Atomic Structure and Chemical Bonds Chapter 6 Atomic Structure and Chemical Bonds

I. Atomic Structure At the center of every atom is a nucleus containing protons and neutrons Electrons travel in an area of space around the nucleus called the electron cloud The electron cloud is mostly empty space and the exact location of any one electron cannot be determined

A. Electrons Electrons have negative charges Electrons travel in predictable areas, but it is impossible to know their exact location

B. Element structure Each element has a specific number of protons, neutrons, and electrons The number of protons and electrons is always the same for a neutral atom of a given element

II. Electron Arrangement A. Electron energy The different areas for an electron in an atom are called energy levels Each energy level represents a different amount of energy

B. Number of electrons Each energy level has a specific number of electrons it can hold The farther an energy level is from the nucleus, the more electrons it can hold

To determine the maximum number of electrons that can occupy an energy level, use the formula, 2n2, where n equals the number of the energy level Level 1 = 2 electrons = 2(12) Level 2 = 8 electrons = 2(22) Level 3 = 18 electrons = 2(32) Level 4 = 32 electrons = 2(42)

= 32 e- = 18 e- = 8 e- = 2 e-

C. Energy steps Electrons in the level closest to the nucleus have the lowest amount of energy Electrons farthest from the nucleus have the highest amount of energy and are the easiest to remove - the closer a negatively charged electron is to the positively charge nucleus, the more strongly it is attracted to the nucleus

= 32 e- = 18 e- = 8 e- = 2 e- Energy

III. Periodic Table and Energy Levels The atomic number for each element is the same as the number of protons in that element The number of protons equals the number of electrons in a neutral atom Therefore, the atomic number also tells you how many electrons are in a neutral element

IV. Electron Configuration Atoms with a complete outer energy level are stable Atoms with exactly eight electrons in an outer energy level are also stable Each period in the periodic table ends with a stable element

V. Element Families Each column of the periodic table contains one element family Members of element families have similar chemical properties because they have the same number of electrons in their outer energy levels

A. Noble Gases Elements in Group 8 have eight electrons in their outer energy level Because their energy levels are stable, they do not combine easily with other elements Helium is also stable because its single energy level is complete with only two electrons

B. Halogens Elements in Group 7 need one more electron to obtain a stable outer energy level The easier it is for a halogen to gain this electron, the more reactive it is The reactivity of the halogens decreases down the group as the outer energy levels get farther from the nucleus - the protons cannot pull electrons in as easily when they are farthest from the nucleus

C. Alkali metals Elements in Group 1 each have one electron in their outer most energy level The easier it is to remove an electron, the more reactive the atom is The reactivity of the alkali metals increases down the group as the outer energy levels get farther from the nucleus - less energy is needed to remove an electron from an energy level that is farther from the nucleus

VI. Electron Dot Diagrams Because it is the outermost electrons that determine an atoms chemical properties, it is helpful to make models that show only those electrons An electron dot diagram is the symbol for the element surrounded by as many dots as there are electrons in its outer energy level

A. How to write them Dots are written in pairs on four sides of the element symbol Group numbers 1 and 2 and 3-8 tell you how many electrons are in the outer shell for those elements

B. Using dot diagrams Dot diagrams help show how many chemical bonds an atom can make A chemical bond is the force that holds two atoms together Atoms bond with other atoms so that each atom has a stable outer shell - each atom likes eight electrons in its outer shell except for hydrogen and helium (they like two)

VII. Ionic Bonds-Loss and Gain Atoms form bonds with other atoms in one of four ways: 1. losing electrons 2. gaining electrons 3. pooling electrons 4. sharing electrons with another atom

An ionic bond forms between two atoms when one atom losses an electron and another atom gains an electron

A. Ions – A question of balance Sodium has only one electron in its outer level The sodium atom loses an electron and becomes more stable with eight electrons in its outer level An ion is an atom that has gained or lost an electron Sodium lost an electron, so it now has one more proton than electron Sodium is now a positive ion, Na+, because it lost an electron

Chlorine has seven electrons in its outer level The chlorine atom gains one electron and becomes more stable with eight electrons in its outer level Chlorine gained an electron, so it now has one more electron than proton Chlorine is now a negative ion, Cl-, because it gained an electron

B. Bond formation The positive sodium ion and the negative chloride ion are strongly attracted to each other This attraction which holds the ions close together creates an ionic bond

A compound is a pure substance containing two or more elements that are chemically bonded The compound made by sodium chloride is table salt (NaCl)

VIII. Metallic Bonding – Pooling Metals form bonds with other metals by forming a shared pool of electrons Metallic bonding allows metals to conduct electricity because the electrons are allowed to move from one atom to the next to transmit current

IX. Covalent Bonds – Sharing The chemical bond that forms between nonmetal atoms when they share electrons Shared electrons are attracted to both nuclei so each atom has a stable outer energy level some of the time A molecule is the neutral particle formed when two atoms share electrons and have an equal number of protons and neutrons

A. Double bonds When two pairs of electrons are involved in a covalent bond ex. In carbon dioxide, CO2, the carbon atom shares two of its electrons with each oxygen atom

B. Triple bonds Three pairs of electrons are shared between two atoms ex. N2

X. Polar and Nonpolar Molecules Some electrons are not shared equally between two atoms Some atoms have a greater attraction for electrons than others do An unequal sharing of electrons makes one side of the bond more negative than the other A polar bond is a bond in which electrons are shared unevenly

A. The polar water molecule In a water molecule, H2O, the oxygen atom has a greater share of the electrons in each bond The oxygen end of a water molecule has a slight negative charge and the hydrogen end has a slight positive charge

Water is said to be polar; it has two oppositely charged ends like poles on a magnet Molecules that do not have these uneven charges are called nonpolar molecules