2011 core ib chemistry - topic 02

Atomic Structure

IB Chemistry Power Points

Topic 02

Atomic Structure

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Review – Basic Atomic Structure

NUCLEUS ELECTRONS

PROTONS NEUTRONS

POSITIVE

CHARGE

ATOM

POSITIVECHARGE

PROTONS

NEUTRALCHARGE

NEUTRONS

NUCLEUS

NEGATIVE CHARGE

ELECTRONS

ATOM

Subatomic components

Relative

Mass

Charge

Proton 1 +1

Neutron 1 0

Electron 5 x 10-4 -1

Review – Basic Atomic Model

A-Z notation

© Addison-Wesley Publishing Company, Inc.

126C

mass number A

atomic number Z

element symbol

The atomic number equals the number of protons. Each element has a unique atomic number.

Mass Number• mass number A = protons +

neutrons• always a whole number


• NOT the value given on the Periodic Table!

Practice: determine the required values and write the chemical symbol in A-Z notation.

• Chlorine-37– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

17

37

17

17

20

Cl3717

Ions

• ions are electrically charged atoms

Neutral atom

negative ionpositive ion

lose electrons

gain electrons

p+ > e- p+ < e-

cation anion

Practice: determine the required values for the negative chloride ion 37

Cl -1

37 Cl-1

– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

17

37

17

18

20

Practice: determine the required values for the positive calcium ion 40

Ca +2

40 Ca+2

– atomic #:

– mass #:

– # of protons:

– # of electrons:

– # of neutrons:

20

40

20

18

20


Isotopes: Atoms of the same element with different mass numbers.

carbon-12 and carbon-14 are

isotopes

similar chemical properties

stable

radioactive

Radioisotopes and Their Uses

Radioisotopes are unstable isotopes that undergo radioactive decay. Radioisotopes have a number of uses:

U-235 is used as fuel in nuclear reactorsCo-60 is used in cancer radiation therapyC-14 is used as a tracer and for archeological datingAm-241 is used in smoke detectors

Mass SpectrometerA mass spectrometer is used to detect, identify and measure the abundance of different atoms, molecules or molecular fragments.

Mass spectrometer studies are used to determine the average atomic mass for an element. The operation of a mass spectrometer can be divided into 5 steps:

1. Vaporization2. Ionization3. Acceleration4. Deflection5. Detection

Chapter 12 13=>

Vaporization: the element to be analyzed is heated and vaporized (gaseous form).

http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html



Chapter 12 14=>

Ionization: the gaseous element is injected slowly into a vacuum chamber where the atoms are bombarded by electrons. This forms ions positive ions X (g) + e- X+

(g) + 2 e-




Chapter 12 15=>

Acceleration: the gaseous ions are accelerated through an electric field (towards a negative plate)




Chapter 12 16=>

Deflection: Ions are deflected in an adjustable magnetic field oriented at right angles to the path. Heavier ions are deflected less.




Chapter 12 17=>

Detection: ions of a specific mass are counted




A sample mass spectrograph

Output provides the abundances of the elemental isotopes of different relative mass

Atomic Mass is Relative

• 12C atom = 1.992 × 10-23 g

• 1 p = 1.007276 amu1 n = 1.008665 amu1 e- = 0.0005486 amu


• atomic mass unit (amu)

• 1 amu = 1/12 the mass of a 12C atom

Average Atomic Mass• a weighted average

of all isotopes of an element

100

(%)(mass(mass)(%) )

Avg.AtomicMass

• this value is found on the Periodic Table

• based on the % abundance data from mass spectrometer

Avg.AtomicMass

Average Atomic Mass

• EXAMPLE: Calculate the average atomic mass of chlorine if its abundance in nature is 75.77% 35Cl, and 24.23% 37Cl.

(35)(75.77) (37)(24.23)

10035.48amu

Average Atomic Mass

equation 1

equation 2

(68.9257)(x) (70.9249)(y)69.7231=

100x + y = 100

Average relative mass of Ga 69.7231 amu

Gallium has two naturally occurring isotopes, Ga-69 and Ga-71, with masses of 68.9257 amu and 70.9249 amu, respectively. Calculate the percent abundances of these isotopes

Solve to get 60.1% Ga-69 and 39.9% Ga-71

All EM radiation is fundamentally the same. The only difference between a gamma ray and a radio wave is the frequency/wavelength/energy.

Visible light is one category of EM radiation. The visible light spectrum is subdivided into six “colors”.

White LightPrism

REDORANGEYELLOWGREENBLUE

VIOLET

A continuous spectrum includes all wavelengths of radiation in a given range.

When white light is passed through a prism a continuous spectrum is produced.

Colored lights do not emit all the wavelengths of the visible light spectrum. For example, a red light emits mostly wavelengths from the red end of the spectrum.

An energized gas sample will emit light of specific wavelengths characteristic of the gas. This is called a line spectrum

Emission spectra are unique for each element

The Bohr model of the atom was developed using information from hydrogen emission spectrum studies. Bohr envisioned an atomic model with:

• a central dense positive nucleus composed of protons and neutrons.

• negative electrons at specific energies orbit the nucleus

• mostly empty space. Nucleus is 10-5 times smaller than atom.

Bohr further stated that the orbiting electrons occupy discrete energy levels. Electrons can only “jump” between energy levels if they absorb or emit a specific amount of energy.

Bohr saw the line spectrum of hydrogen as a direct result of energized electrons releasing a specific amount of energy by emitting a photon of light at a certain wavelength.

The different lines in the hydrogen spectrum were evidence for a number of different energy levels.

lower energylonger

wavelength

higher energyshorter

wavelength

Visible spectrum for

hydrogen atom convergence

Lower energy = more stable electron orbit

Electrons fill the lowest energy orbitals first.

Each orbital has a maximum possible number of electrons.

As you should recall:

1st energy level (ground state) = 2 electrons

2nd energy level = 8 electrons

3rd energy level = 8 electrons

The electronic structure of an atom

A carbon atom has six electrons

1st energy level holds 2

2nd energy level takes the remaining 4

The electron structure for carbon would be written as 2,4

The electrons in the outermost energy level are called valence electrons. Carbon has 4 valence electrons.

Try writing the electron structure for calciumA calcium atom has 20 electrons

1st energy level holds 2

2nd energy level holds 8

3rd energy level holds 8

4th energy level holds last 2

The electron structure for calcium would be written as 2,8,8,2

Connect to Periodic Table

1234

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2011 core ib chemistry - topic 02

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