1.1 Atom
All matter is composed of atoms，existing individually or in combination with each other. An atom is an extremely small electrically-neutral particle. It is the smallest unit involved in the chemical change of matter. Atoms can be treated as distinct particles because they behave as such chemically，but atoms themselves are composed of even smaller subparts，including electron，proton and neutron.
The electron is a particle with a negative electrical charge and has a mass of 9.107×10-28 grams. This mass is 1/1837 the mass of a hydrogen atom，and corresponds to 0.0005486 atomic mass unit （AMU）. One atomic mass unit has a mass of 1.660×10-24 grams. Although the actual electrical charge of an electron is known，its value is too cumbersome for practical use. The electron，therefore，has been assigned a relative electrical charge of -1. The size of an electron has not been determined exactly，but its diameter is believed to be less than 10-12 centimeters.
The proton is a particle with a positive electrical charge. It has an actual mass of 1.672×10-24 grams，1837 times the mass of an electron. This mass is slightly less than that of a hydrogen atom.① On the relative scale，the proton has a mass of approximately one atomic mass unit. The electrical charge of the proton is equal in magnitude but opposite to that of the electron; hence，the relative charge of a proton is +1.
The other major component of the atom，the neutron，is a neutral particle，with neither a positive nor a negative electrical charge. The mass of a neutron，1.675×10-24 grams，although slightly greater than that of a proton，is practically equal to the mass of a proton. The assigned relative mass of the neutron is approximately one atomic mass unit.
An atom is composed of a positively-charged nucleus orbited by negatively-charged electrons. The nucleus is the core of an atom，containing two kinds of particles，the neutron and proton. Because the neutron is electrically neutral and proton is electrically positive，the nucleus has a positive charge. Both the neutron and proton give the nucleus its mass. The particles that orbit the nucleus are electrons. Much lighter，negatively-charged electrons occupy a relatively large space around the nucleus. The charge of one electron is equal in magnitude to that of one proton. The number of electrons orbiting a nucleus is exactly equal to that of protons contained in the nucleus. The equal and opposite charge cancel each other，and the atom as a whole is neutral. The electrons are bound in the atom by electrostatic attraction. The atom remains neutral unless some external force causes a change in the number of electrons.② If an atom is expanded to the size of one of our largest football stadium，the nucleus would be about the size of a marble at the center.
The number of protons in the nucleus plays such an important role in identifying the atom that it is given a special name，the atomic number. The symbol Z is often used for atomic number （or number of protons）. Hydrogen has an atomic number of 1 and chlorine has an atomic number of 17. The atomic number is also equal to the number of electrons.
The atomic mass numberis equal to the sum of the total number of protons （Z） and the total number of neutrons （N） as shown in equation，where A stands for the atomic mass number. Not all atoms of the same element have the same atomic mass number，although the Z is the same，the N and thus the A are different.③ Atoms of the same element with different atomic mass numbers are called isotopes.
A=Z+N