Chapter 3: Atoms and the periodic table Section 2: A Guided Tour of the Periodic Table

Goals/Objectives After completing the lesson, students will be able to . . . Relate the organization of the periodic table to the arrangement of electrons within an atom; Explain why some atoms gain or lose electrons to form ions; Determine how many protons, neutrons, and electrons an isotope has, given its symbol, atomic number, and mass number; Describe how the abundance of isotopes affects an element’s average atomic mass.

Organization of the Periodic Table The periodic table groups similar elements together, which makes it easier to predict the properties of the element based on where it is located on the table. Elements are represented by their symbols and are arranged in an order based on the number of protons found in the nucleus of its atoms.

Organization of the Periodic Table Elements are listed in this order, because the periodic law states that properties will occur in a regular pattern. Periodic Law—Properties of elements tend to change in a regular pattern when elements are arranged in order of increasing atomic number, or number of protons in their atoms.

Turn to page 78-79 in the textbook.

Using the Periodic Table to Determine Electronic Arrangement Periods—A horizontal row of elements in the periodic table. As you move left to right in the periodic table, not only do the number of protons increase by one, but so do the electrons. Protons = positive charge Electrons = negative charge Neutrons = neutral charge (no charge) One way to remember that there are the same number of protons and electrons is the fact that elements on the periodic table are neutral (their positive charges cancel out with the same number of negative charges). Atoms have “energy levels” where the electrons are found. Valence electrons—An electron in the outermost energy level of an atom. Valence electrons determine the chemical properties of atoms.

Elements in the Same Group Have Similar Properties Group (Family)—A vertical column of elements in the periodic table. Elements in groups have the same number of valence electrons, so they have similar properties. THEY ARE NOT EXACTLY THE SAME . . . Just similar. They have different number of protons in their nuclei and different number of electrons in their filled inner energy levels

Some Atoms Form Ions Ionization—The process of adding electrons to or removing electrons from an atom or group of atoms. Some energy levels are only partially filled, which makes it easier for them to gain or lose electrons in order to have a full outer energy level. If an atom gains or loses an electron, it no longer has the same number of electrons as it does protons. Because the charges do not cancel completely like before, the ion that forms has a net electric charge. Ion—An atom or group of atoms that has lost or gained one or more electrons and therefore has a net electric charge.

Ionization Cation Anion In elements with high reactivity, electrons are easily shared. Cation—An ion with a positive charge. This indicates that an electron (negative charge) was “given up,” or removed from this atom. When an element gives up an electron, there is a positive charge and is indicated with putting a “positive” (plus sign) after the chemical symbol. Example: If lithium (Li) gave up an electron, it would be written Li+ Anion—An ion with a negative charge. This indicates that an electron was “gained” to help complete the outermost energy level. This usually occurs within elements that it’s easier to “fill up” the energy level as opposed to “giving up” electrons in order to accomplish the same goal (stable energy levels). When an element gains an electron, there is a negative charge and is indicated with putting a “negative” (minus sign) after the chemical symbol. Example: If Fluorine (F) gains an electron, it would be written F-

How Do the Structures of Atoms Differ? Atoms of different elements have their own unique structures and, because of this, they have different properties. Atomic Number—The number of protons in the nucleus of an atom. Since atoms have the same number of electrons as the protons, the atomic number indicates the number of electrons found in the atom as well. The atomic number for a given element never changes.

How Do the Structures of Atoms Differ? Mass Number—The total number of protons and neutrons in the nucleus of an atom. This number does not include the number of electrons in the atom, because protons and neutrons provide most of the atom’s mass. Although atoms of an element always have the same atomic number, they can have different mass numbers. This occurs when there is a different number of neutrons in the nucleus.

Isotopes Isotopes—Any atom having the same number of protons but different numbers of neutrons. If you want to refer to a certain isotope, you write it like this: AXZ. X = Chemical Symbol Z = Atomic Number A = The number of protons and neutrons combined. In a normal hydrogen element, it would be written 1H1

Isotopes (Q & A) How many isotopes can one element have? Can an atom have just any number of neutrons? No; there are "preferred" combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best. Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons than protons in order to stick together. Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable.

Isotopes (Q &A) I'm not sure what you mean by "unstable." Do atoms just fall apart if they don't have the right number of neutrons? Well, yes, in a way. Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves.

Calculating the Number of Neutrons in an Atom In order to calculate the number of neutrons in the isotopes, subtract their atomic numbers (found in the periodic table) from their given mass numbers. Example: Carbon-14 Subtract their atomic number (6) from the mass number (14). 14 – 6 = 8

The Mass of an Atom The mass of a single atom is very small, which causes it to be quite hard to work with. Atomic masses are usually expressed in atomic mass units (amu). Atomic Mass Unit—A quantity equal to one- twelfth of the mass of a carbon-12 atom. Average Atomic Mass—The weighted average of the masses of all naturally occurring isotopes of an element.

Summary Elements are arranged in order of increasing atomic number so that elements with similar properties are in the same column, or group. Elements in the same group have the same number of valence electrons. Reactive atoms may gain or lose valence electrons to form ions. An atom’s atomic number is its number of protons. An atom’s mass number is its total number of subatomic particles in the nucleus. Isotopes of an element have different numbers of neutrons, and therefore have different masses. An element’s average atomic mass is a weighted average of the masses of its naturally occurring isotopes.