chapter 2 atoms, molecules, and ions lecture presentation © 2012 pearson education, inc

Chapter 2

Atoms, Molecules,and Ions

Lecture Presentation

© 2012 Pearson Education, Inc.


Dalton's Postulates

Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.


Dalton's Postulates

Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.


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.


Symbols of Elements

Elements are symbolized by one or two letters.


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.

Key Points

• The elements of The Periodic Table are classified into three groups – metals, nonmetals, and metalloids

• Elements in a group have similar chemical properties

• Elements in a in a period have different chemical properties

Figure 2.11 Metals, metalloids, and nonmetals.

Chromium

CopperCadmium

Lead

Bismuth

Boron

SiliconArsenic

Antimony

TelluriumCarbon(graphite)

Sulfur

Chlorine Bromine

Iodine

The modern periodic table.Alkali metals

Alkaline earth metals Halogens

Noble Gases

Transition Metals

Figure 2.10 The modern periodic table.

+1+2 +3 -3 -2 -1

0

NC


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


Diatomic Molecules

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

Chemical Formulas• Molecular Formula

– It only shows the exact number of atoms of each element in the smallest possible units of a substance

– Ex. H2O, O2, N2

• Empirical Formula– Tells us what elements are present and the simplest whole

number ratio of their atoms– Not necessarily the actual number of atoms in a given

molecule.N2H4 molecular formulaNH2 empirical formula


Ionic compounds (such as NaCl) are generally formed between metals and nonmetals.


Chemical FormulasMolecular (covalent or binary) compounds are composed of molecules and almost always contain only nonmetals.


Ions

• When atoms lose or gain electrons, they become ions.– Cations are positive and are formed by elements on the

left side of the periodic chart (donate electrons to nonmetals)

– Anions are negative and are formed by elements on the right side of the periodic chart (gain electrons from metals)

cation – ion with a positive chargeIf a neutral atom loses one or more electronsit becomes a cation.

anion – ion with a negative chargeIf a neutral atom gains one or more electronsit becomes an anion.

Na11 protons11 electrons Na+ 11 protons

10 electrons

Cl17 protons17 electrons Cl-

17 protons18 electrons

2.5


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.

• The sum of the charges on the cation and the anion have to add up to zero, thus the compound is electronically neutral– Ex Al2O3

• Predicting Ionic Charge– The number of electrons an atom loses or gains is

related to its position on the periodic table.

Naming Ionic Compounds cont..

• Certain Metals especially transition metals can form more than one type of cation

• We use the Stock System to name them– Mn 2+ MnO (manganese (II) oxide)– Mn 3+ Mn2O3 (manganese (III) oxide)– Mn 4+ MnO2 (manganese (IV) oxide)

Naming Ionic Compounds cont..• Binary compounds: made from just 2 elements

– Cation (electron donating number fixed) is named first and the name of the element stays the same

– Cation (electron donating more than one) follow the mentioned above rules

– Anion (electron accepting) is named second and you add -ide

• Ex. Oxygen turns to Oxide• Ex Nitrogen turns to Nitride

Sample Problem 2.5 Naming Binary Ionic Compounds

PROBLEM: Name the ionic compound formed from the following pairs of elements:

(a) magnesium and nitrogen

SOLUTION:

(b) iodine and cadmium

(c) strontium and fluorine (d) sulfur and cesium

(a) magnesium nitride

(b) cadmium iodide

(c) strontium fluoride

(d) cesium sulfide

Sample Problem 2.6 Determining Formulas of Binary Ionic Compounds

PROBLEM: Write empirical formulas for the compounds named in Sample Problem 2.5.

SOLUTION:

Mg2+ and N3-; three Mg2+(6+) and two N3-(6-); Mg3N2

Cd2+ and I-; one Cd2+(2+) and two I-(2-); CdI2

Sr2+ and F-; one Sr2+(2+) and two F-(2-); SrF2

Cs+ and S2-; two Cs+(2+) and one S2- (2-); Cs2S

(a) magnesium nitride

(b) cadmium iodide

(c) strontium fluoride

(d) cesium sulfide

Naming Ionic Compounds cont..• Ternary Compounds: made up three elements

– If cation (electron donating) has a fixed number of electrons to give the name will stay the same and it is named first

– If the cation (electron donating) different number of electrons to give the name will stay the same but you will mention the number of electrons (roman numerals) that are being donating and it is still named first

– If the anion (electron accepting) 1 element then you will add –ide

– If the anion (electron accepting) more than one element refer to table 2.5 page 63, it is mentioned after the cation

Sample Problem 2.8 Determining Names and Formulas of Ionic Compounds Containing Polyatomic Ions

SOLUTION:

PROBLEM: Give the systematic names or the formula or the formulas for the names of the following compounds:

(a) Fe(ClO4)2 (b) sodium sulfite

(a) ClO4- is perchlorate; iron must have a 2+ charge. This is iron(II)

perchlorate.

(b) The anion sulfite is SO32- therefore you need 2 sodiums per sulfite.

The formula is Na2SO3.

(c) Hydroxide is OH- and barium is a 2+ ion. When water is included in the formula, we use the term “hydrate” and a prefix which indicates the number of waters. So it is barium hydroxide octahydrate.

(c) Ba(OH)2 8H2O

Molecular Compounds

• They are usually composed of nonmetal elements

• The chemical bond that forms between the elements is called “covalent bond”– Most of them are binary compounds – Named in the same manner as ionic compounds

Naming Molecular Compounds

• The first name the element that is first in the formula, its name does not change

• The second element in the formula is named second and –ide is added – HCl (hydrogen chloride)– HBr (hydrogen bromide)

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) .

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

– CO2: carbon dioxide– CCl4: carbon tetrachloride


Nomenclature of Binary Compounds

• 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


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

Sample Problem 2.11 Determining Names and Formulas of Binary Covalent Compounds

SOLUTION:

PROBLEM: (a) What is the formula of carbon disulfide?

(c) Give the name and formula of the compound whose molecules each consist of two N atoms and four O atoms.

(b) What is the name of PCl5?

(a) Carbon is C, sulfide is sulfur S and di-means 2 - CS2.

(b) P is phosphorous, Cl is chloride, the prefix for 5 is penta-. Phosphorous pentachloride.

(c) N is nitrogen and is in a lower group number than O (oxygen). Therefore the formula is N2O4 - dinitrogen tetraoxide.

Acids• Acid: a substance that yields hydrogen ions (H +,

equivalent to a proton) when dissolved in water– Formulas for acids contain one or more hydrogen as well

as an anionic group– The name assigned to the compound depends on its

physical state

• Anions whose names end in “ide” form acids with a “hydro” prefix and an “ic” ending– HCl Hydrochloric acid

Naming Acids

Sample Problem 2.10 Determining Names and Formulas of Anions and Acids

SOLUTION:

PROBLEM: Name the following anions and give the names and formulas of the acids derived from them:

(a) Br - (b) IO3 - (c) CN - (d) SO4

2- (e) NO2 -

(a) The anion is bromide; the acid is hydrobromic acid, HBr.

(b) The anion is iodate; the acid is iodic acid, HIO3.

(c) The anion is cyanide; the acid is hydrocyanic acid, HCN.

(d) The anion is sulfate; the acid is sulfuric acid, H2SO4.

(e) The anion is nitrite; the acid is nitrous acid, HNO2.

chapter 2 atoms, molecules, and ions lecture presentation © 2012 pearson education, inc

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