# Atomic Structure Distinguishing Among Atoms Prentice-Hall Chapter 4.3 Dr. Yager.

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Atomic Structure Distinguishing Among Atoms Prentice-Hall Chapter 4.3 Dr. Yager

Objectives  Explain what makes elements and isotopes different from each other.  Calculate the number of neutrons in an atom.  Calculate the atomic mass of an element.  Explain why chemists use the periodic table.

Elements are different because they contain different numbers of protons. The atomic number of an element is the number of protons in the nucleus of an atom of that element. Note: Electrons equal the number of protons in a neutral atom

19 5 16 B 5 23

The total number of protons and neutrons in an atom is called the mass number. The number of neutrons in an atom is the difference between the mass number and atomic number.

Au is the chemical symbol for gold. Mass Number Atomic Number

Mini-Quiz

Isotopes Isotopes are atoms that have the same number of protons but different numbers of neutrons. Because isotopes of an element have different numbers of neutrons, they also have different mass numbers.

Despite these differences, isotopes are chemically alike because they have identical numbers of protons and electrons.

Mini-Quiz

Atomic Mass It is useful to compare the relative masses of atoms to a standard reference isotope. Carbon-12 is the standard reference isotope. Carbon-12 has a mass of exactly 12 atomic mass units. An atomic mass unit (amu) is defined as exactly one twelfth of the mass of a carbon- 12 atom.

Atomic Mass The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element. A weighted average mass reflects both the mass and the relative abundance of the isotopes as they occur in nature.

Weighted Average Mass of a Chlorine Atom

The atomic mass of copper is 63.546 amu. Which isotope is more abundant, Copper-63 or Copper-65?

Calculating the Relative Abundance of Cu 62.9 amu (x) + 64.9 amu (1- x) = 63.546 amu 62.9 amu (x) + 64.9 amu (1- x) = 63.546 amu x = 69.2% x = 69.2% 1- x = 30.8% 1- x = 30.8%

1. Isotopes of an element have 1. Isotopes of an element have a)the same mass number. b)different atomic numbers. c)the same number of protons but different numbers of neutrons. d)the same number of protons but different numbers of electrons.

1. Isotopes of an element have 1. Isotopes of an element have a)the same mass number. b)different atomic numbers. c)the same number of protons but different numbers of neutrons. d)the same number of protons but different numbers of electrons.

2. How many neutrons are in sulfur-33? 2. How many neutrons are in sulfur-33? a)16 neutrons b)33 neutrons c)17 neutrons d)32.06 neutrons

2. How many neutrons are in sulfur-33? 2. How many neutrons are in sulfur-33? a)16 neutrons b)33 neutrons c)17 neutrons d)32.06 neutrons

3. If sulfur contained 90.0% sulfur-32 and 10.0% sulfur-34, its atomic mass would be a)32.2 amu. b)32.4 amu. c)33.0 amu. d)35.4 amu.

3. If sulfur contained 90.0% sulfur-32 and 10.0% sulfur-34, its atomic mass would be a)32.2 amu. b)32.4 amu. c)33.0 amu. d)35.4 amu.

Mass Spectrometer

The Mass Spectrum for Boron With 123 atoms of Boron: 23 are Boron-10 and 100 are Boron-11 The total mass is (23 x 10) + (100 x 11) = 1330 amu The average mass is 1330 / 123 = 10.8 amu Therefore the atomic mass is 10.8 amu

The Mass Spectrum for Zirconium (Zr) Isotopes of Zr Zr-90 51.5% Zr-91 11.2% Zr-92 17.1% Zr-94 17.4% Zr-96 2.8% Atomic Mass = 0.515(90) + 0.112(91) + 0.171(92) + 0.174(94) + 0.028(96)

Listing of known isotopes

Periodic Table A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties. A periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements).

Periodic Table

Period Each horizontal row of the periodic table is called a period. Within a given period, the properties of the elements vary as you move across it from element to element.

A Period

Group Each vertical column of the periodic table is called a group, or family. Elements within a group have similar chemical and physical properties.

A Group

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