Science succeeds after 20 years in the hunting of the quark
Everything is made up of tiny particles called atoms. At one time people thought that the atom was the smallest particle that could exist. The name atom comes from a Greek word that means 'cannot be divided'. But scientists like Marie Curie showed that atoms must be made up of smaller particles. The smaller particles were shown to be of three different types, with different masses and electrical charges. They became known as sub-atomic particles.
1 Complete the table below. (You will probably need to look up the information in a textbook.)
Sub-atomic particle Relative mass Electrical charge electron proton neutron
All neutrons have the same mass, and all protons have the same mass. The mass of a neutron is approximately equal to the mass of a proton. The mass of an electron, by contrast, is so small that it makes very little difference to the mass of an atom. So the total mass of an atom is very close to the mass of its protons plus the mass of its neutrons.
The number of protons plus the number of neutrons is a good guide to the mass of an atom, and scientists have called this the mass number of an atom:
mass number = number of protons + number of neutrons.
An atom contains the same number of electrons as protons. Electrons and protons carry an equal and opposite electrical charge, and neutrons carry no charge. So the atom itself is uncharged because the charges on the electrons balance the charges on the protons. The number of protons (or electrons) in an atom is called the atomic number.
The number of neutrons in an atom can be worked out if the mass number and the atomic number are known, because
number of neutrons = mass number - atomic number.
Most of an atom is empty space. The protons and neutrons are held firmly together at the centre of the atom, in the nucleus. The electrons orbit the nucleus, constantly moving like planets around the Sun. The electrons are held in orbit in different shells. Electrons in shells close to the nucleus have less energy than those in shells further away. The shells are said to have different energy levels. They are numbered starting with the one closest to the nucleus, so the higher a shell's number, the higher its energy level.
There is a maximum number of electrons that each shell can hold. For example, the first shell can hold up to two electrons; the second and third shells can hold up to eight electrons each. An electron cannot go into a higher shell until the previous shell is full.
An oxygen atom has eight electrons, so two fill the first shell and the other six go into the second shell. Oxygen has an electronic arrangement of 2,6.
2 An atom of sodium has 11 electrons and a mass number of 23. a) How many neutrons does it have? b) What is its atomic number? c) What is the total charge on an atom of sodium?
3 Draw a diagram, similar to the one given for oxygen, to show the arrangement of electrons in an atom of a) helium (atomic number 2) b) carbon (atomic number 6) c) nitrogen (atomic number 14).
4 If an atom of an element has enough electrons to fill one or more shells completely, the element is said to be stable. Which of the following elements would you expect to be stable? a) Element X: electronic arrangement 2,1 b) Element Y: electronic arrangement 2,8 c) Element Z: electronic arrangement 2,8,1.
5 Scientists now know that the proton-electron-neutron model of the atom is oversimplified. The article tells you more about the current theories on the structure of an atom. a) What is a quark? b) What 'flavours' do quarks come in? c) How was the top quark created? d) What was surprising about the mass of the top quark? e) Why was it important for the theory of the atom that the top quark was found?
