4.1 Defining the Atom

• Democritus believed that atoms were indivisible and indestructible.
• By using experimental methods, Dalton transformed Democritus's ideas on atoms into a scientific theory.
• Scientists can observe individual atoms by using instruments such as scanning tunneling microscopes.

4.2 Structure of the Nuclear Atom

• Three types of subatomic particles are electrons, protons, and neutrons.
• In the nuclear atom, the protons and neutrons are located in the nucleus. The electrons are distributed around the nucleus and occupy almost all the volume of the atom.

4.3 Distinguishing Among Atoms

• Elements are different because they have different numbers of protons.
• The number of neutrons in an atom is the difference between the mass number and atomic number.
• Because isotopes of an element have different numbers of neutrons, they also have different mass numbers.
• To calculate the atomic mass of an element, multiply the mass of each isotope by its natural percent abundance (expressed as a decimal), and then add the products.
• The periodic table lets you easily compare the properties of one element (or a group of elements) to another element (or group of elements).