gc-s009-substances

Henry R. Kang (1/2010)

General Chemistry

Lecture 9

Molecule and Ion


Contents

• Substances Atoms, molecules, and ions

• Chemical Formula and Models• Molecular (or Covalent) Compounds• Ionic Compounds

Cations Anions


Substances

Substances

• Substances are pure and can not be separated by physical means.

• Substances are subdivided into elements and compounds. Elements are in the forms of atoms and allotropes.

Atoms can not be separated by chemical means.

Compounds are aggregates of atoms from two or more different elements in a definite arrangement.Compounds can be separated by chemical means.

Compounds are further classified in two groups: molecular (or covalent) compounds and ionic compounds. Henry R. Kang (1/2010)

Atoms, Molecules, and Ions• Atom is a single entity of an element.

Six noble gases (He, Ne, Ar, Kr, Xe, and Rn) are the only elements exist as single atoms in nature. They are monatomic gases.

• Molecule is a substance of two or more atoms chemically united in a definite arrangement such as H2 and CO2.

Each molecule is an individual entity.

Molecules can be separated by chemical means into pure components.

• Ion is an atom or an aggregate of atoms chemically bounded together that has a net positive or negative charge such as Li+, Fe3+, Cl and SO4

2.Henry R. Kang (1/2010)

Molecular Substances

• Molecule is a substance of two or more atoms in a definite arrangement held together by chemical bonds. Two or more elements joined in a fixed ratio such as HBr,

NO2, and H2CO3. They are classified as compound.

Two or more atoms of the same element such as Cl2, O2, O3, and P4. They are classified as element or allotrope of an element.

• A diatomic molecule contains only two atoms. H2, N2, O2, F2, Cl2, Br2, I2, HF, HCl, HBr, HI, CO, NO, etc.

• A polyatomic molecule contains three or more atoms. O3, S8, H2O, CO2, NH3, CH4, N2O4, H3PO4, etc.


Molecule and Element• Some elements exist as molecules.

• Examples: H2, N2, O2, O3, F2, Cl2, Br2, I2, P4, S8,

and C60 (buckminsterfullerene).

Each mole contains only one element.

• Allotrope Allotrope is one of several distinct

forms of an element.

Examples: Oxygen (O2) and ozone (O3) are allotropes

of the element O.

Diamond, graphite, and C60 are allotropes of the element carbon.

S

ClCl

P

P

P

P

SS

S

SSS

S


Chemical Formulasand

Models


Chemical Formulas: Definition

• Chemical formulas are used to express the chemical composition of substances.

• Definition Chemical formula is a notation to represent a

substance (either molecular or ionic compound) that uses atomic symbols together with numerical subscripts to indicate the relative numbers of atoms of different elements in the substance.

• Examples: CH4 (methane), N2O5 (dinitrogen pentoxide), NaCl

(sodium chloride), and K2SO4 (potassium sulfate). Henry R. Kang (1/2010)

Types of Chemical Formulas

• Molecular formula Represent the chemical composition of molecules.

• Structural formula Indicate the structure of atoms within a molecule.

• Empirical formula The smallest integer ratio of atoms in a substance. The greatest common denominator for the numbers of all

different atoms in the substance is 1.

• Ionic formula (for ionic compounds) A combination of cation (or cations) and anion (or anions). Ionic formula is usually the same as its empirical formula.


Molecular Formulas: Definition

• Molecular formula is a subset of the chemical formula that represents a molecular (or covalent) compound, having an exact number of atoms for each element in the molecule.

• Examples:

H2, O2, O3, H2O, CO2, NO2, N2O4, CH3OH, CHCl3, etc.

Note that O2 (oxygen gas) and O3 (ozone) have the same empirical formula O; but they are different compounds with different structures and different properties.

Also, NO2 (nitrogen dioxide) and N2O4 (dinitrogen tetroxide) have the same empirical formula NO2; but they are different compounds with different structures and different properties.


Structure Formulas: Definition• Structure formula is a form of the molecular formula

that indicates the arrangement of atoms within a molecule by connecting atoms with a line (or lines) to represent a chemical bond.

• Examples: HCl, HOH, O=C=O, HNH

| H

• Structure formula gives the bonding sequence of atoms in the molecule.

• Example: The 3 atoms in CO2 is bonded O=C=O, not O=O=C.


Types of Formulas & Models• Chemical models are used as aids in visualizing the sizes

and shapes of molecules.

Nitrogen Amide ion Ammonia Ammonium ion

Molecular formula

N2 NH2– NH3 NH4

+

Structure formula

NN HNH HNHH

HHNH

H

Ball-and-stick

model

Space-filling model

Comparisons of Chemical Models

Ball-and-Stick Model Space-Filling Model

Advantage

Represent chemical bonds.Give approximations to actual bond angles.Easy to visualize the 3D geometric shape.Easy to construct.

Balls are proportional in size to atoms.More closely represent molecular size.

Disadvantage

All atoms, except H, have the same size. Sticks have only a few different lengths; thus, bond lengths are not in proportion.

Bonds are not visible.Difficult to see the 3D structure.Not easy to construct.


Ionic Substances


Ions• Ion is an atom or a group of atoms

bonded chemically to have a net positive or negative charge.

• Symbol:

XC, where C is the charge with sign.

• Ionic compounds consist cations and anions.

• Cation is an ion with a positive charge.

If a neutral atom loses one or more electrons, it becomes a cation.

• Anion is an ion with a negative charge.

If a neutral atom gains one or more electrons, it becomes an anion.

3 protons3 electrons

FF

Li Li+




Lose an electron

Gain an electron


Types of Ions

• Monatomic ions contain only one atom. Examples: Na+, Ca2+, Al3+, Cl–, O2–, N3–

• Polyatomic ions contain two or more atoms. Examples: OH–, O2

2–, CN–, NH2–, NH4

+, Hg2

2+, NO3–, CO3

2–, HCO3–, SO4

2–

Note that the atoms can come from the same element such as O2

2– and Hg22+.


Charges of Monatomic Anions

• Nonmetallic elements of the main group usually have a charge equal to the group number minus 8. Charge = (Group number) – 8

• Examples: Cl– (group 7A): 7 – 8 = -1 O2– (group 6A): 6 – 8 = -2 N3– (group 5A): 5 – 8 = -3


Charges of Monatomic Cations:

• Metallic elements of the main group usually have a charge equal to the group number in the periodic table. Charge = (Group number)

Examples: Na+ (Group 1A), Ca2+ (Group 2A), and Al3+ (Group 3A).

Exceptions: Metallic elements of high atomic number can have more than one cation. The common cation has a charge = (Group number – 2) Examples: Tl3+ (Group 3A) and Tl+ (Group 3A – 2);

Pb4+ (Group 4A) and Pb2+ (Group 4A – 2).

• Most transition elements have more than one cation (see next slide). Generally, they have a common cation with +2 charge.


Charges of Monatomic Ions Shown in the Period Table

3Li+

37Rb+

19K+

11Na+

55Cs+

20Ca2+

10

3231Ga3+

3635Br-

34Se2-

33

15P3-

16S2-

17Cl-

18

42

14Si4+

38Sr2+

41

57

4039

56Ba2+

49In3+

50Sn2+

51 52Te2-

53I-

54

7372 74

43 44 45 46 47Ag+

48Cd2+

76 79 83 84 85 8675 80Hg2

2+

Hg2+

78 81Tl+

Tl3+

77 82Pb2+

Pb4+

24Cr3+

232221

12Mg2+

4Be2+

1H+

9F-

2

30Zn2+

29Cu+

Cu2+

28Ni2+

27Co2+

Co3+

26Fe2+

Fe3+

25Mn2+

Mn4+

5B3+

13Al3+

6C4-

7N3-

8O2-


Examples of Ions

• How many neutrons, protons, and electrons in 2713Al3+?

#neutron = mass# – atomic# = 27 – 13 = 14

#proton = atomic# = 13

#electron = atomic# – charge = 13 – 3 = 10

• How many neutrons, protons, and electrons in 7834Se2?

#neutron = mass# – atomic# = 78 – 34 = 44

#proton = atomic# = 34

#electron = atomic# – charge = 34 – (-2) = 36

XAZ

Element SymbolMass Number

Atomic Number

CCharge with sign


Ionic Compounds

• Ionic compounds consist of a combination of cations and anions. Usually, the most reactive metals

(Groups 1A, 2A, and 3A) and the most reactive nonmetals (Groups 5A, 6A, and 7A) combine to form ionic compounds.

• Examples: KI, CaF2, AlBr3, etc.

Note that the sequence of writing an ionic compound is the cation first followed by the anion.


Ionic Compounds: Structure

• Unlike the molecular compounds, the ionic compounds do not consist of discrete molecular units.

• Instead, they form 3D connected networks with repeating orderly patterns of ions. Example: the table salt, NaCl, is a clear crystal, containing numerous Na+

cations and Cl– ainons; each Na+ ion is surrounded by 6 Cl– ions (top, bottom, right, left, front, and back) and vice versa for Cl– ion that is surrounded by 6 Na+ ions.

• Therefore, the formula of ionic compound is written by giving the smallest integer ratios of different ions in the compound, called “formula unit”. Examples: NaCl, KF, CaCl2, Fe2(SO4)3

• The sum of the charges on the cation(s) and anion(s) in a neutral ionic compound must be zero.


Formula of Ionic Compounds• The sum of the charges on the cation(s) and anion(s) in each

formula unit must be zero. If the charges on the cation and anion are numerically different, then the

subscript of the cation is equal to the charge on the anion and the subscript of the anion is equal to the charge on the cation

The subscripts must always be reduced to the smallest ratios (Namely, the formula unit)

If the charges are numerically equal, no subscripts are needed Sr2+ and O2-, we have Sr2O2. We must reduce to the smallest ratios, thus SrO

Cr2O3

Cr3+ O2

2×(+3) = +63×(-2) = -6

CaCl2 Na2CO3

Ca2+ Cl Na+ CO32

1×(+2) = +22×(-1) = -2

Sum = 0

2×(+1) = +21×(-2) = -2

Sum = 0 Sum = 0

Li2OLi+ O2

2×(+1) = +21×(-2) = -2

Sum = 0


Example 1 of Ionic Substances• Write the ions for the following elements: Li, Ba, Al, Pb, P, S, Br.

• Answer: Li+, Ba2+, Al3+, Pb4+, P3, O2, and Br

• Fill the blanks in the table to give the formulas of ionic compounds formed by the cation and anion in the corresponding column and row.

Li+ Ba2+ Al3+ Pb4+

Br Li Br Ba Br2 Al Br3 Pb Br4

O2 Li2O Ba O Al2O3 PbO2

P3 Li3P Ba3P2 Al P ?

Li+ Ba2+ Al3+ Pb4+

Br

O2

P3


Example 2 of Ionic Compounds• Fill the blanks in the table to give the formulas of ionic

compounds formed by the cation and anion in the corresponding column and row.

NH4+ Cu2+ Fe3+

NO3

SO42

PO43

NH4+ Cu2+ Fe3+

NO3 NH4NO3 Cu(NO3)2 Fe(NO3)3

SO42 (NH4)2SO4 Cu SO4 Fe2 (SO4)3

PO43 (NH4)3PO4 Cu3(PO4)2 Fe PO4


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