2.4 The structure of the atom Atomic structure How can atoms join together to make molecules? What makes certain atoms more ready to do this? Why do hydrogen atoms pair up but helium atoms remain single? To find answers to questions like these, we need first to consider the structure of atoms in general. Dalton thought they were solid, indivisible particles. But research since then has shown that atoms are made up of various subatomic particles. J. J. Thompson discovered the electron (in 1897) and the proton. Crucial experiments were then carried out in Rutherford’s laboratory in Manchester in 1909 that showed that the atom is largely empty space. Rutherford calculated that an atom is mostly space occupied by the negatively charged electrons, surrounding a very small, positively charged nucleus. The nucleus is at the centre of the atom and contains almost all the mass of the atom. By 1932, when the neutron was discovered, it was clear that atoms consisted of three subatomic particles – protons, neutrons and electrons. These particles are universal – all atoms are made from them. The atom remains the smallest particle that shows the characteristics of a particular element. Measuring the size of atoms Modern methods such as scanning tunnelling microscopy have allowed us to ‘see’ individual atoms in a structure. However, atoms are amazingly small! A magnification of 100 million times is necessary to show the stacking pattern of the atoms that make up a gold bar. A single atom is so small that it cannot be weighed on a balance. However, the mass of one atom can be compared with that of another using a mass spectrometer. The element carbon is chosen as the standard. The masses of atoms of all other elements are compared to the mass of a carbon atom. This gives a series of values of relative atomic mass for the elements. Carbon is given a relative atomic mass of exactly 12, which can be written as carbon-12. Table 2.6 gives some examples of the values obtained for other
Activity 2.6 Discovering the structure of the atom Skills Research skills
ICT skills
The discovery of the nature of the subatomic particles that make up all atoms took place in a relatively short space of time around the beginning of the twentieth century. Investigate this key period in the history of science using library and internet sources. Devise a PowerPoint or poster presentation on the significant discoveries and the scientists involved. Key scientists to research are J. J. Thompson, Ernest Rutherford and James Chadwick.
Questions A1 What was remarkable about the structure of the atom suggested by the Geiger–Marsden experiments? A2 What is it about the nature of the neutron that made it the last of the particles to be discovered?
Element
Atomic symbol
Relative atomic mass
carbon
C
12
hydrogen
H
1
oxygen
O
16
calcium
Ca
40
copper
Cu
64
gold
Au
197
Table 2.6
The relative atomic masses of some elements.
elements. It shows that carbon atoms are 12 times as heavy as hydrogen atoms, which are the lightest atoms of all. Calcium atoms are 40 times as heavy as hydrogen atoms.
Original material © Cambridge University Press 2014
Chapter 2: The nature of matter
41