Atomic Structure

IB Chemistry Power Points

Topic 02

Atomic Structure

2.1: The nuclear atom 2.1 The nuclear atom Understandings: • Atoms contain a positively charged dense nucleus composed of protons and neutrons (nucleons). • Negatively charged electrons occupy the space outside the nucleus. • The mass spectrometer is used to determine the relative atomic mass of an element from its isotopic composition.

• Use of the nuclear symbol notation to deduce 𝑋𝑋 𝑍𝑍 𝐴𝐴the number of protons, neutrons and electrons in atoms and ions.

• Calculations involving non-integer relative atomic masses and abundance of isotopes from given data, including mass spectra.

2.1 Application and skills

ATOM.. Atomos

• An atom is the smallest particle of an element that retains its identity in a chemical reaction.

• Although early philosophers and scientists could not observe individual atoms, they were still able to propose ideas about the structure of atoms.

Once upon a time…

Democritus atomic theory • Democritus reasoned that

atoms were indivisible and indestructible.

– Although, Democritus’s ideas agreed with later scientific theory, they did not explain chemical behavior

– They also lacked experimental support because Democritus’s approach was not based on the scientific method.

Democritus who?

John Daltons Atomic theory • By using experimental methods, Dalton transformed Democritus’s ideas

on atoms into a scientific theory.• Dalton studied the ratios in which elements combine in chemical

reactions.• All elements are composed of tiny indivisible particles called atoms.• Atoms of the same element are identical. The atoms of any one element are

different from those of any other element.• Atoms of different elements can physically mix together or can chemically

combine in simple whole-number ratios to form compounds.• Chemical reactions occur when atoms are separated from each other, joined,

or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

Thomson’s Plum Pudding model

Discovered Electrons • So maybe the atom is

divisible after all • Thomson discovered the

“electron” by conducting his cathode ray experiment.

• He then proposed the plum pudding model of the atom.

Rutherford’s Gold-Foil Experiment

Rutherford’s results were that most alpha particles went straight through, or were slightly deflected.

• What was surprising is that a small fraction of the alpha particles bounced off the gold foil at very large angles.

• Some even bounced straight back toward the source.

• Which lead to the discovery of the nucleus.

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

© Addison-Wesley Publishing Company, Inc.

• 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

© Addison-Wesley Publishing Company, Inc.

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 19=>

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 20=>

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-

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

Chapter 12 21=>

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

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

Chapter 12 22=>

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

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

Chapter 12 23=>

Detection: ions of a specific mass are counted

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

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

© Addison-Wesley Publishing Company, Inc.

• 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

2.2 Electron Configuration

• Understandings • Emission spectra are produced when photons are emitted

from atoms as excited electrons return to a lower energy level.

• The line emission spectrum of hydrogen provides evidence for the existence of electrons in discrete energy levels, which converge at higher energies.

• The main energy level or shell is given an integer number, n, and can hold a maximum number of electrons, 2n2.

• A more detailed model of the atom describes the division of the main energy level into s, p, d and f sub-levels of successively higher energies.

• Sub-levels contain a fixed number of orbitals, regions of space where there is a high probability of finding an electron.

• Each orbital has a defined energy state for a given electronic configuration and chemical environment and can hold two electrons of opposite spin.

• Applications -and Skills

• Description of the relationship between colour, wavelength, frequency and energy across the electromagnetic spectrum.

• Distinction between a continuous spectrum and a line spectrum.

• Description of the emission spectrum of the hydrogen atom, including the relationships between the lines and energy transitions to the first, second and third energy levels.

• Recognition of the shape of an s atomic orbital and the px, py and pz atomic orbitals.

• Application of the Aufbau principle, Hund’s rule and the Pauli exclusion principle to write electron configurations for atoms and ions up to Z = 36.

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

1

2

3

4

5

6

7

6

7

1A2A

3B4B5B6B7B8B8B8B1B2B

3A4A5A6A7A8Agroup # = # valence (outside) e-

d p

f

sRow

=# shells

Electron Configuration

1s1

row #shell #

possibilities are 1-77 rows

subshellpossibilities are

s, p, d, or f4 subshells

group ## valence e-

possibilities are:s: 1 or 2

p: 1-6d: 1-10f: 1-14

Total e- should equalAtomic #

What element has an electron configuration of 1s1?

Practice:Ask these questions every time you have to write an electron

configuration• Lithium:

1. find the element on the periodic table2. what is the period number?3. how many shells?4. what is the group number?5. how many valence electrons?6. what subshell(s) does Li have? 7. what is the electron configuration?

atomic # = 32

2

11

s

1s2 2s1

Practice:Ask these questions every time you have to write an electron

configuration• Boron:

1. find the element on the periodic table2. what is the row #?3. how many shells?4. what is the group #?5. how many valence electrons?6. what subshell(s) does B have? 7. what is the electron configuration?

atomic # = 52

2

33

p

1s2 2s2 2p1

Order of Electron Subshell Filling:It does not go “in order”

1s2

2s2 2p6

3p6

4p6

5p6

6p6

7p6

3s2

4s2

5s2

6s2

7s2

3d10

4d10

5d10

6d10

4f14

5f14

1s2 2s22p6 3p63s2 4s2 4p65s23d10 5p6 6s24d10 6p67s25d104f14 7p66d105f14

1

2

3

4

5

6

7

6

7

per

iod

# =

# e

- sh

ells 1A

2A

3B4B5B6B7B8B8B8B1B2B

3A4A5A6A7A8Agroup # = # valence e-

d

f

3d

4d

5d

6d

4f

5f

Subshells d and f are “special”

Electron Configuration

1s1

row #shell #

possibilities are 1-77 rows

subshellpossibilities are

s, p, d, or f4 subshells

group ## valence e-

possibilities are:s: 1 or 2

p: 1-6d: 1-10f: 1-14

Total e- should equalAtomic #

What element has an electron configuration of 1s1?

Practice:Ask these questions every time you have to write an electron

configuration• Lithium:

1. find the element on the periodic table2. what is the row #?3. how many shells?4. what is the group #?5. how many valence electrons?6. what subshell(s) does Li have? 7. what is the electron configuration?

Practice:Ask these questions every time you have to write an electron

configuration• Boron:

1. find the element on the periodic table2. what is the row #?3. how many shells?4. what is the group #?5. how many valence electrons?6. what subshell(s) does B have? 7. what is the electron configuration?

Order of Electron Subshell Filling:It does not go “in order”

1s2

2s2 2p6

3p6

4p6

5p6

6p6

7p6

3s2

4s2

5s2

6s2

7s2

3d10

4d10

5d10

6d10

4f14

5f14