives • Relate the organization of the periodic table to the arrangement of electrons within an atom. • Explain why some atoms gain or lose electrons to form ions. • Determine how many protons, neutrons, and electrons an atom has, given its symbol, atomic number, and mass number. • Describe how the abundance of isotopes affects an element’s average atomic mass. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Organization of the Periodic Table • The periodic table groups similar elements together. • This organization makes it easier to predict the properties of an element based on where it is in the periodic table. • Elements are listed in order of number of protons, because the periodic law states that when elements are arranged this way, similarities in their properties will occur in a regular pattern. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Organization of the Periodic Table, continued • The periodic table helps determine electron arrangement. • Horizontal rows in the periodic table are called periods. • Just as the number of protons an atom has increases as you move from left to right across a period, so does its number of electrons. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Organization of the Periodic Table, continued • Elements in the same group have similar properties. • A group is a vertical column of elements in the periodic table. • Atoms of elements in the same group have the same number of valence electrons, so these elements have similar properties. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Some Atoms Form Ions • An ion is an atom or group of atoms that has lost or gained one electrons and has a negative or positive charge. • A lithium atom loses one electron to form a 1+ charged ion: • A fluorine atom gains one electron to form a 1 charged ion: Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table How Do the Structures of Atoms Differ? • The atomic number, Z, of an atom equals the number of protons in the nucleus. • The mass number, A, of an atom equals the number of protons plus the number of neutrons in the nucleus. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Nucleus Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Atomic Number Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Mass Number Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table How Do the Structures of Atoms Differ? continued • An isotope is an atom that has the same number of protons as other atoms of the same element do but that has a different number of neutrons. • Example: Hydrogen has three isotopes, shown below. • Some isotopes are more common than others. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table How Do the Structures of Atoms Differ? continued • If you know the atomic number and mass number of an atom, you can calculate the number of neutrons it has. • Example: uranium-235 has a mass number of 235. Like all uranium atoms, it has an atomic number of 92. The number of neutrons it has is therefore: Mass number (A): Atomic number (Z): Number of neutrons: Chapter menu 235 –92 143 Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table Isotopes Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Chapter 4 Section 2 A Guided Tour of the Periodic Table How Do the Structures of Atoms Differ? continued • Because the mass of a single atom is so tiny, atomic masses are usually expressed in atomic mass units. • An atomic mass unit (amu) is equal to one twelfth of the mass of a carbon-12 atom. • The average atomic mass for an element is a weighted average of the masses of all naturallyoccurring isotopes of an element. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.