Lecture Presentation Chapter 2 Atoms, Molecules, and Ions Dr. Subhash C. Goel South GA State College...

Lecture Presentation

Chapter 2

Atoms, Molecules,and Ions

Dr. Subhash C. GoelSouth GA State College

Douglas, GA© 2012 Pearson Education, Inc.

Copyright © Cengage Learning. All rights reserved. 2 | 3

Law of Multiple Proportions

When two elements form more than one compound, the masses of one element in these compounds for a fixed mass of the other element are in small, whole-number ratios.

Atoms,Molecules,and Ions© 2012 Pearson Education, Inc.

Law of Conservation of Mass

The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place.

Atoms,Molecules,and Ions


Structure of the AtomThe atom is composed of two parts:

Nucleus

Central core

Positively charged

Contains most of the atom’s mass

Electrons

Very light

Negatively charged

Exist in the region around the nucleus


Discovery of the Electron

J. J. Thomson used an apparatus similar to the one shown on the next slide, a cathode ray tube.

He discovered that the particles that make up the cathode ray are negative and are part of all matter. This finding is illustrated on the following slides.

As a result, Thomson concluded that atoms are not indivisible, as Dalton had postulated.


Cathode Ray Tube


Cathode rays are attracted to the positive end of the magnet and repelled by the negative end.


The Electron

Thomson measured the charge/mass ratio of the electron to be 1.76 108 coulombs/gram (C/g).

Robert Millikan calculated the charge on the electron. He determined the charge on various drops of oil and found the smallest increment in charge—that is, the charge on the electron. This is illustrated on the next slide.


Radioactivity

• Radioactivity is the spontaneous emission of radiation by an atom.

• It was first observed by Henri Becquerel.• Marie and Pierre Curie also studied it.


The Atom, circa 1900

• The prevailing theory was that of the “plum pudding” model, put forward by Thomson.

• It featured a positive sphere of matter with negative electrons imbedded in it.

1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron (-)3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

2.2


The Nuclear Atom

• Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom.

• Most of the volume of the atom is empty space.

Chadwick’s Experiment (1932)(1935 Noble Prize in Physics)

H atoms - 1 p; He atoms - 2 p

mass He/mass H should = 2

measured mass He/mass H = 4

+ 9Be 1n + 12C + energy

neutron (n) is neutral (charge = 0)

n mass ~ p mass = 1.67 x 10-24 g2.2


Subatomic Particles

• Protons and electrons are the only particles that have a charge.

• Protons and neutrons have essentially the same mass.

• The mass of an electron is so small we ignore it.


Exercise

1. How many protons, neutrons, and electrons are in (a) an atom of 197Au, (b) an atom of strontium-90?

2. How many protons, neutrons, and electrons are in (a) a 138Ba atom, (b) an atom of phosphorus-31?

© 2012 Pearson Education, Inc.


Atomic Symbol

An atomic symbol is a one- or two-letter notation used to represent an atom corresponding to a particular element.

The first letter must be UPPERCASE.

The second letter, when present, must be lowercase.

Symbols come from the element name; several names come from Latin.


Symbols of Elements

All atoms of the same element have the same number of protons, which is called the atomic number, Z.


Symbols of Elements

The mass of an atom in atomic mass units (amu) is the total number of protons and neutrons in the atom.


Isotopes

• Isotopes are atoms of the same element with different masses.

• Isotopes have different numbers of neutrons.

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

2.3

How many protons, neutrons, and electrons are in C14

6 ?

How many protons, neutrons, and electrons are in C11

6 ?

Exercise

1. Magnesium has three isotopes with mass numbers 24, 25, and 26. (a) Write the complete chemical symbol (superscript and subscript) for each. (b) How many neutrons are in an atom of each isotope?

2. Give the complete chemical symbol for the atom that contains 82 protons, 82 electrons, and 126 neutrons.


Atomic Mass

Atomic and molecular masses can be measured with great accuracy using a mass spectrometer.


Average Mass

• Because in the real world we use large amounts of atoms and molecules, we use average masses in calculations.

• Average mass is calculated from the isotopes of an element weighted by their relative abundances.


Exercise

Naturally occurring chlorine is 75.78% 35Cl (atomic mass 34.969 amu) and 24.22% 37Cl (atomic mass 36.966 amu).Calculate the atomic weight of chlorine.



Periodic Table

• The periodic table is a systematic catalog of the elements.

• Elements are arranged in order of atomic number.


Periodic Table

• The rows on the periodic chart are periods.• Columns are groups.• Elements in the same group have similar chemical

properties.


Periodicity

When one looks at the chemical properties of elements, one notices a repeating pattern of reactivities.


Groups

These five groups are known by their names.


Periodic Table

Nonmetals are on the right side of the periodic table (with the exception of H).

Metalloids border the stair-step line (with the exception of Al, Po, and At).

Metals are on the left side of the chart.


Exercise

1. Which two of these elements would you expect to show the greatest similarity in chemical and physical properties: B, Ca, F, He, Mg, P?

2. Locate Na (sodium) and Br (bromine) in the periodic table. Give the atomic number of each and classify each as metal, metalloid, or nonmetal.



Chemical Formulas

The subscript to the right of the symbol of an element tells the number of atoms of that element in one molecule of the compound.


Chemical Formulas

Molecular compounds are composed of molecules and almost always contain only nonmetals.


Diatomic Molecules

• These seven elements occur naturally as molecules containing two atoms:– Hydrogen– Nitrogen– Oxygen– Fluorine– Chlorine– Bromine– Iodine


Types of Formulas

• Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound.

• Molecular formulas give the exact number of atoms of each element in a compound.


Exercise

1. Write the empirical formulas for (a) glucose, a substance also known as either blood sugar or dextrose, molecular formula C6H12O6; (b) nitrous oxide, a substance used as an anesthetic and commonly called laughing gas, molecular formula N2O.

2. Give the empirical formula for diborane, whose molecular formula is B2H6.



Types of Formulas

• Structural formulas show the order in which atoms are bonded.

• Perspective drawings also show the three-dimensional array of atoms in a compound.


An ion is an electrically charged particle obtained from an atom or chemically bonded group of atoms by adding or removing one or more electrons.


A cation is a positively charged ion formed by losing one or more electrons.

A cation is named by its element name followed by the word ion.

For example, Na+ is the sodium ion, etc.


An anion is a negatively charged ion formed by gaining one or more electrons.

An anion is named by changing the last part of the element name to –ide followed by the word ion. For example, F- is the fluoride ion, O2- is the oxide ion, N3- is the nitride ion, etc.


An ionic compound is composed of cations and anions.

Ions are arranged in a repeating three-dimensional pattern, forming a crystal.

The formula of an ionic compound gives the smallest possible integer number of ions in the substance (without writing charges) so that the combination is electrically neutral.

The formula gives the formula unit of the compounds. A formula unit is not a molecule!


Writing Formulas

• Because compounds are electrically neutral, one can determine the formula of a compound this way:– The charge on the cation becomes the subscript

on the anion.– The charge on the anion becomes the subscript

on the cation.– If these subscripts are not in the lowest whole-

number ratio, divide them by the greatest common factor.

Formula of Ionic Compounds

Al2O3

2.6

2 x +3 = +6 3 x -2 = -6

Al3+ O2-

CaBr2

1 x +2 = +2 2 x -1 = -2

Ca2+ Br-

Na2CO3

2 x+1 = +2 1 x -2 = -2

Na+ CO32-

?


What is the formula of the ionic compound of Ca2+ and PO4

3-?

The common multiple of the charges is 6, so we need three Ca2+ and two PO4

3-. The resulting formula is

Ca3(PO4)2


Common Cations


Common Anions

13 protons, 10 (13 – 3) electrons

34 protons, 36 (34 + 2) electrons

Do You Understand Ions?

2.5

How many protons and electrons are in ?Al2713

3+

How many protons and electrons are in ?Se7834

2-

Exercise

• Predict the charge expected for the most stable ion of barium and the most stable ion of oxygen.

• Answer:

Exercise

1. Which of these compounds would you expect to be ionic: N2O, Na2O, CaCl2, SF4?

2. Which of these compounds are molecular: CBr4, FeS, P4O6, PbF2?


Inorganic Nomenclature

• Write the name of the cation.• If the anion is an element, change its

ending to -ide; if the anion is a polyatomic ion, simply write the name of the polyatomic ion.

• If the cation can have more than one possible charge, write the charge as a Roman numeral in parentheses.

Chemical Nomenclature• Ionic Compounds

– often a metal + nonmetal– anion (nonmetal), add “ide” to element name

BaCl2 barium chloride

K2O potassium oxide

Mg(OH)2 magnesium hydroxide

KNO3 potassium nitrate

2.7

• Transition metal ionic compounds– indicate charge on metal with Roman numerals

FeCl2 2 Cl- -2 so Fe is +2 iron(II) chloride

FeCl3 3 Cl- -3 so Fe is +3 iron(III) chloride

Cr2S3 3 S-2 -6 so Cr is +3 (6/2) chromium(III) sulfide

2.7

?


What are the names of the following ionic compounds?

BaO

Cr2(SO4)3

BaO is barium oxide.Cr2(SO4)3 is chromium(III) sulfate or chromic sulfate.

?


What are the chemical formulas for the following ionic compounds?

potassium carbonate

manganese(II) sulfate

The ions K+ and CO32- form K2CO3

The ions Mn2+ and SO42- form MnSO4


Exercise

1. Write the empirical formula of the compound formed by (a) Al3+ and Cl– ions, (b) Al3+ and O2–

ions, (c) Mg2+ and NO3– ions.

2. Write the empirical formula for the compound formed by (a) Na+ and PO4

3–, (b) Zn2+ and SO42–

(c) Fe3+ and CO32–.


Exercise

1. Name the ionic compounds (a) K2SO4, (b) Ba(OH)2, (c) FeCl3.

2. Name the ionic compounds (a) NH4Br, (b) Cr2O3, (c) Co(NO3)2.


Patterns in Oxyanion Nomenclature

• When there are two oxyanions involving the same element:– The one with fewer oxygens ends in -ite.– The one with more oxygens ends in -ate.

• NO2− : nitrite; SO3

2− : sulfite

• NO3− : nitrate; SO4

2− : sulfate


Patterns in Oxyanion Nomenclature

• Central atoms on the second row have bond to at most three oxygens; those on the third row take up to four.

• Charges increase as you go from right to left.


Patterns in Oxyanion Nomenclature• The one with the fewest oxygens has the prefix hypo-

and ends in -ite.– ClO− : hypochlorite

• The one with the second fewest oxygens ends in -ite.

– ClO2− : chlorite

• The one with the second most oxygens ends in -ate.

– ClO3− : chlorate

• The one with the most oxygens has the prefix per- and ends in -ate.

– ClO4− : perchlorate


Acid Nomenclature

• If the anion in the acid ends in -ide, change the ending to -ic acid and add the prefix hydro- .– HCl: hydrochloric acid– HBr: hydrobromic acid– HI: hydroiodic acid


Acid Nomenclature

• If the anion in the acid ends in -ite, change the ending to -ous acid.– HClO: hypochlorous acid

– HClO2: chlorous acid


Acid Nomenclature

• If the anion in the acid ends in -ate, change the ending to -ic acid.– HClO3: chloric acid

– HClO4: perchloric acid


Nomenclature of Binary Compounds

• The less electronegative atom is usually listed first.

• A prefix is used to denote the number of atoms of each element in the compound (mono- is not used on the first element listed, however) .


Nomenclature of Binary Compounds

The ending on the more electronegative element is changed to -ide.

CO2: carbon dioxide

CCl4: carbon tetrachloride

If the prefix ends with a or o and the name of the element begins with a vowel, the two

successive vowels are often elided into one.

N2O5: dinitrogen pentoxide

?


What are the names of the following compounds?

OF2

S4N4

BCl3

OF2 is oxygen difluoride

S4N4 is tetrasulfur tetranitride

BCl3 is boron trichloride

?


What are the formulas for the following binary molecular compounds?

carbon disulfide

nitrogen tribromide

dinitrogen tetrafluoride

The formula for carbon disulfide is CS2.

The formula for dinitrogen tetrafluoride is N2F4.

The formula for nitrogen tribromide is NBr3.


Nomenclature of Organic Compounds

• Organic chemistry is the study of carbon.• Organic chemistry has its own system of

nomenclature.



The simplest hydrocarbons (compounds containing only carbon and hydrogen) are alkanes.



The first part of the names just listed correspond to the number of carbons (meth- = 1, eth- = 2, prop- = 3, etc.).



• When a hydrogen in an alkane is replaced with something else (a functional group, like -OH in the compounds above), the name is derived from the name of the alkane.

• The ending denotes the type of compound.– An alcohol ends in -ol.

Lecture Presentation Chapter 2 Atoms, Molecules, and Ions Dr. Subhash C. Goel South GA State College...

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