CCEA ADVANCED SUBSIDIARY
CHEMISTRY
1.1
Atomic Structure
Electrons, protons
and neutrons as the constituent particles of the atom. Their location in the
atom, their relative masses and charges. Atomic number, mass number and
isotopes.
When you have finished this section you should be able to:
·
Describe the properties of
protons, neutrons and electrons in terms of their relative charge and relative
mass ;
·
Understand the importance of
these particles in the structure of the atom ;
·
Define the terms atomic
number, Z and mass number, A ;
·
Use values for atomic number
and mass number to calculate the number of protons and neutrons in the nucleus
;
·
Explain the existence of
isotopes
·
Use isotopic symbols to
describe the composition of the nucleus.
All atoms are electrically neutral. The number of electrons in the shells is the same as the number of protons in the nucleus. The mass is made up almost entirely from the masses of the protons and neutrons. The masses of the proton and neutron are virtually identical.
|
Particle |
Relative Mass |
Relative Charge |
|
Proton (p) |
1 |
+1 |
|
Neutron (n) |
1 |
0 |
|
Electron (e) |
0 (1/1837) |
-1 |
J.J. Thompson (1897) experiments with cathode ray tubes. He discovers a beam of rays emitted from the cathode when an electric current passes through a gas at low pressure. The rays are deflected by electric and magnetic fields and consist of a stream of electrons.
Electric discharges through gases at low pressure produces a stream of particles from the anode. Work with electric and magnetic fields shows them to be positively charged. Hydrogen gives the lightest particles, which are assumed to be protons.
Chadwick (1932) bombards beryllium with a-particles producing fast moving particles, which are not affected by electric or magnetic fields. The particles are neutrons.
Atoms consist of a very small, dense nucleus, around which the electrons circulate in a comparatively large volume. The volume of the nucleus is about 10-44 m3 and it is composed of two different types of particles, protons and neutrons. These are known collectively as nucleons.
An atom is small, but its nucleus is smaller still. The radius of an atom is of the order of 10-10 m, but the radius of a nucleus is of the order of 10-15 m.
Do the next exercises will help you to appreciate the difference in size.
Suppose a football, diameter 22 cm, is scaled up so that it becomes as big as the earth, diameter 13000 km.
Calculate whether an atom of diameter 0.32 nm (3.2 x 10-10 m) will become as big as:
A a pin head, diameter 1mm
C a football, diameter 22cm
D a weather balloon, diameter 1.8 m
If the nucleus of an atom were scaled up to the size of a pin head (say 1 mm diameter), how big would the atom be?
Since the mass of an atom is concentrated in its nucleus, the nucleus must be extremely dense. Estimate how dense it is by doing the next exercise.
For atoms of elements at the beginning of the Periodic Table the volume of the nucleus, VN, is given by:
VN = 1.73 x (relative atomic mass) x 10-45 m3
Use this expression to calculate the density of the sodium nucleus:
(a) in kg m-3
(b) in tonnes cm-3
(Remember that 1 mol of sodium atoms weighs 23.0 g and contains 6.02 x 1023 atoms)
Calculate:
(a) the volume occupied by a sodium atom (radius 1.86 x 10-10 m)
(b) the fraction of the volume occupied by the nucleus.
(Hint: assume that both the atom and its nucleus are spheres with volume given by 4πr3/3 )
With all this empty space why does any object appear solid? The electrons in an atom move very rapidly around the nucleus. The electrons effectively form a shield around the nucleus, marking the limits of the atom’s volume and making it seem solid.
Atomic number and mass number give us important information about an atom and are particularly useful in distinguishing one isotope of an element from another.
The atomic number (Z) of an atom is the number of protons in the nucleus.
The mass number (A) is the total number of particles in the nucleus.
Isotopes are atoms of an element that have different numbers of neutrons in the nucleus i.e. they have the same atomic number but different mass numbers.
e.g. 3517Cl and 3717 Cl.
The table shows the mass number and number of neutrons in the nucleus, for four atoms, W, X, Y and Z.
W X Y Z
Mass number 36 39 40 40
Neutrons in nucleus 18 20 21 22
a) Write down the atomic numbers of the four atoms.
b) Which of the four atoms are isotopes of the same element?
c) Use your Periodic Table to write isotopic symbols (e.g. 2713Al) for the four atoms.
Relative
atomic mass, relative isotopic mass and relative molecular mass. The carbon-12
standard. The use of the mass spectrometer to obtain accurate atomic masses.
(Details of the workings of the mass spectrometer are not required). Deduction
of Relative Molecular Mass from a molecular ion peak. (Limited to ions with
single charges).
When you have finished this section you should be able to:
·
Calculate the masses of coins
relative to a chosen standard ;
·
Express masses in a variety of
units ;
·
Define the terms relative
atomic mass (Ar), relative isotopic mass and relative molecular mass
(Mr) in terms of carbon-12 ;
Atoms are so small that their masses, expressed in grams, are difficult to work with. Some examples are listed in Table 1 below
Table 1
|
Element |
Average mass of atom g |
|
H |