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

3517Cl3717Cl

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.

Radioactive Isotopes

• nuclear power generation• sterilization of surgical instruments in

hospitals• crime prevention• finding cracks and stress in metals and• preservation of food

Carbon Dating

• By measuring the relative isotopic mass of C-14 in a given organic – possible to calculate the age of a living organism from the % of C-14 remaining in the sample. (how decay and half life).

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.

Bohr Model• In 1913, Neils Bohr proposed that the electrons in atoms are a

in orbits of differing energy around the nucleus (think of planets orbiting around the sun

Bohr Model• Electron can gain energy by

moving to a higher energy level or

• lose energy by moving to lower energy level

• Transitions of electrons between two discrete energy levels will produce a line spectrum.

• Line spectrum – evidence for electrons being in energy levels (shells).

Emission Spectrum• An electron in a higher energy level (larger value of n) would fall into a lower energy (stable).• This process would involve a loss of energy. • To conserve energy, a particle called a photon carries away the excess energy in this transition.• The energy of this photon = difference in the energy levels of the electron transition ( E2 – E1 )• To identify this photon with the light

found when atoms emit light,we need to know the frequency of the emitted light.

Energy = h x frequency h : Planck's constant

For a given atom, there can be many types of transitions from higher to lower energies, and thus many different energies of emitted photons and, subsequently, many different frequencies of light waves.

• When electrons return to the first level (n = 1) the series of lines occurs in the ultraviolet region (Lyman series )as this involves the largest energy change.

• The visible region (Balmer series )spectrum is formed by electrons dropping back to the n = 2 level and the first series in the infrared (Paschen series) is due to electrons falling to the n = 3 level. The lines in the spetrum converge because the energy levels themselves converge.

Page 56

• Such series of distinct spectral lines are observed experimentally when a gas such as hydrogen is heated up –

the added heat energy excites the electrons, which subsequently fall into lower energy levels,

emitting photons of various energies and, thus, light of various frequencies. • The observed frequencies correspond very well to what is

predicted in the Bohr model.

Line Spectrum

• When energy is supplied to individual elements they emit a spectrum which only contains emssions at particular wavelengths.

• Each element has its own characteristic spectrum known as a line spectrum.

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

Page 63

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.

Electron Arrangements

• Electrons in an atom are arranged in energy levels(shells).

• Max no. of electrons in each shell = 2n2

Main energy level no. 1 2 3 4 5

Max no. of electrons 2 8 18 32 50

• The energy levels are called shells with numbered 1, 2, 3, 4, etc. (principal quantum numbers, n)

• n correspond to the no. of rows (periods) in the PT.

• For a given element, electrons are added to the shells as follows:

- Up to 2 electrons in shell 1- Up to 8 electrons in shell 2- Up to 18 electrons in shell 3

Modern Atomic Structure

n = 1

n = 2

n = 3n = 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

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

Atomic no. (electron config.)

H 1He 2

Li 3

Be 4B 5

C 6N 7O 8F 9Ne 10Na 11

Mg 12

Al 13Si 14P 15