Tro3 lecture 04

Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA Introductory Chemistry , 3 rd Edition Nivaldo Tro Chapter 4 Atoms and Elements 2009, Prentice Hall

Experiencing Atoms There are about 91 elements found in nature. Over 20 have been made in laboratories. Each has its own, unique kind of atom. They have different structures. Therefore they have different properties. Tro's "Introductory Chemistry", Chapter 4

Dalton’s Atomic Theory Each Element is composed of tiny, indestructible particles called atoms. Tiny, hard, indivisible, spheres. All atoms of an element are identical. They have the same mass, volume, and other physical and chemical properties. So, atoms of different elements are different. Every carbon atom is identical to every other carbon atom. They have the same chemical and physical properties. However, carbon atoms are different from sulfur atoms. They have different chemical and physical properties. Tro's "Introductory Chemistry", Chapter 4

Dalton’s Atomic Theory Atoms combine in simple, whole-number ratios to form molecules of compounds. Because atoms are unbreakable, they must combine as whole atoms. The nature of the atom determines the ratios in which it combines. Each molecule of a compound contains the exact same types and numbers of atoms. Law of Constant Composition Chemical formulas Tro's "Introductory Chemistry", Chapter 4

Some Notes on Charges There are two kinds of charges, called positive and negative. Opposite charges attract. + attracted to –. Like charges repel. + repels +. – repels –. To be neutral, something must have no charge or equal amounts of opposite charges. Tro's "Introductory Chemistry", Chapter 4

Structure of the Nucleus Rutherford proposed that the nucleus had a particle that had the same amount of charge as an electron but opposite sign. Based on measurements of the nuclear charge of the elements. These particles are called protons . Protons have a charge of +1 c.u. and a mass of 1 amu. Since protons and electrons have the same amount of charge, for the atom to be neutral, there must be equal numbers of protons and electrons. Tro's "Introductory Chemistry", Chapter 4

Some Problems How could beryllium have 4 protons stuck together in the nucleus? Shouldn’t they repel each other? If a beryllium atom has 4 protons, then it should weigh 4 amu, but it actually weighs 9.01 amu! Where is the extra mass coming from? Each proton weighs 1 amu. Remember: The electron’s mass is only about 0.00055 amu and Be has only 4 electrons—it can’t account for the extra 5 amu of mass. Tro's "Introductory Chemistry", Chapter 4

There Must Be Something Else There To answer these questions, Rutherford proposed that there was another particle in the nucleus—it is called a neutron . Neutrons have no charge and a mass of 1 amu. The masses of the proton and neutron are both approximately 1 amu. Tro's "Introductory Chemistry", Chapter 4

The Modern Atom We know atoms are composed of three main pieces—protons, neutrons, and electrons. The nucleus contains protons and neutrons. The nucleus is only about 10 -13 cm in diameter. The electrons move outside the nucleus with an average distance of about 10 -8 cm. Therefore, the radius of the atom is about 10 5 times larger than the radius of the nucleus. Tro's "Introductory Chemistry", Chapter 4

The Nature of Electrical Charge Electrical charge is a fundamental property of protons and electrons. Positively and negatively charged objects attract each other. Like charged objects repel each other. + to +, or  to  . When a proton and electron are paired, the result is a neutral charge. Because they have equal amounts of charge. Tro's "Introductory Chemistry", Chapter 4

Tro's "Introductory Chemistry", Chapter 4

Practice—An Atom Has 20 Protons. Determine if Each of the Following Statements Is True or False? If it is a neutral atom, it will have 20 electrons. If it also has 20 neutrons, its mass will be approximately 40 amu. If it has 18 electrons, it will have a net 2  charge. Tro's "Introductory Chemistry", Chapter 4 True True False

Elements Each element has a unique number of protons in its nucleus. All carbon atoms have 6 protons in their nuclei. The number of protons in the nucleus of an atom is called the atomic number . Z is the short-hand designation for the atomic number. Because each element’s atoms have a unique number of protons, each element can be identified by its atomic number . The elements are arranged on the Periodic Table in order of their atomic numbers. Each element has a unique name and symbol. The symbol is either one or two letters One capital letter or one capital letter + one lower case letter.

The Periodic Table of Elements Tro's "Introductory Chemistry", Chapter 4 Atomic number Atomic mass Element symbol

Review What is the atomic number of boron, B? What is the atomic mass of silicon, Si? How many protons does a chlorine atom have? How many electrons does a neutral neon atom have? Will an atom with 6 protons, 6 neutrons, and 6 electrons be electrically neutral? Will an atom with 27 protons, 32 neutrons, and 27 electrons be electrically neutral? Will an Na atom with 10 electrons be electrically neutral? Tro's "Introductory Chemistry", Chapter 4

Review What is the atomic number of boron, B? 5 What is the atomic mass of silicon, Si? 28.09 amu How many protons does a chlorine atom have? 17 How many electrons does a neutral neon atom have? 10 Will an atom with 6 protons, 6 neutrons and 6 electrons be electrically neutral? Yes Will an atom with 27 protons, 32 neutrons, and 27 electrons be electrically neutral? Yes Will an Na atom with 10 electrons be electrically neutral? No Tro's "Introductory Chemistry", Chapter 4

Periodicity Tro's "Introductory Chemistry", Chapter 4 = Metal = Metalloid = Nonmetal

Practice—Classify Each Element as Metal, Nonmetal, or Metalloid. Xenon, Xe Tungsten, W Bromine, Br Arsenic, As Cerium, Ce Tro's "Introductory Chemistry", Chapter 4 Nonmetal Metal Nonmetal Metalloid Metal

The Modern Periodic Table Elements with similar chemical and physical properties are in the same column. Columns are called Groups or Families . Designated by a number and letter at top. Rows are called Periods . Each period shows the pattern of properties repeated in the next period. Tro's "Introductory Chemistry", Chapter 4

The Modern Periodic Table, Continued Main group = representative elements = “A” groups. Transition elements = “B” groups. All metals. Bottom rows = inner transition elements = rare earth elements. Metals Really belong in periods 6 and 7. Tro's "Introductory Chemistry", Chapter 4

Tro's "Introductory Chemistry", Chapter 4 = Halogens = Lanthanides = Actinides = Alkali metals = Alkali earth metals = Noble gases = Transition metals

Important Groups— Alkali Metals Group IA = Alkali metals. Hydrogen is usually placed here, though it doesn’t belong. Soft, low melting points, low density. Flame tests: Li = red, Na = yellow, and K = violet. Very reactive, never found uncombined in nature. Tend to form water soluble compounds that are crystallized from seawater then molten salt electrolyzed. Colorless solutions. React with water to form basic (alkaline) solutions and H 2 : 2 Na + 2 H 2 O  2 NaOH + H 2 Releases a lot of heat. Tro's "Introductory Chemistry", Chapter 4 lithium sodium potassium rubidium cesium

Important Groups—Alkali Earth Metals Group IIA = Alkali earth metals. Harder, higher melting, and denser than alkali metals. Mg alloys used as structural materials. Flame tests: Ca = red, Sr = red, and Ba = yellow-green. Reactive, but less than corresponding alkali metal. Form stable, insoluble oxides from which they are normally extracted. Oxides are basic = alkaline earth. Reactivity with water to form H 2 : Be = none, Mg = steam, Ca, Sr, Ba = cold water. Tro's "Introductory Chemistry", Chapter 4 magnesium calcium beryllium strontium barium

Important Groups—Halogens Group VIIA = Halogens. Nonmetals. F 2 and Cl 2 gases, Br 2 liquid, and I 2 solid. All diatomic. Very reactive. Cl 2, and Br 2 react slowly with water: Br 2 + H 2 O  HBr + HOBr React with metals to form ionic compounds. hydrogen halides all acids: HF weak < HCl < HBr < HI. bromine iodine chlorine fluorine

Important Groups—Noble Gases Group VIIIA = Noble gases. All gases at room temperature. Very low melting and boiling points. Very unreactive, practically inert. Very hard to remove electron from or give an electron to. Tro's "Introductory Chemistry", Chapter 4

Charged Atoms The number of protons determines the element. All sodium atoms have 11 protons in the nucleus. In a chemical change, the number of protons in the nucleus of the atom doesn’t change. No transmutation during a chemical change!! During radioactive and nuclear changes, atoms do transmute. Atoms in a compound are often electrically charged, these are called ions . Tro's "Introductory Chemistry", Chapter 4

Ions Atoms acquire a charge by gaining or losing electrons. Not protons! Ion charge = # protons – # electrons. Ions with a positive charge are called cations . More protons than electrons. Form by losing electrons. Ions with a negative charge are called anions . More electrons than protons. Form by gaining electrons. Chemically, ions are much different than the neutral atoms. Because they have a different structure. Tro's "Introductory Chemistry", Chapter 4

Atomic Structures of Ions Nonmetals form anions. For each negative charge, the ion has 1 more electron than the neutral atom. F = 9 p + and 9 e  ; F ─ = 9 p + and 10 e  . P = 15 p + and 15 e  ; P 3─ = 15 p + and 18 e  . Anions are named by changing the ending of the name to –ide . fluorine F + 1e   F ─ fluoride ion oxygen O + 2e   O 2─ oxide ion The charge on an anion can often be determined from the group number on the periodic table. Group 7A  1  , Group 6A  2  . Tro's "Introductory Chemistry", Chapter 4

Atomic Structures of Ions, Continued Metals form cations . For each positive charge the ion has 1 less electron than the neutral atom. Na atom = 11 p + and 11 e  ; Na + ion = 11 p + and 10 e  . Ca atom = 20 p + and 20 e  ; Ca 2+ ion = 20 p + and 18 e  . Cations are named the same as the metal. sodium Na  Na + + 1e  sodium ion calcium Ca  Ca 2+ + 2e  calcium ion The charge on a cation can often be determined from the group number on the periodic table. Group 1A  1+, Group 2A  2+, (Al, Ga, In)  3+. Tro's "Introductory Chemistry", Chapter 4

Example 4.5: Find the number of protons and electrons in the Ca 2+ ion. Tro's "Introductory Chemistry", Chapter 4

Apply the solution map. Determine the atomic number of the element. Example: Find the number of protons and electrons in the Ca 2+ ion. Information: Given: Ca 2+ = calcium Find: #p and #e Solution Map: ion symbol  element  atomic number  #p  #e Tro's "Introductory Chemistry", Chapter 4 Ca = calcium Atomic number = 20

Apply the solution map. Determine the number of electrons in an atom of the element. Example: Find the number of protons and electrons in the Ca 2+ ion. Information: Given: Ca 2+ = calcium, Z = 20 Find: #p = 20 and #e Solution Map: ion symbol  element  atomic number  #p  #e Tro's "Introductory Chemistry", Chapter 4 Ca = calcium Atomic number = 20 #p = atomic number = 20 Ion charge = #p - #e +2 = 20 - #e -18 = - #e 18 = #e

Practice—Fill in the Table. Tro's "Introductory Chemistry", Chapter 4

Practice—Fill in the Table, Continued. Tro's "Introductory Chemistry", Chapter 4

Valence Electrons and Ion Charge The highest energy electrons in an atom are called the valence electrons . Metals form cations by losing their valence electrons to get the same number of electrons as the previous noble gas. Main group metals. Li + = 2 e  = He; Al 3+ = 10 e  = Ne. Nonmetals form anions by gaining electrons to have the same number of electrons as the next noble gas. Cl  = 18 e  = Ar; Se 2  = 36 e  = Kr. Tro's "Introductory Chemistry", Chapter 4

Ion Charge and the Periodic Table The charge on an ion can often be determined from an elements position on the periodic table. Metals are always positive ions, nonmetals are negative ions. For many main group metals, the cation charge = the group number. For nonmetals, the anion charge = the group number – 8. Tro's "Introductory Chemistry", Chapter 4

Tro's "Introductory Chemistry", Chapter 4 Li + Na + K + Rb + Cs + Be 2+ Mg 2+ Ca 2+ Sr 2+ Ba 2+ Al 3+ Ga 3+ In 3+ O 2  S 2  Se 2  Te 2  F  Cl  Br  I  N 3  P 3  As 3  1A 2A 3A 7A 6A 5A

Structure of the Nucleus Soddy discovered that the same element could have atoms with different masses, which he called isotopes . There are two isotopes of chlorine found in nature, one that has a mass of about 35 amu and another that weighs about 37 amu. The observed mass is a weighted average of the weights of all the naturally occurring atoms. The atomic mass of chlorine is 35.45 amu. Tro's "Introductory Chemistry", Chapter 4

Isotopes All isotopes of an element are chemically identical. Undergo the exact same chemical reactions. All isotopes of an element have the same number of protons. Isotopes of an element have different masses. Isotopes of an element have different numbers of neutrons. Isotopes are identified by their mass numbers . Protons + neutrons. Tro's "Introductory Chemistry", Chapter 4

Isotopes, Continued Atomic Number. Number of protons. Z Mass Number = Protons + Neutrons. Whole number. A Percent natural abundance = Relative amount found in a sample.

Neon Tro's "Introductory Chemistry", Chapter 4 9.25% 22 12 10 Ne-22 or 0.27% 21 11 10 Ne-21 or 90.48% 20 10 10 Ne-20 or Percent natural abundance A, mass number Number of neutrons Number of protons Symbol

Isotopes Cl-35 makes up about 75% of chlorine atoms in nature, and Cl-37 makes up the remaining 25%. The average atomic mass of Cl is 35.45 amu. Cl-35 has a mass number = 35, 17 protons and 18 neutrons (35 - 17). Tro's "Introductory Chemistry", Chapter 4 Atomic symbol A = Mass number Z = Atomic number A X Z = X-A Cl 35 17

Example 4.8: How many protons and neutrons in the chromium isotope ? Tro's "Introductory Chemistry", Chapter 4

Apply the solution map. Determine the number of neutrons in an atom of the element. Information: Given: = calcium Find: #p and #n Solution Map: ion symbol  element  atomic number  #p  #n Example: How many protons and neutrons in the chromium isotope ? Tro's "Introductory Chemistry", Chapter 4 Cr = chromium Atomic Number = 24 #p = atomic number = 24 Mass Number = #p + #n 52 = 24 + #n 28 = #n

Practice—Complete the Following Table. Tro's "Introductory Chemistry", Chapter 4

Practice—Complete the Following Table, Continued. Tro's "Introductory Chemistry", Chapter 4

Tro3 lecture 04

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