– ATOMS –
Atoms are the smallest particles of matter that have the properties of the chemical elements – hydrogen, oxygen, iron, and so on. They are so small that it is impossible to see them even with a high-powered microscope. Everything on Earth is made up of atoms in different chemical combinations. Water, for instance, is a compound of two elements, two atoms of hydrogen and one atom of oxygen. However, some elements, such as gold and diamonds exist uncombined.
Ninety-two elements occur naturally. They range from the lightest, hydrogen, to the heaviest, uranium. Each of the elements has been assigned a number – 1 for hydrogen, 8 for oxygen, 29 for copper, 92 for uranium. They are usually arranged on a chart called the periodic table, which puts elements with the same chemical properties in the same column. Thus, all inert gases, such as helium, appear in one column in the periodic table.
The formulation of the atomic theory is one of the great achievements of science. It has enabled us to understand the properties of the elements, the basic building blocks of all matter, so that we know which elements can combine with each other. The science of chemistry is based on our understanding of atoms and their behaviour in interacting with one another. Another science called nuclear physics came into being to study the structure of the atom itself. As scientists investigated the atom, it became apparent that the atom was not a solid piece of matter, but was made up of even smaller particles. The first subatomic particle that scientists identified was the electron, a tiny piece of matter with a negative electric charge.
The weight of an electron was very small indeed – approximately one eighteen-hundredth of the weight of a hydrogen atom, the lightest of all the elements. Scientists came to believe that the electrons orbited the nucleus of the atom, in which almost all of the weight of the atom was concentrated. It is now known that electrons revolve around the nucleus at incredibly fast rates of speed. For many years scientists did many different kinds of experiments and all had the same idea about the structure of atoms.
However, when they managed to obtain more evidence, they had to modify the atomic theory. There was not just one kind of particle in the nucleus of an atom; there were two. One of these has a positive electric charge and is called a proton. The other is neutral, that is, it has no electric charge. For this reason, it was called a neutron.
A. Complete the following sentences.
1. Helium (line 13) is a(n) .
2. Elements (line 17) are .
3. An electron (line 25) is a(n) .
4. If something is neutral (line 38), it .
B. What do.the following refer to?
1. They’ (line 8): Elements which
2. ‘It’ (line 16):
3. ‘all’ (line 34): all
4. ‘these’ (line 37): these
C. Mark the statements as True (T) or False (F).
1. The theory about the structure of atoms has changed through years.
2. An electron is heavier than a hydrogen atom. 3. Electrons turn around the nucleus at a low speed.
1. In what way are gold and diamonds different from other elements?
2. What does nuclear physics study?
E. Complete the following statement.
The periodic table arranges elements according to…………………………….