# Chapter 3 Atoms and Elements 3.5 Atomic Number and Mass Number 1.

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Chapter 3 Atoms and Elements 3.5 Atomic Number and Mass Number 1

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 symbol of an element. 2

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. 3

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 4

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 5

Mass Number The mass number represents the number of particles in the nucleus. is equal to the number of protons + the number of neutrons. 6 6

Atomic Models 7

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 8

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 9

3.6 Isotopes and Atomic Mass Isotopes are atoms of the same element that have different mass numbers. have the same number of protons, but different numbers of neutrons. 10

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. 11

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 - ). 12

Information from Atomic Symbols Examples of number of subatomic particles for atoms Atomic symbol 16 31 65 O P Zn 8 15 30 8 p + 15 p + 30 p + 8 n16 n 35 n 8 e - 15 e - 30 e - 13

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 6 6 6 protons ______ ______ ______ neutrons ______ ______ ______ electrons______ ______ ______ 14

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 - ___________ 15

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 7 8 16

Isotopes of Magnesium 17 TABLE 3.7

Isotopes of Sulfur A sample of naturally occurring sulfur contains several isotopes with the following abundances Isotope % abundance 32 S 95.02 33 S 0.75 34 S 4.21 36 S 0.02 18 32 S, 33 S, 34 S, 36 S 16 16

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. has the smallest unit of amu 19 Na 22.99

Isotopes of Some Elements and Their Atomic Mass 20 Most elements have two or more isotopes that contribute to the atomic mass of that element. 8

Examples Using the periodic table, specify the atomic mass of each element. A.calcium__________ B. aluminum __________ C. lead__________ D. barium__________ E. iron__________ 21

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. 22

Atomic Masses carbon-12:98.89 % natural abundance12 amu carbon-13:1.11 % natural abundance13.0034 amu Why is the atomic mass of the element carbon 12.01 amu? = 12.01 amu mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111 ) = 11.87 amu + 0.144 amu

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. 24

Calculating Atomic Mass for Cl 35 Cl has atomic mass 34.97 amu (75.76%) and 37 C has atomic mass 36.97 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 25

Examples 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 68.926) atoms and 39.90% of 71 Ga (atomic mass 70.925) atoms. What is the atomic mass of gallium? 26

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