chapter 1 - atom structure

8/20/2019 Chapter 1 - Atom Structure

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FEG 2113

CHEMISTRY I

CHAPTER 1

Atomic Structure


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Atomic Theory of Matter

The theory that atoms are the fundamental

building blocks of matter reemerged in the early

19th century, championed by John Dalton.


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Atomic Composition

3 subatomic particles made up all atoms:

Electrically positive protons

Electrically neutral neutrons

Electrically negative electrons


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Structure of An Atom

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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.


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Table: Properties & Location of Protons,

Neutrons & Electrons In Atom

Subatomic

Particle

Symbol Relative

electrical

charge

Location

Proton p+

+ 1 In thenucleus

Electron e- - 1 Outside

the

nucleusNeutron n0 0 In the

nucleus

1 atomic mass unit (amu) = 1.6605 x 10-24

g


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1 atomic mass unit (amu) = 1 gmol-1


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Symbols of Elements

Elements are symbolized by one or two

letters.


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Atomic Number

All atoms of the same element have the same

number of protons:The atomic number (Z)


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Atomic Mass

The mass of an atom in atomic mass units

(amu) is the total number of protons andneutrons in the atom.


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ATOMIC NUMBER (Z)

• Number of protons in the nucleus of an atom• Atomic number (Z) = Number of protons

MASS NUMBER (A)• Sum of the number of protons and neutrons

in the nucleus of an atom

• Mass number (A) = Number of protons + Number of neutrons

= Atomic Number (Z) + Number of neutrons


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Mass number

Element symbol

Atomic number

A

X

Z

Example :

What is the atomic number and the massnumber of the element FLUORINE thatcontains 9 protons and 10 neutrons ? Writethe element symbol.

• Atomic number =

• Mass number =


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Isotopes

• Isotopes are same atomic number with different

masses.

• Isotopes have different numbers of neutrons.

11

6C12

6C13

6C14

6C


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Exercise

How many neutrons are in each isotope ofoxygen? Write the symbol of each isotope.

Oxygen (atomic no.) = 8

a) Oxygen-16 b) Oxygen-17 c) Oxygen-18


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Example :

Isotope Isotope mass (amu) Abundance

(%)63

29Cu 62.9298 69.09

6529Cu 64.9278 30.91

Average atomic mass for Cu

=

=

=


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Properties subatomic particle

Subatomic

Particle

Symbol Relative

electrical

charge

Mass

(g)

Mass

(amu)

Location

Proton p+ + 1 1.6726x 10-24

1 In thenucleus

Electron e- - 1 9.1094

x 10-28

0.0005 Outside

the

nucleus

Neutron n0 0 1.6749

x 10-24

1 In the

nucleus


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Relative Atomic Masses, Ar• Masses of fundamental atomic particles often expressed in atomic

mass units (amu)

• Relative atomic mass - The mass of an atom is measured relative tothe mass of an atomic standard, Carbon-12

• Relative atomic mass (symbol: Ar ) is a dimensionless physical

quantity, the ratio of the average mass of atoms of an element (froma given source) to 1/12 of the mass of an atom of carbon-12

• 1 carbon atom has a mass of 12.000 amu

• Atomic mass of an element − the average relative mass of theisotopes of that element compared to atomic mass of carbon-12 (12amu)

http://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Carbon-12http://en.wikipedia.org/wiki/Carbon-12http://en.wikipedia.org/wiki/Carbon-12http://en.wikipedia.org/wiki/Carbon-12http://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Atomhttp://en.wikipedia.org/wiki/Mass


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• 1 amu 1/12 of the mass of an atom of carbon

with 6 protons and 6 neutrons (a carbon-12 atom)

• 1 amu = 1.66054 × 10 -24 g


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Relative Molecular Masses, Mr

• The mass of one MOLECULE of thesubstance compared to 1/12 the mass of

one ATOM of carbon-12 isotope. Its

symbol is Mr .• Mr is calculated by adding together the

relative atomic masses of all the atoms

present in the molecular formula of thesubstance.


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Periodic Table

• It is a systematic

catalog of the

elements.• Elements are

arranged in order

of atomic number.


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Periodicity

When one looks at the chemical properties of

elements, one notices a repeating pattern ofreactivities.


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Periodic Table

• The rows on the

periodic chart are

periods.

• Columns are groups.

• Elements in the same

group have similar

chemical properties.


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• Mole is a chemical unit used in quantitative measurement of particles

involved in chemical reactions

• A mole is the amount of a substance thatcontains as many elementary entities

(atoms, molecules, ions or other particles)

as there are atoms in exactly 12 g of the

carbon-12 isotope.

Mole Concept


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2009, Prentice-Hall, Inc.

Using Moles

Moles provide a bridge from the molecular

scale to the real-world scale.


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Avogadro’s Number

• One mole always contains the samenumber of particles, no matter what the

substance is.

• 1 mole = 6.0221415 x 1023

particles • This value is known as Avogadro’s

number in honour of Amedeo Avogadro,

an Italian lawyer and physicist (1776-1856)


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Empirical Formula

• Simplest formula

• Gives the smallest whole-number ratio ofatoms present in a compound

Molecular Formula

• True formula

• Total number of atoms of each element present in one molecule of a compound

• Knowing the relative numbers of atoms ofeach element in a molecule


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Molecular Formula

• Molecular formula = [Empirical formula]n

where n should be integers (n = 1, 2, 3…)

• To determine molecular formula from

empirical formula, the molar mass mustbe obtained from experiment


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Empirical Formula from % Composition

For a compound composed of atoms of A & B,

% A

% B

g A

g B

x mol A

x mol B

x mol A

y mol B AxBy

Ratio gives

formula

Find mole ratio

Convert weight

% to mass (g)Convert mass (g)

to moles (mol)


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Example 1:

Finding Empirical Formula

25.00g of orange compound, contains 6.64g

of potassium, 8.84g of chromium and 9.52g

of oxygen

K, Cr, O

Given the molar mass

K = 39.40 g/molCr = 52.00 g/mol

O = 16.00 g/mol


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• Remember that we learn in school:

)gmol(eightmolecularw

)g(mass)mol(mol

1


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Atom K Cr O

Mass (g) 6.64 g 8.84 g 9.52 g

Mol

Atom Ratio

Whole-

numberMol Ratio

Empirical

formula


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Atom K Cr O

Mass (g) 6.64 g 8.84 g 9.52 g

Mol 6.64 g39.40 g/mol

= 0.170 mol

8.84 g52.00 g/mol

= 0.170 mol

9.52 g16.00 g/mol

= 0.595 mol

Mol Ratio 0.1700.170

= 1

0.1700.170

= 1

0.5950.170

3.5

Whole-

number

Mol Ratio

1 x 2 = 2 1 x 2 = 2 3.5 x 2 = 7

Empirical

formula

K2Cr 2O7


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Example 2 : Finding Empirical

formula & Molecular formula

Eugenol is the major component in oil of

cloves. It has a molar mass of 164.2 g/mol

and is 73.14 % C and 7.37 % H, the

remainder is oxygen. What are the empiricaland molecular formulas of eugenol?

[ Assumption : Mass % mass (g)]

The mass of O in a 100.0 g sample :

73.14 g C + 7.37 g H + mass of O = 100.00 g

Mass of O = 19.49 g O


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Atom C H O

Mass (%) 73.14 % 7.37 % 19.49 %

Mass (g)

Mol

Mol Ratio

Empirical

formula


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Atom C H O

Mass (%) 73.14 % 7.37 % 19.49 %

Mass (g) 73.14 g 7.37 g 19.49 g

Mol 73.14 g

12.011 g/mol

= 6.089 mol

7.37 g

1.008 g/mol

= 7.312 mol

19.49 g

15.999 g/mol

= 1.218 mol

Mol Ratio 6.089 mol

1.218 mol

= 4.999 5

7.312 mol

1.218 mol

= 6.003 6

1.218 mol

1.218 mol

= 1

Empirical

formulaC5H6O


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The molecular mass of eugenol = 164.2 g/mol

• [ ]n = 164.2 g/mol


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The molecular mass of eugenol

= 164.2 g/mol

• [C5H6O]n = 164.2 g/mol

• [(5x12.011 g/mol) + (6x1.008 g/mol) +(1x15.999 g/mol)] n = 164.2 g/mol

• (60.055 + 6.048 + 15.999) n = 164.2

• n = 164.2 / 82.102 = 1.99995 2• Molecular formula = [C5H6O]2

= C10H12O2


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Learning Outcomes

• Determine the empirical and molecularformulae of compounds and calculate

their relative formula/molecular mass

• Use atomic number and mass number to

differentiate different element and

different isotopes of the same element

chapter 1 - atom structure

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