Chapter TwoATOMS, MOLECULES AND IONS

Section 2.3 The modern view of atomic structure:

Subatomic particles in the atom:Proton, neutron, and electron.Charge of electron = - 1.602 10-19 CCharge of proton = + 1.602 10-19 CNeutrons are not charged.Every atom has an equal number of electrons and protons, so atoms are electrically neutral.Electrons reside in a vast space in the atom.Protons and neutrons reside in the nucleus of the atom.Electrons are attracted to the protons in the nucleus by electrostatic forces (between particles of opposite electrical charges)

Atoms have extremely small masses. The mass of the heaviest known atom, forexample, is approximately 4 10-22 g . Because it would be cumbersome to expresssuch small masses in grams, we use the atomic mass unit (amu), where 1 amu = 1.66054 10-24 gA proton has a mass of 1.0073 amu, a neutron 1.0087 amu, and an electron 5.486 10-4amu (See TABLE 2.1)

Most of atoms have diameters between 1 10-10 m and 5 10-10 m and . A convenient عليه متفقnon–SI unit of length used for atomic dimensions is the angstrom (Å), where1 Å = 110-10 m . Thus, atoms have diameters of approximately 1-5 Å .

SAMPLE EXERCISE 2.1 Atomic SizeThe diameter of a US dime is 17.9 mm, and the diameter of a silver atom is 2.88 Ǻ . How many silver atoms could be arranged side by side across the diameter of a dime? silver atoms could be arranged side by side across the diameter of a 0.2 mm dime? Solution:

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General Chemistry A: First 2013/2014

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Practice exercise 2.1 The diameter of a carbon atom is 1.54 Ǻ (a) Express this diameter in picometers. (b) How many carbon atoms could be aligned side by side across the width of a pencil line that is0.20 mm wide?

Atomic Numbers, Mass Numbers, and IsotopesAtomic Number: Number of protons in the nucleus = Number of electrons ( in a neutral atom)Mass Number: total number of protons and neutrons in the atom.Isotopes: Atoms of identical atomic numbers but different mass numbers (different number of neutrons). Carbon has four isotopes: all have the same number of electrons = same number of protons but they have different numbers of neutrons.

Sample exercise 2.2How many protons, neutrons, and electrons are in (a) 197Au (b) strontium-90

Solution:

(a) Atomic number of Au = number of es = number of protons = 79 (from periodic table), and the mass number of Au = 197

Number of neutrons = 197 – 79 = 118

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(b) Atomic number of Sr = number of es = number of protons = 38 (from periodic table), and the mass number of Sr = 90

(c) Number of neutrons = 90 – 38 = 52 neutrons.

Atomic WeightsUnit : amu = 1.66054 10-24 gMass of a hydrogen atom = 1.66054 10-24 gMass of an oxygen atom = 2.6560 10-23 g

Average atomic mass = % of the first isotope its at.wt. + % of the second isotope its at.wt.

For example, Carbon has two isotopes

98.93 %of 12C (12 amu), and 1.07% of 13C (13.00335 amu)

The average atomic mass of C = (0.9893)(12 amu) + (0.0107)(13.00335 amu) =12.01 amu

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AMPLE Calculating the Atomic Weight of an Element from Isotopic AbundancesNaturally 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.Solution:

Atomic weight = (0.7578)(34.969 amu) + (0.2422)(36.966 amu)= 26.50 amu + 8.953 amu= 35.45 amu

The atomic weight, which is actually the average atomic mass, is between the masses of the two

isotopes and is closer to the value of 35Cl,

the more abundant isotope (75.78 %).

Problem (2.92)

There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of Br2 molecules, and the mass of a Br2 molecule is the sum of the massesof the two atoms in the molecule. The mass spectrum of Br2

consists of three peaks:

Mass (amu)

Relative Size

157.836 0.2569159.834 0.4999161.832 0.2431

(a) What is the origin of each peak (of what isotopes does each consist)? (b) What is the mass of each isotope?(c) Determine the average molecular mass of a Br2 molecule. d) Determine the average atomic mass of a bromine atom. (e) Calculate the abundances of the two isotopes.

Solution

a) Bromine molecule (Br2) consists of two bromine atoms (Br-Br). These two atoms may be

originated from the same type of isotopes Br2(11) and Br2(22) or from two types of isotopes,

Br2(12).

The first peak will represent the lighter Br2 molecule, the third peak will represent the heavier

Br2 molecules and the middle peak will represent the intermediate Br2 molecule which is Br2(12)

.

b) Mass of Br(1) is 157.836/2 = 78.918 amu.

Mass of Br(2) is 161.832/2 = 80.916 amu.

c) Average molar mass of Br2 molecules

= 0.2569 (157.836) + 0.4999 (159.834) + 0.2431 (161.832) = 159.79 amu

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d) Average atomic mass of a bromine atom = Average molecular weight of Br2/2

Average atomic mass of a bromine atom = 159.79/2 = 79.895 amu

e) Let x = Abundance of Br(1) and (1-x) is the abundance of Br(2).

79.895 = x (78.918) + (1-x) (80.916) x = 0.5110

The abundance of Br isotope (type 1) is 51.10 % and the abundance of the ther isotope is 48.90 %

The periodic tableIs the most significant tool that is used by chemists to organize and to remember the chemical

facts: similarities, differences, reactivities, properties, …etc.

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Periods: Seven horizontal rows labeled by Arabic numerals (1-7).

Period No. of elements in the period

First 2

Second 8

Third 8

Fourth 18

Fifth 18

Sixth32 = 18 +14

(to fit in a page)

SeventhIncomplete

12+... + 14

Groups: Eight vertical groups, of specific names, and are labeled as:

1A, 1B, 2A, 2B, …, 8 A, and 8B. or, IA, IB, IIA, IIB …, VIII A

Roman numeral rather than Arabic ones are often used in this scheme.

Elements in a group have similar properties. Because they have the same arrangement of es at

the periphery of their atoms.

Metallic elements: share characteristic properties, such as: luster, high electrical and heat

conductivity, all (except Hg) are solids.

In the periodic table: on the left and in the middle.

Non metals: Generally differ from the metals in appearance, and in other physical properties.

Some of the non metals are gaseous, some are solid, and one is liquid (bromine).

Nonmetallic elements are separated from the metallic ones, by a stepped line started from Boron

(B) to Astatine (At).

Metalloids: lie along the stepped المتعرج line that runs from boron (B) to Astatine (At) الخط

Have properties fall between those of metals and those f non metals.

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Molecule: an assembly of two or more atoms tightly bound together.Diatomic molecule: is made up of two atoms. O2, H2, Cl2, …etcTriatomic molecule: is made up of three atoms. O3

Polyatomic molecule: is made up of more than three atoms.Molecular compounds: are composed of molecules contain more than one type of atom. For example: H2O, C2H5OH, Na2CO3, … etc

Molecular formula: a formulum that indicates the actual numbers and types of atoms in the molecule.

Molecular formula: that formula which indicates the actual number and type of atoms in a molecule.Empirical formula: indicates the relative number atoms in a molecule.

CH is the empirical formula of benzene (C6H6).HO is the empirical formula of hydrogen peroxide (H2O2).BH3 is the empirical formula of diborane (B2H6).CH2O is the empirical formula of glucose (C6H12O6) .

Picturing molecules:Structural formula: shows the arrangement of bonded atoms, which atom is attached to which within the molecule.

There are different representation for structural formula can be used :

CH4 (molecular formula)

Ionic compounds: combination of metal and non metal as: NaCl, BaCl2, Na2CO3, …Molecular compounds: combination of non metals only as : NO, H2O, CH4, …

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Structural formula Perspective drawing ball and stick space filling

Sample exercise 2.9Which of the following compounds would you expect to be ionic?

N2O, Na2O, CaCl2, SF4.Ionic: Na2O and CaCl2. The other two compounds are molecular.

Names and formulas of ionic compounds:1. Positive Ions (cations)(a) Cations formed from metal atoms: Na+, Zn2+, Al3+, … (b) Cations with different charges:

Cu(I), and Cu(II) Fe(II), and Fe(III)Older method: add …ous, or …ic to the elements Lattin name.

(c) Cations formed from nonmetal atoms: ended by … ium.NH4

+: ammonium ion H3O+: hydronium ion

See table 2.4

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2. Negative Ions (Anions): (a) monatomic anions ended by … ide.

H- Hydride ion, O2- : Oxide ion, N3- nitride: ion.

A few simple polyatomic anions also having names ending in ide, such as: OH- hydroxide CN- cyanide O2

2- peroxide (b) polyatomic anions (oxyanions)

these anions containing oxygen have names ending in ate or ite

NO3- nitrate ion SO4

2- sulphate ion SeO42- selenate ion

NO2- nitrite ion SO3

2- sulphite ion SeO32- selenite ion

ClO4- perchlorate ion BrO4

- perbromate ion.ClO3

- chlorate ion BrO3- bromate ion.

ClO2- chlorite ion BrO2

- bromite ion.ClO- hypochlorite ion BrO- hypobromite ion.

(c) Anions derived by adding H+ to an oxyanion named by adding the word hydrogen or dihydrogen:

HCO3- hydrogen carbonate H2PO4

- dihydrogen phosphate

Common anions are listed in table 2.5

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3. Ionic compounds:The cation name is followed by the anion nameCaCl2 calcium chlorideAl(NO3)3 aluminium nitrateCu(ClO4)2 copper perchlorate (or cupric perchlorate)

SAMPLE EXERCISE 2.12Name the following ionic compounds: (a) K2SO4 (b) Ba(OH)2 (c) FeCl3

(a) K2SO4 : potassium sulphate(b) Ba(OH)2 : barium hydroxide(c) FeCl3 : Iron(III) chloride (or Ferric chloride)

PRACTICE EXERCISE Give the formula for (a) magnesium sulphate, (b) silver sulphide, (c) lead(II) nitrate

(a) magnesium sulphate: MgSO4(b) silver sulphide: Ag2S(c) lead(II) nitrate: Pb (NO3)2

Names and Formulas of acids:

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Names and formulas of binary compoundsCl2O dichlorine monoxide NF3 nitrogen trifuoride N2O4 Dinitrogen tetroxideP4O10 tetraphosphorus decasulphate

SAMPLE EXERCISEName the following compounds: (a) SO2, (b) PCl5, (c) N2O3

(a) SO2: sulfur dioxide(b) PCl5: phosphorus pentachloride(c) N2O3: dinitrogen trioxide

SOME SIMPLE ORGANIC COMPOUNDSAlkanes: structural formula

Derived alkanes

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Different forms of propanol:

Selected questions:Start from 2.17 2.18, 23, 25, 32, 35, 49, 62, 65-72.

HW2 : 65-72 واحد ورقة في

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