Atomic Structure

Relative masses/charges of protons, neutrons and electrons

Sub-atomic particle

Relative Mass

Relative Charge

Proton (p) 1 +1Neutron (n) 1 0Electron (e) 5 x 10-4 -1

Protons Neutrons Electrons Electronic Configuration

168O

2311Na

2311Na

16 28O

Isotopes• Isotopes are atoms of the same element with

the same ________(same number of protons) but different __________(different no. of neutrons).

Isotope Protons Neutrons Electrons

3517Cl

3717Cl

Pg 57 Test yourself

Isotopes• react the same way (same chemical properties)

N2 + 3H2 2NH3

N2 + 3D2 2ND3

• have different physical properties has a boiling pt of -2530C whereas has a boiling pt of -2500C

12H

22H

Why?

Chemical properties – the same number and arrangement ofelectrons

Physical properties – different masses hence move at diferent speed

Radioactive Isotopes• Many isotopes are radioactive as the nuclei of

these atoms break down spontaneously, emitting radiation.

• 3 different forms of radiation - Gamma radiation : highly penetrating - Alpha radiaton : can be stopped by a few cm

of air- Beta radiation : can be stopped by a thin

sheet of aluminium.

The mass spectrometer

Atoms/molecules are bombarded by high energy electron.Atoms are ionised by knocking one or moreelectrons off to give a positive ion.M(g) + e M+(g) + 2e

Ions are deflected bya magnetic field.

The beam of ions is detected electrically

Positive ions are acceleratedin an electric field.

http://www.chemguide.co.uk/analysis/masspec/howitworks.html

Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on:• the mass• positive charge on the ion.

The lower the mass/charge (m/z) ratio,the more the ions are deflected

Lighter ions are deflected more than heavier ones.Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge.

Lightest ions

Heaviest ions

40 220Ca

3717Cl

Which ion will deflect the most in a mass spectrometer?

137 256Ba

The mass spectrum shows that iron has 4 isotopes as follows:

Calculate the relative atomic mass of iron.

The mass spectrometer measures the relative abundance of different isotopes (atoms) of an element. The output is a mass spectrum.

Pg 61 Test yourself

An element has an atomic number of 24. The natural element consists of four isotopes.

The mass spectrum of the element X produced the following peaks of three of its isotopes on the chart recorder.

(a) What is element X classified in the Periodic Table? (b) Calculate the isotopic mass of the 4th isotope if the relative atomic mass

of element X is 52.06.

Rutherford’s planetary model

• Electron should emit energy and spirally fall on the nucleus. But the atome is stable.

Niels Henrik Bohr• Tackled the question of why the electrons did not fall on the nucleus.• Conducted experiments on the emission spectra of hydrogen atoms.• The light emitted created a line spectrum.

Excitation of Electrons

• When an atom receives enough energy, the electron can jump to higher levels, farther from the nucleus.

• The electrons are in __________ states.• Once excited (unstable), the electron rapidly falls to a lower

level.• The excited state had an excess energy. So, the atom must __________ energy.• We observe the emission of a ______________________.

The Bohr Model• The electron can circle the nucleus only in

__________ orbits designated by a quantum number, n.

• Those orbits correspond to specific _______ distance from the nucleus and correspond to a very specific ________ state of the electron.

• The electron CANNOT exist at an other distances or have any other energy except the allowed ones.

The Bohr Model• The quantum number can have integer values,

n = 1 corresponding to the orbit __________to the nucleus.

• When an electron resides in the orbit designated by n = 1 it is said to be in the ___________ state.

• Excited electrons give out energy when they return to their ground (stable) state.

• Loss of energy seen as visible light or other radiation.

• Captured on photograhic plates as emission spectral.

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The visible hydrogen spectrum• The spectrum consists of discrete lines and that the lines

converges towards the high energy (violet) end of the spectrum.

The lines in the spectrum get closer together at higher frequency / energy

Energy = h x frequency Energy = h x speed of light / wavelength

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Continuous Spectrum

• White light is made up of all the colours of the spectrum.

• When it is passed through a prism, a continuous spectrum of all the colours can be obtained.

• A continuous spectrum contains all wavelengths from a band of the electromagnetic spectrum.

In summary• The H atom has only ertain allowed energy levels

called _________ states.• The atom does not ______ energy while in one of

these stationary states. • The atom changes to another stationary state only by

absorbing or emitting a ________ whose energy equals the difference between the 2 states.

• Bohr was thus able to explain the distinct lines (not continuous) in the atomic spectra of hydrogen

Precedents to Quantum Theory

• However, Bohr’s theory of distinct energy levels could explain electron behavaiour in atoms beyond __________.

• At this time, a lot of work about the nature of light was being investigated.

• Light was seen both as a wave and a particle – the idea of duality.

Quantum Mechanical Model

• Erwin Schrodinger• Werner Heisenberg• Wolfgang Pauli• Friedrich Hund

• In order to explain the nature of the electron, the idea of electrons being particles was changed to that of a dual nature (De Brogile)

Erwin Schrodinger

• The position of the electron was determined using a ________ equation (incoporate both the wave & particle properties of the electron)

Quantum Mechanical Model

• Erwin Schrodinger’s work showed that electrons do not circle orbits.

• Since the position of an electron varies, the term “orbital” was used, instead of shells.

• Electrons can be found with a high probability in specific regions of space called “orbitals”

• An orbital is defined as the region withn which there is a ______% probability of locating a particular electron in a free atom.

Quantum Mechanical Model

• In quantum theory, the electron shells are not fixed orbits but clouds of probability.

• Cannot measure the exact location of the electron.

Quantum Mechanical Model

• The electron is located within a sphere (or shell) around the nucleus – the probability of finding it near the nucleus is higher, but never 100%.

• The quantum shells are called orbitals.

Quantum Mechanical Model

• Modern quantum theories lead to stable locations of electrons, which are not exact planetary orbits but are characterized by specific quantum numbers.

Principal Quantum Number, n

• Values of n = 1,2,3,... (positive integers only)• Determines orbital size i.e. the larger the value

of n, the ___________ the orbital.• Also, determines the __________ level of the

electrons it contains and the average ___________ from the nucleus.

Principal Quantum Number, n

• Each shell is characterized by a different principal quantum number (n).

• Larger n => __________shell is from nucleus and ___________ in energy.

Principal Quantum Number, n

• n also determines the ____________ number of electrons in the shell : 2n2

Shell 1 2 3 4 5

Max no. of electrons 2 8 18 32 50

Modern Atomic Structure

n = 1

n = 2

n = 3

n = 4

Hein, Arena, Foundations of College Chemistry, 2000, page 202

1s

2s

3s

4s

2p

3p

4p

3d

4d 4f

Sublevel designation

An orbital for a hydrogenatom. The intensity of the dots shows that the electronspends more time closer tothe nucleus.

The first four principal energylevels in the hydrogen atom.Each level is assigned aprincipal quantum number n.

The types of orbitals on eachof the first four principalenergy levels.

Energy Level Diagram of a Many-Electron Atom

ArbitraryEnergy Scale

18

18

32

8

8

2

1s

2s 2p

3s 3p

4s 4p 3d

5s 5p 4d

6s 6p 5d 4f

NUCLEUS

O’Connor, Davis, MacNab, McClellan, CHEMISTRY Experiments and Principles 1982, page 177

Atomic no. (electron config.)

H 1He 2

Li 3

Be 4

B 5

C 6

N 7O 8F 9Ne 10Na 11

Mg 12

Al 13Si 14P 15