Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 13 



world politics is a tool of atomic energy. The most startling display of energy 

 that had ever been known to the world occurred on August 6, 1945, when an 

 atomic bomb exploded over Hiroshima, Japan, and uranium atoms (U-235) 

 broke apart and unloosed their extraordinary power. 



Structure of Matter 



The physical states of matter are more or less easily changed by conditions 

 about them. In shifting temperatures, the state of water may be a gas, fluid, or 

 solid, i.e., vapor, rain, and sleet in quick succession. The composition of mat- 

 ter is not thus easily changed, the elements and their compounds, the atoms 

 and molecules. Atoms are the incredibly minute, organized units of matter that 

 are the building blocks of elements. 



An element is composed of one kind of atom for which it is named, oxygen, 

 carbon, calcium, and so on. One hundred elements are known, mainly dis- 

 covered in nature: certain radioactive ones have also been created experimen- 

 tally. The elements are distributed unevenly. Four of them, oxygen, carbon, 

 hydrogen, and nitrogen constitute 96 per cent of living matter; less than 20 

 make up 99 per cent of the atmosphere, the ocean, and the earth's crust. 



Molecules are usually the units peculiar to an element or a compound. 

 Molecules of elements contain two or more atoms of the same kind. Molecules 

 of compounds have two or more different kinds of atoms. The molecule of 

 water has two atoms of hydrogen and one of oxygen (Fig. 2.3). 



Molecules are continually attracted to one another by intermolecular force 

 that is electrical rather than gravitational. They are in constant motion, in a 

 random jumpy dance. They are too small to be visible and the dance cannot 

 be seen but can be felt as heat. When a substance is cold, e.g., ice, the dance 

 is slow; when hot, e.g., boiling water, the dance is extraordinarily rapid. Turn 

 an electrical current through a cold iron and the dance of the molecules is 

 changed from the slow to rapid rate. The motion never stops. The lounger in 



Hydrogen 



H 



ooo 



MOLECULES 



Oxygen 



Water 



Fig. 2.3. Diagram of the formation of a molecule of water by the sharing of elec- 

 trons between two atoms of hydrogen and one of oxygen. Electrons are the particles 

 that take part in chemical reactions. 



