How Do the Structures of Atoms Differ?

• The atomic number,Z, of an atom equals the number of protons in the nucleus.
• The mass number,A, of an atom equals the
  number of protons plus the number of neutrons in
  the nucleus.
• An isotope is an atom that has the same number of protons as other atoms of the same element do but that has a different number of neutrons.

Example: Hydrogen has three isotopes

• Some isotopes are more common than others.
• If you know the atomic number and mass number of an atom, you can calculate the number of
  neutrons it has.
• Example: uranium-235 has a mass number of 235. Like all uranium atoms, it has an atomic number of 92. The number of neutrons it has is therefore:

Mass number (A): 235

Atomic number (Z): –92

Number of neutrons: 143

• Because the mass of a single atom is so tiny, atomic masses are usually expressed in atomic mass units.
• An atomic mass unit (amu) is equal to one twelfth of the mass of a carbon-12 atom.
• The average atomic mass for an element is a weighted average of the masses of all naturally-occurring isotopes of an element.

How Are Elements Classified?

• The elements are classified into three groups.
• Most elements are metals, elements that are shiny and conduct heat and electricity well.
• Nonmetals, all except hydrogen of which are found on the right side of the periodic table, may be solids, liquids, or gases at room temperature.
• Between these groupings are semiconductors, elements that can conduct electricity under
  certain conditions.
• The periodic table below shows the distribution of metal, nonmetals, and semiconductors in the periodic table.
• The alkali metals, found in Group 1 of the periodic table, are very reactive.
• The alkaline-earth metals, which include calcium, are found in
  Group 2 of the periodic table, and are somewhat less reactive than the
  alkali metals.
• The transition metals, such as gold, iron, and mercury, occupy Groups
  3–12 of the periodic table.
• Carbon is found in three different forms and can form many compounds.
• Nonmetals and their compounds are plentiful on Earth.
• Halogens, such as chlorine, are located in Group 17 of the periodic table.
• Noble gases, such as neon, make up Group 18 of the periodic table.
  They are unreactive.
• Semiconductors are intermediate conductors of heat and electricity.
• Silicon is the most familiar semiconductor.
• Silicon is an important part of computer chips, as well as other semiconductor devices such as transistors, LED display screens, and solar cells.
• The mole is useful for counting small particles.
• A mole (abbreviation: mol) is the number of particles that is the same as the number of atoms of carbon in 12 g of carbon-12.
• Avogadro’s constant is the number of particles per mole of a substance: 6.022 × 10 23
• Moles and grams are related.
• The mass in grams of 1 mol of a substance is called its molar mass.
• For example, 1 mol of carbon-12 atoms has a molar mass of 12.00 g.
• The molar mass of an element is its average atomic mass, which
  is listed in the periodic table.