Tutorial 1 - Simple Atomic Structure

 Contents

Constituents of the Atom

The simplest model of the atom is shown in the diagram below:

This is the layout of a lithium atom, with three protons, three electrons, and four neutrons.  The protons and neutrons are, of course, found in the nucleus.  They are called nucleons.  The electrons are found in shells orbiting the nucleus.

 Question 1 How many protons neutrons and electrons are there in the lithium atom?

It is important to understand:

• The nucleus is very small compared to the atom, about 10 000 times smaller.  The diameter of an atom is in the order of 10-10 m, whereas the diameter of the nucleus in the order of about 10-15  m.

• The atom is a very dynamic entity.  The diagram shows a stylised representation; the reality is that nucleons are moving about and changing shape all the time.

 Property Electron Proton Neutron Charge -1 e +1 e 0 Mass 9.11 × 10-31 kg 1.67 × 10-27 kg 1.67 × 10-27 kg Relative Mass 1/1836 1.0000 1.0004

Notice that:

• The electron and the proton have the same value of charge, but the signs are different.  We also use a quantity for the charge called electronic charge unit, e.

• 1e = 1.602 × 10-19 C.

• The neutron has a very slightly higher mass than the proton.

 Question 2 A carbon atom has 6 protons and 6 neutrons.  Draw out the carbon atom in a similar way to the lithium atom in the diagram above.
 Question 3 What is the total charge of a carbon nucleus? (a) in electronic charge units (b) in coulombs Question 4 What is the total charge of the electrons? (a) in electronic charge units (b) in coulombs

Charged Atoms

Atoms are neutral because the positive charge and the negative charge cancel out.  If an electron is removed, the atom becomes positively charged and we call the charged atom an ion.  If an electron is added, we get a negative ion.  The protons never move.

The movement of electrons between atoms is at the heart of chemical reactions.

Isotopes

Different atoms are distinguished by their numbers of protons and neutrons.  We write the symbols using the following notation:

•   A is called the nucleon number, or the mass number.  It is the total number of nucleons.

•   Z is the proton number or the atomic number, which is the number of protons.  The number of protons determines the element.

Be careful not to confuse atomic number with the symbol A.  We will refer to A as the nucleon number in these notes and Z as the proton number.

We can determine the number of neutrons simply by subtracting the proton number from the nucleon number.

No of neutrons = A – Z

Atomic particles are always in whole numbers.

• Isotopes have the same numbers of protons, but different numbers of neutrons.

• Isotopes have the same physical and chemical properties.

• If the proton number is altered, the element changes.

• Some isotopes are radioactive, as the nuclei are unstable.

Chemical reactions involve the electrons of the outer shells.  Nuclei are not involved in any way, and remain totally unaltered even in the fiercest chemical reactions.

Question 5

 Carbon 14 is an isotope of Carbon.  Complete the table: Protons Neutrons Electrons Write out carbon-14 in isotope form

Carbon-14 looks like this:

Carbon 14 is an unstable isotope.  It decays so that one of the neutrons turns into a proton.

 Question 6 Can you draw out the new atom?  How many protons, neutrons and electrons does it have?  What is it?

Charge to mass ratio

This is an important quantity to particle physicists.  If two particles have the same charge to mass ratio, they will be deflected by the same amount by magnetic fields or electric fields (you will study these at A2).  Charge to mass ratio is also called specific charge.

The charge to mass ratio is given by this equation:

Charge to mass ratio = charge (C) ÷ mass (kg)

The units are coulombs per kilogram (C kg-1)

The electron has the highest charge to mass ratio, as it's the smallest particle.  For an electron, the calculation is quite simple:

Charge to mass ratio = 1.6 × 10-19 C ÷ 9.11 × 10-31 kg = 1.76 × 1011 C kg-1

For a nucleus (without the electrons) we have to be a bit more careful, as shown in the worked example:

 Worked Example What is the charge to mass ratio of a helium nucleus (alpha particle)? Answer We will assume that the proton and the neutrons have the same mass, 1.67 × 10-27 kg.   In a helium nucleus, there are 2 protons and 2 neutrons, making up 4 nucleons.   Charge to mass ratio = 2 × 1.6 × 10-19 C                               4 × 1.67 × 10-27 kg                                                          = 4.85 × 107 C kg-1

For a neutral atom, the charge to mass ratio is zero, because there is no charge.

For the Lithium ion, Li+, there is a charge of 1.6 × 10-19 C.  There are 7 nucleons (3 protons, and 4 neutrons).

Charge to mass ratio = 1 × 1.6 × 10-19 C    = 1.37 × 107 C kg-1

7 × 1.67 × 10-27 kg

 In the exam, read the question carefully.  Make sure that you are doing a charge to mass ratio, not the charge on the ion.  Specific charge is the same thing as the charge to mass ratio.  The unit for charge to mass is C kg-1 not N kg-1.

 Question 7 A sulphur atom has a nucleon number of 32.  What is the specific charge of an S2- ion? Mass of a nucleon = 1.67 × 10-27 kg; e = 1.6 × 10-19 C.
##### Rutherford Scattering (Extension)

In the early part of the last century, the accepted model of the atom was proposed by J J Thompson in his plum pudding model.  This consisted of a matrix of protons in which were embedded electrons.  Although this may sound ridiculous today, scientists previously believed that the atom could not be broken down at all.

Ernest Rutherford (1871 – 1937) used alpha particles to study the nature of atomic structure with the following apparatus:

Rutherford was using alpha particles (helium nuclei) as nuclear bullets to smash up the atoms; he wanted to see atoms bursting like watermelons.  But…

His observations are best illustrated with this diagram

Instead of bits of atom, Rutherford found that a small proportion of the alpha particles were deflected, while an even smaller proportion bounced right back.  From analysis of these observations he concluded:

• Most of the atom was empty space.

• The positive charge was concentrated in a very small space

• The radius of the nucleus was in the order of 3 × 10-14 m.

• The alpha particles that were deflected back had to be travelling in a line with the nucleus.

Rutherford’s estimates were not far out.  Later research has shown the nuclear radius to be in the order of 1.5 × 10-14 m.  However the boundary is not sharp, but rather fuzzy, as the nucleus is very dynamic.

 Question 8 What led Rutherford to conclude that the nucleus was very tiny and had a positive charge?