Chapter 3 Atoms and Elements 3.4 The Atom 1

The Law of Multiple Proportions and Dalton’s Atomic Theory Elements are made up of tiny particles called atoms. Each element is characterized by the mass of its atoms. Atoms of the same element have the same mass, but atoms of different elements have different masses. The chemical combination of elements to make different chemical compounds occurs when atoms join in small whole-number ratios. Chemical reactions only rearrange how atoms are combined in chemical compounds; the atoms themselves don’t change.

Subatomic Particles Atoms contain subatomic particles. Protons have a positive (+) charge. Electrons have a negative (-) charge. Neutrons are neutral. Like charges repel and unlike charges attract. 3

Atomic Structure: Electrons Cathode-Ray Tubes: J. J. Thomson ( ) proposed that cathode rays must consist of tiny negatively charged particles. We now call them electrons.

Rutherford’s Gold Foil Experiment In Rutherford’s gold foil experiment, positively charged particles were aimed at atoms of gold. mostly went straight through the atoms. were deflected only occasionally. Conclusion: There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close. 5

Rutherford’s Gold Foil Experiment 6 Copyright © 2009 by Pearson Education, Inc.

Structure of the Atom An atom consists of a nucleus that contains protons and neutrons. of electrons in a large, empty space around the nucleus. 7

Atomic Mass Scale On the atomic mass scale, 1 atomic mass unit (amu) has a mass equal to 1/12 of the mass of the carbon-12 atom. a proton has a mass of about 1 (1.007) amu. a neutron has a mass of about 1 (1.008) amu. an electron has a very small mass, amu. 8

Particles in the Atom 9

Examples Identify each statement as describing a 1) proton, 2) neutron, or 3) electron. A. found outside the nucleus B. has a positive charge C. is neutral D. found in the nucleus 10

Examples Is each of the following statements true or false? Protons are heavier than electrons Protons are attracted to neutrons Electrons are small that they have no electrical charge The nucleus contains all the protons and neutrons of an atom 11

Chapter 3 Atoms and Elements 3.5 Atomic Number and Mass Number 12

Atomic Number The atomic number is specific for each element. is the same for all atoms of an element. is equal to the number of protons in an atom. appears above the smbol of an element. 13

Atomic Number and Protons Examples of atomic number and number of protons: Hydrogen has atomic number 1; every H atom has one proton. Carbon has atomic number 6; every C atom has six protons. Copper has atomic number 29; every Cu atom has 29 protons. Gold has atomic number 79; every Au atom has 79 protons. 14

Examples State the number of protons in each. A. A nitrogen atom 1) 5 protons 2) 7 protons 3) 14 protons B. A sulfur atom 1) 32 protons 2) 16 protons 3) 6 protons C. A barium atom 1) 137 protons 2) 81 protons 3) 56 protons 15

Electrons in An Atom An atom of an element is electrically neutral; the net charge of an atom is zero. has an equal number of protons and electrons. number of protons = number of electrons Aluminum has 13 protons and 13 electrons. The net (overall) charge is zero. 13 protons (13+) + 13 electrons (13 -) = 0 16

Mass Number The mass number represents the number of particles in the nucleus. is equal to the number of protons + the number of neutrons. 17

Atomic Models 18

Examples An atom of zinc has a mass number of 65. A. How many protons are in this zinc atom? 1) 302) 353) 65 B. How many neutrons are in the zinc atom? 1) 302) 353) 65 C. What is the mass number of a zinc atom that has 37 neutrons? 1) 372) 653) 67 19

Examples An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 142) 163) 34 B. Its mass number is 1) 142) 163) 34 C. The element is 1) Si2) Ca3) Se 20

Chapter 3Atoms and Elements 3.6 Isotopes and Atomic Mass 21  24 Mg 25 Mg 26 Mg

Isotopes are atoms of the same element that have different mass numbers. have the same number of protons, but different numbers of neutrons. 22

Atomic Symbol An atomic symbol represents a particular atom of an element. gives the mass number in the upper left corner and the atomic number in the lower left corner. 23

Atomic Numbers carbon-14 C 14 6 atomic number mass number carbon-12 C 12 6 atomic number mass number 6 protons 6 electrons 8 neutrons 6 protons 6 electrons 6 neutrons

Information from Atomic Symbols The atomic symbol for a specific atom of an element gives the number of protons (p + ), number of neutrons (n), and number of electrons (e - ). 25

Information from Atomic Symbols Examples of number of subatomic particles for atoms Atomic symbol O P Zn p + 15 p + 30 p + 8 n16 n 35 n 8 e - 15 e - 30 e - 26

Examples Naturally occurring carbon consists of three isotopes: 12 C, 13 C, and 14 C. State the number of protons, neutrons, and electrons in each of the following: 12 C 13 C 14 C protons ______ ______ ______ neutrons ______ ______ ______ electrons______ ______ ______ 27

Examples Write the atomic symbols for atoms with the following subatomic particles: A. 8 p +, 8 n, 8 e - ___________ B. 17p +, 20n, 17e - ___________ C. 47p +, 60 n, 47 e - ___________ 28

Examples 1. Which of the pairs are isotopes of the same element? 2. In which of the pairs do both atoms have 8 neutrons? A. 15 X 15 X 8 7 B. 12 X 14 X 6 6 C. 15 X 16 X

Isotopes of Magnesium 30

Isotopes of Magnesium 31

Isotopes of Sulfur A sample of naturally occurring sulfur contains several isotopes with the following abundances Isotope % abundance 32 S S S S S, 33 S, 34 S, 36 S Copyright © 2009 by Pearson Education, Inc.

Atomic Mass The atomic mass of an element is listed below the symbol of each element on the periodic table. gives the mass of an “average” atom of each element compared to 12 C. is not the same as the mass number. 33 Na 22.99

Isotopes of Some Elements and Their Atomic Mass 34 Most elements have two or more isotopes that contribute to the atomic mass of that element.

Atomic Mass for Cl The atomic mass of chlorine is due to all the Cl isotopes. not a whole number. the average of two isotopes: 35 Cl and 37 Cl. 35

Learning Check Using the periodic table, specify the atomic mass of each element. A.calcium__________ B. aluminum__________ C. lead__________ D. barium__________ E. iron__________ 36

Calculating Atomic Mass The calculation for atomic mass requires the percent(%) abundance of each isotope. atomic mass of each isotope of that element. sum of the weighted averages. mass of isotope(1)x (%) + mass of isotope(2) x (%)

Atomic Masses carbon-12:98.89 % natural abundance12 amu carbon-13:1.11 % natural abundance amu Why is the atomic mass of the element carbon amu? = amu mass of carbon = (12 amu)(0.9889) + ( amu)( ) = amu amu

Calculating Atomic Mass for Cl 35 Cl has atomic mass amu (75.76%) and 37 C has atomic mass amu (24.24%). Use atomic mass and percent of each isotope to calculate the contribution of each isotope to the weighted average. Atomic mass 35 Cl x % abundance = Atomic mass 37 Cl x % abundance = Sum is atomic mass of Cl is 39

Learning Check Gallium is an element found in lasers used in compact disc players. In a sample of gallium, there is 60.10% of 69 Ga (atomic mass ) atoms and 39.90% of 71 Ga (atomic mass ) atoms. What is the atomic mass of gallium? 40