Chapter 2 Elements and Compounds. Malone and Dolter - Basic Concepts of Chemistry 9e2 Setting the Stage – Search for Water A key molecule we search for

Chapter 2Chapter 2

Elements and CompoundsElements and Compounds

Malone and Dolter - Basic Concepts of Chemistry 9e

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Setting the Stage – Search for Water

A key molecule we search for on other planets is water, as its presence suggests the possibility of life

Water is only one example of matter (albeit an important one)

Matter is anything that has mass and occupies space

Chemistry is the study of matter and the changes that it undergoes


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Setting a Goal - Part A The Elements and Their Composition

You will become familiar with the basic components of matter and the properties that make each type of matter unique


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Objectives for Section 2-1

Distinguish between elements and compounds

Match the names and symbols of the common elements


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2-1 The Elements

An element is the most basic form of matter that exists under ordinary circumstances

A compound is a unique substance that is composed of two or more elements that are chemically combined


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Elements Free in Nature

Only a few elements are found in their free state (not combined with other elements)


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Elemental Distribution

The Earth has 88 elements present in measurable amounts, but 10 constitute ~98.9% of the crust

There are 112+ known elements (26 spontaneously decay into other elements)

The human body is 93% carbon, hydrogen and oxygen


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Distribution of Elements – the Earth

O 45.2%

Si 27.2%

Al 8%

Fe 5.8%

Ca 5.06%Mg 2.77%

Na 2.32%

K 1.68%

Ti 0.86%H, Mn, P, all otherelements 1.1%


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Distribution of Elements - Human

O 64.6% C 18.0%

H 10.0%

N 3.1%Ca 1.9%

P 1.1%

Cl, K, Mg, Na, S 1.2%Trace elements(Cu, Fe, Zn etc) 0.1%


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Names and Symbols of the Elements

Symbol - usually the first and second letters of the name For two letter symbols, the first letter is

capitalized but the second is not

e.g. Li, Be, Ca, Ba, He, Ne Two letters are used for most elements.

Exceptions: B, C, F, H, I, K, N, O, P,

S, U, V, W, Y


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Names and Symbols of the Elements….continued

When the names of two or more elements begin with the same two letters, later letters are used in the symbols.

Examples Ruthenium (Ru) Rubidium (Rb) Cerium (Ce) Cesium (Cs) Thorium (Th) Thalium (Tl) (Ta is Tantalum)


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Names and Symbols of the Elements….continued.

When the name and symbol use different letters, that means the element was known in antiquity, and the Latin (or other ancient language) name is used as the basis of the symbol. The only exception is Tungsten (W), which uses the German name (Wolfram)


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Examples

Ag (Argentium; silver)Au (Aurum; gold)Cu (Cuprum; copper)Fe (Ferrum; iron)Hg (Hydragyrum; mercury)K (Kalium; potassium)Na (Natrium; sodium)Pb (Plumbum; lead)Sb (Stibonium; antimony)Sn (Stannum; tin)


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Objective for Section 2-2

List the postulates of the Atomic Theory


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2-2 The Composition of Elements: Atomic Theory of John Dalton

Matter is composed of small indivisible particles called atoms

Atoms of the same element are identical and have the same properties

Chemical compounds are composed of atoms of different elements combined in small whole-number ratios

Chemical reactions are merely the rearrangement of atoms into different combinations


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John Dalton (1766-1844),founder of the modern atomic theory of matter


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Molecules and Compounds

Atoms are the smallest fundamental particle of an element that have the properties of that element

Molecules - formed by the chemical combination of two or more atoms

Molecular compounds - molecules of atoms of two different elements

Covalent bonds - the force holding the atoms together in a molecular compound


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List the components of an atom, their relative masses, charges, and location in the atom


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2-3 Composition of the Atom

Atoms are composed of three subatomic particles proton - particle with a +1 charge; p electron - particle with a -1 charge; e neutron - particle with no charge; n

The proton and the neutron have roughly the same mass (~1 amu or 1.67 10-24 g)

The mass of the electron is substantially smaller than the masses of the proton or the neutron


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Electrical Nature of Matter

Electrostatic forces attraction between opposite charges repulsion between same charges

Ions are charged atoms or molecules cations - positively charged anions - negatively charged

Compounds consisting of ions are known as ionic compounds, and the forces holding them together are called ionic bonds


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Nuclear Model of the Atom

It was shown that the protons in the atom are found in a very small part of the atom called the nucleus

The protons and the neutrons of the atom are found in the nucleus (and are therefore termed nucleons)

The electrons are found outside of the nucleus


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Nuclear Model of the Atom


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Objectives for Section 2-4

Define the terms atomic number, mass number, and isotope

Using the table of elements, determine the number of protons, neutrons, and electrons in any isotope of an element

Demonstrate how the atomic mass of an element is determined from isotopes and their percent abundance


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2-4 Atomic Number, Mass Number, and Atomic Mass

Atomic number, Z, is the number of protons in the nucleus

The mass number, A, is the total number of nucleons

An isotope is an atom of a specific element with a specific mass number


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Isotopes

Atoms may have the same atomic number but different mass numbers (meaning the same number of protons but different numbers of neutrons)

The atomic number determines the identity of the element

Atoms of the same element with differing mass numbers are termed isotopes


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Isotopes Most elements occur naturally as a mixture of isotopes The specific isotope is written using isotopic notation This isotope will be referred to by the mass number

e.g. copper-63

Cu63

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simply Cu63

or more


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Isotopic Mass

Mass of an isotope compared to a standard The accepted standard is 12C, which is defined as

having a mass unit of exactly 12 amu An amu is defined as one-twelfth the mass of an

atom of 12C The atomic mass of an element listed in the

periodic table is the weighted average of the atomic masses of all isotopes present in nature


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Calculating Atomic Masses

For boron, 19.9% occurs as 10B and 80.1% occurs as 11B. The isotopic mass of 10B is 10.013 amu and 11B is 11.009 amu

10B: 0.199 10.013 amu = 1.99 amu 11B: 0.801 11.009 amu = 8.82 amu Atomic mass of boron = 10.81 amu


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Calculating Atomic Masses (2)

From the following data, determine the atomic mass of lithium.

Atomic mass = 6.015 x 0.0742 + 7.016 x 0.9258 = 6.941

Isotope Mass (amu) %Abundance

6Li 6.015 7.42

7Li 7.016 92.58


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Calculating Atomic Masses (3)

Bromine has two natural isotopes, 79Br and 81 Br. From its atomic mass, estimate the abundance of the two isotopes.

From the front cover, the atomic mass of bromine is 79.904, hence the abundance is approximately 50% each.


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Setting a Goal - Part BCompounds and Their Composition

You will learn to distinguish between ionic and molecular compounds based upon their chemical structure and general properties


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Objectives for Sections 2-5 and 2-6

Describe the difference between a molecular and ionic compound


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2-5 Molecular Compounds

A molecule is formed by the chemical combination of two or more atoms

Molecules are the basic particles of molecular compounds

The atoms in a molecule are held together by a force called the covalent bond


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Formulas

A compound is represented by using the symbols for the elements of which it is composed

Subscripts are used to indicate how many atoms of a particular element exist in the compound

If there is only one atom of a particular element, the one is assumed


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Formulas, cont’d

Note that changing the subscripts changes the compound consider H2O and H2O2; SO2 and SO3

Two different compounds can, however, share the same molecular formula dimethyl ether and ethyl alcohol

(ethanol) both have the formula C2H6O


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Ethyl Alcohol and Dimethyl Ether

Ethyl alcohol on the left; dimethyl ether on the right

These species are termed structural isomers Formulas that show the order and arrangement of

specific atoms are known as structural formulas

C C

H

H

H

H

O

H

H CH

H

H

O C

H

H

H


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Isomers

Isomers are different compounds (with different structural formulas) but sharing the same molecular formula

E.g. H-O-CN cyanic acid

H-N=C=O isocyanic acid

H-CNO fulminic acid

Each has the molecular formula HCNO


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Molecular Elements: Allotropes

Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine, exist as diatomic molecules

Some elements exist in a variety of forms, called allotropes

e.g. Carbon – graphite; diamond; buckminsterfullerene (etc)

Phosphorus - red (Pn) and white (P4)

Sulfur – S8 and S4


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Write the formulas of simple ionic compounds given the charges on the ions


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2-6 Ionic Compounds

Recall that some atoms and molecules can exist as charged species

Cations are positively charged ions Anions are negatively charged ions


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How Charged Species Arise

Neutral atoms and molecules have the same number of protons and electrons

Cations have fewer electrons than protons Anions have more electrons than protons

Na Na+ e-+

Cation

Cl + e-Cl-

Anion


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Ionic Compounds

Formula unit - simplest whole-number ratio of ions in an ionic compound

For example: Ca2+ + Br-

the resulting formula should be electrically neutral

two Br- are needed for each Ca2+

the resulting formula is CaBr2


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Polyatomic Ions

These are cations or anions consisting of groups of atoms that are covalently bonded to each other

Examples are NO3-, SO4

2-, ClO4-, MnO4

-

When more than one appears in a formula unit, the polyatomic ion is put in between parentheses, and a subscript is used to indicate the number of the ions that appear in the formula unit


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Examples of Polyatomic Ions

Ba(ClO4)2, CaCO3


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Further Examples of Polyatomic Ions

NO2+ (nitronium ion: important in the

nitration of organic compounds) C6H5CH2

+ (benzyl cation; a carbocation)

VO2+ (vanadyl cation; an important cation

of vanadium)


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Formulas of Ionic Compounds with Polyatomic Ions

These are determined the same way as for monoatomic ions

The total charge must be zero For Fe3+ and NO3

-

1 Fe3+ + 3 NO3- = Fe(NO3)3


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Further Examples

For Fe2+ and PO43-

3 Fe2+ and 2 PO43- = Fe3(PO4)2

For Fe3+ and C2O42- (oxalate)

2 Fe3+ and 3 C2O42- = Fe2(C2O4)3

Pb2+ and N3- (azide)

Pb2+ and 2 N3- = Pb(N3)2


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Chapter 2 Elements and Compounds. Malone and Dolter - Basic Concepts of Chemistry 9e2 Setting the Stage – Search for Water A key molecule we search for