Part I Atoms, Molecules and Stoichiometry

Ch 1: p.1

L.S.T. Leung Chik Wai Memorial School

F.6 Chemistry

Atomic Structure, Radioactivity and Relative masses

PART I ATOMS, MOLECULES AND STOICHIOMETRY

Chapter 1 : Atomic Structure, Radioactivity and Relative Masses

1.1 The Atomic Structure

Atoms – They are the smallest particles of an element that can take place in chemical changes.

Fundamental particles in an atom – Protons and neutrons make up the nucleus, which is surrounded by the electrons.

The size of the atom – X ray work shows that the diameter of atoms are of order of 2x10-10 m, which is 0.2 nm (1 nm = 10-9 m)

The masses of the atom – Ranges from 10-27 to 10-25 kg.

I. Discovery

Electron – Proposed by G.J. Stoney (1874)

Discovered by Thompson (1897)in the Cathode Ray Tube experiment.

Proton - Proposed by Goldslein (1886)

Discovered by Rutherford (1899) in the Gold Foil Experiment.

Neutron - Discovered by Sir James Chadwick (1932)

Bombarding Be with a particles to emit neutrally electrically particles.

II. Properties of the fundamental particles of an atom

Mass / Charge
Proton / 1 unit (1.6725 x 10-24 g) / +1
Neutron / 1 unit (1.6725 x 10-24 g) / 0
Electron / 1/1837 unit (negligible) / -1

Note:

<1> The nucleus is a dense, positively charged body.

<2> In between the electrons and nucleus is space.

<3> The number of protons should be equal to the number of electrons because the atom should be electrically neutral.

<4> The number of protons is approximately equal to the number of neutrons.

<5> Atoms can link up to form molecules which are more stable.

Examples: O2 , N2 , CO2 , H2O

<6> Ions are produced when atoms or molecules gain or lose electrons.

Examples:

Cation: Na – e à Na+

Anion: Cl + e à Cl-

III. The Nucleus of the Atom

Nuclear Structure

(A) Nuclear charge – It is the total positive charges on the protons.

e.g. He atom has +2 unit of nuclear charge.

Atomic number (Z) – It is the number of protons in the nucleus.

Mass number (A) – It is the sum of number of protons and neutrons.

Isotopes: Atoms of same element that have the same number of protons but different number of neutrons. (or same atomic number but different mass number)

Naturally occurring Isotopes of some common elements:

16O 35Cl 1H 12C

17O 37Cl 2H 13C

18O 3H 14C

Isotopic abundance: It is the abundance of each isotope in a mixture of isotopes for an element.

(B) Isotopic Mass and Relative Atomic Mass

It is defined that the mass of a 12C nuclide is equal to 12.000 atomic mass units (a.m.u.)

That is

The masses of the three fundamental particles:

Mass of proton = 1.007177 a.m.u.

Mass of neutron = 1.008665 a.m.u.

Mass of electron = 0.0005486 a.m.u.

Isotopic mass of a nuclide:

e.g. 3517Cl : It has 17 protons and 18 neutrons

Isotopic mass of 3517Cl = 17 x 1.007177 + 18 x 1.008665

= 35.277979 a.m.u.

Relative isotopic mass = isotopic mass / (1/12 x mass of 12C )

e.g. The relative isotopic mass of 3517Cl = 35.277979 a.m.u. / 1 a.m.u.

= 35.277979

* The actual value for isotopic mass of the 3517Cl nuclide is 34.96885

Relative Atomic mass (Ar) : It is the average of the relative isotopic masses weighted to take account of the isotopic abundances.

e.g. Relative atomic mass of chlorine:

Isotopes / Isotopic relative isotopic mass / Abundance
35Cl / 34.97 / 75%
37Cl / 36.95 / 25%

Ar of Cl = 35.4545

This technique has two important uses:

<1> The determination of relative isotopic masses and abundances of isotopes.

<2> The determination of relative molecular masses and structure of organic compounds.

In mass spectrometry, the device used is mass spectrometer.

How a mass spectrometer works:

<1> A gaseous sample of material is allowed to enter the ionization chamber.

<2> An electron gun bombards the sample with electrons, forming positively charged ions. (mainly single charge : +1 )

X(g) + e (fast) à X+(g) + 2e (slow)

<3> The ions are accelerated by an electric field and deflected along a circular path by a magnetic field.

The lighter the particles, the greater degree of deflection.

<4> The intensity of the ion beam is detected electrically, amplified and finally recorded.

It is produced by increasing the magnetic field so that ions of higher mass / charge ratios are gradually brought into the line with the detector.

Height of the peak = relative isotopic abundance

The peak with the highest m/e ratio will most likely correspond to the ‘molecular ion’, i.e. the molecule which has lost only a single electron. In most mass spectra, the values of m/e ratio can be converted to the relative masses of the particles if the charges on the ions are taken to be one.

Name: ______Class No.:______Date:______Marks:______

Exercises

1. What do you understand by the terms (a) relative atomic mass , (b) isotope ?

Outline, with the aid of a labeled diagram, the use of the mass spectrometer in the determination of relative atomic mass.

( 10 marks )

1.2 Radioactivity

(A) Discovery
<1> Henri Becquerel (1896)
- Uranium salts caused photographic plate to ‘fog’ even covered by a layer of black paper.

<2> Mme Curie (1898)
- Radioactivity of pitchblende (mainly U3O8 ): two radioactive elements : polonium and radium.
- She identified two radiation from radium:
a -radiation and b- radiation

<3> Pierre ( 1900 )

- g- radiation

(B) Types of ionizing radiation : a- ,b-, g- radiation

Properties

Nature / Charge / Mass / Velocity / Relative penetration
a / He nuclei / +2 / 4 units / 0.05c / 1
b / Stream of fast moving e / -1 / 1/1836 units / 3-99% c / 100
g / Electromagnetic radiation / No charge / Nil / c / 10000

<1> a- radiation: Stream of high velocity of He nuclei

When there is a loss of an a- particle of an element,

Atomic number: decrease by ______(loss of ____ protons )

Mass number : decrease by ______( loss of ____ protons and _____ neutrons )

e.g. The nuclide radium-226 loses an a- particles to form the nuclide radon –222

Nuclear equation :

<2> b- radiation – Stream of electrons at high speed.
The electrons are emitted from unstable nuclei as a result of the splitting of a neutron.

10n à 11p + 0-1e

When there is a loss of b- particle,

atomic number : increase by ______mass number: ______

e.g. The nuclide thorium-234 emits an b- particle to form a nuclide of protactinium-234

Nuclear equation:

<3> g- radiation: High energy electromagnetic radiation

It has high penetrating power and low ionization power. g-rays are emitted when a nuclide emits a- or b- particles.

Note: g- rays are also emitted in a process called electron capture.

e.g. 3718Ar + 0-1e à 3717Cl

Name: ______Class No.:______Date:______Marks:______

Exercise

1. In 1909, Geiger and Marsden reported the results of experiments in which a beam of a-particles was directed at very thin metallic foils. The large majority of the a-particles went through the foils without deflection, but some were deflected from their straight-line parts and a very small number were deflected or scattered backwards.

(a) What is an a-particles? State its mass number and its charge.

(b) Why were most a-particles not deflected?

(d) What do these experiments illustrate about the structure of the metal atoms?

(e) State one other method of deflecting a-particles.

( 6 marks )

The radioactivity can be detected and measured by use of a Geiger-Muller tube. It consists of a chamber filled with argon. The radiation passes through a thin mica window and ionize the argon. The positive argon ions move to a cathode and the electrons to the anode. This creates an electrical pulse, which is amplified. The pulse registers as clicks, light flashes or meter readings on the counter.

(D) Stable and Unstable isotopes

Note:

<1> Naturally occurring elements consist of one stable isotope in higher abundance.

<2> Unstable isotopes are radioactive and known as radioactive isotope or radioisotope.

<3> The stability of an isotope depends on the neutron/proton ratio.

Stable isotopes (elements) of low atomic number (<20) should be equal to 1:1

For isotopes with higher atomic numbers, the ratio increases up to 1.6 to maintain stability. (see fig 1.15)

(E) Decay series

A decay series can be represent graphically by plotting number of neutrons against number of protons or by plotting mass number against atomic number.

(F) Natural Radioisotopes

Uranium minerals 238U are the greatest sources of natural radioisotopes.

Example: Carbon-14 emits b-radiation.

(G) Artificial Nuclear Transformations

Example: The bombardment of aluminium nuclei with a-particles to form a phosphorus-30 nuclei as well as a neutron.

Note:

<1> Neutrons are particular useful in such nuclear reaction because it is neutral.

<2> New elements are made after the transformation.

Such nuclear reactions can be expressed by nuclear equations.

The symbol AZX is used to represent the nuclide X with atomic number Z and mass number A. The symbols used to represent the various types of radiation or particles are listed below.

Particles/ Radiation / Symbols
Alpha particle / a or 42He
Beta particle / b or 0-1e
Neutron / 10n

Examples:

94 Be + 42He à ______+ 10n

2311Na + 10n à ______+ 11H

147N + 21H à ______+ 10n

<3> The sums of the atomic number and mass number must always be balanced on the left and right hand side of such nuclear equation.

Mass number: Left: ______right : ______

94Be + 11H à 63Li + 42He

Atomic number: Left: ______right: ______

Name: ______Class No.:______Date:______Marks:______

Exercise

1. State what type of emission occurs at each of the following stages in the decay chain for:

22888Ra à 22889Ac à 22890Th à 22488Ra

(a) (b) (c)

Indicate by nuclear equations (balanced atomic and mass number) the nuclear reaction when:

(d) 5927Co absorbs a neutron.

(e) 14N reacts with an a-particle and the proton is emitted.

(f) 156C decays with the emission of a b-particles.

( 6 marks )

(H) Uses of Isotopes (Radioisotopes)

1. Leak detection: A radioactive source is introduced into systems like storage tanks and buried pipelines, and a detector is used to locate the position of leakage.

Leakage checking

2. Radiotherapy: g radiation is used in cancer treatment.

3. Nuclear power: Nuclear reactions result in the release of a vast quantity of energy, which is used to generate electricity.

4. As tracers: Radioactive isotopes can be used as tracers to study metabolism in living organism.