Electrons are located in an area around the nucleus called the electron cloud. The electron cloud is made up of different energy levels. The electrons in the outermost energy level of an atom are called valence electrons. The number of valence electrons in an atom determines how and whether or not the atom will form chemical bonds with other atoms. Energy Levels

Atoms of most elements are more stable when they have 8 valence electrons. Atoms usually react in a way that makes it more stable: –either it gains electrons, increasing the number of valence electrons to 8 (or 2 in the case of hydrogen) –or, it gives up loosely held valence electrons Stable Atoms

Chemical bonds form when the valence electrons of one atom interact with the valence electrons in another atom. When atoms bond, electrons may be transferred from one atom to another, or they may be shared between atoms. Chemical Bonds

One way to depict the number of valence electrons is through an electron dot diagram. –Diagram includes the symbol for the element –Each dot stands for one valence electron Electron Dot Diagrams (Lewis Dot Diagram)

Periods: –from left to right the atomic number increases –as the atomic number increases, the number of electrons also increases Groups: –from top to bottom, elements in a group always have the same number of valence electrons –the elements within a group have similar properties because they all have the same number of valence electrons Bonding & the Periodic Table

Valence Electrons & the Periodic Table

Group 1 elements (Alkali Metals) have 1 valence electron and can become chemically more stable if they lose one valence electron. Because of this, they are very reactive.

Group 2 elements (Alkaline Earth Metals) lose 2 electrons easily and are almost as reactive as Group 1.

Group 3 to 12 elements (Transition Metals) – In general, the reactivity of metals decreases from left to right across the Periodic Table.

Metalloids can either lose or share electrons – can behave as a metal or nonmetal depending on the conditions.

Nonmetals become more stable when they gain or share electrons – all nonmetals have 4 or more valence electrons. They combine with metals by gaining electrons. They combine with other nonmetals by sharing electrons.

Group 17 elements (Halogens) have 7 valence electrons – a gain of just one makes them stable – because of this, they react easily with other elements that can give or share one electron.

Group 18 elements (Noble Gases) have 8 valence electrons, except for He which has 2, are stable, and not reactive.

Ionic Bonds An ionic bond is the attraction between two oppositely charged ions. Involves the transferring of electrons Ionic bonds form as a result of the attraction between positive and negative ions.

Covalent Bonds The chemical bond formed when two atoms share electrons is called a covalent bond. The force that holds atoms together in a covalent bond is the attraction of each atom’s nucleus for the shared pair of electrons. –Covalent bonds usually form between atoms of nonmetals –Ionic bonds usually form when a metal combines with a nonmetal

Atoms of some elements pull more strongly on shared electrons than do atoms of other elements. As a result, the electrons are pulled more toward one atom, causing the bonded atoms to have slight electrical charges.

Covalent Bonds A covalent bond in which electrons are shared equally is called a nonpolar bond. A covalent bond in which electrons are shared unequally is called a polar bond.