IPS Unit 9 Chemical Bonding and Formulas Section 1

Compounds Some of the matter around you is in the form of uncombined elements such as copper and oxygen. Like many other elements, these will unite chemically to form a new compound when the conditions are right. The compound formed when elements combine often has properties that aren’t anything like those of the individual elements. A chemical formula tells what elements a compound contains and the exact number of the atoms of each element in a unit of that compound. it uses atomic symbols and numerical subscripts H2O NaCl Pb(NO3)2

Formulas A subscript written after a symbol tells how many atoms of that element are in a unit of the compound. If a symbol has no subscript, the unit contains only one atom of that element. A subscript written after a parentheses, means you need to multiply everything inside the parentheses by the subscript.

Atomic Stability The electric forces between oppositely charged electrons and protons hold atoms and molecules together; and thus are the forces that cause compounds to form. Atoms of noble gases are unusually stable because they have 8 electrons in their outer energy level. (with the exception of Helium which has 2) Because of their stability, the Noble Gases rarely form compounds. To understand the stability of the Noble Gases, it is helpful to look at electron dot diagrams. Electron dot diagrams show only the valence electrons.

Valence Electrons Valence electrons are electrons in the outer most energy level Only use Main Group elements Range is from 1 to 8 The group number is the number of valence electrons (minus 10 in the p block)

Valence Electrons Elements in the same group/family have the same number of valence electrons Valence electrons determine how an element will bond with other elements

Valence Electrons Practice How many valence electrons does each atom have? 1 2 Li Ca F 7 3 Al O 6 N 5

Electron Dot Diagrams Electron Dot Diagrams are notations in which only the valence electrons of an element are shown, indicated by dots placed around the element’s symbol The elements symbol is used The number of electrons you need to use is the number of valence electrons for that element Dots, or x’s, are placed around the symbol Only place electrons in one of the four directions: top, right, bottom, left Never place electrons all around the symbol or at the corners Place electrons one at a time on each side before putting a second electron on a side

Electron Dot Diagrams 1 valence electron X 3 valence electrons X To show how electron dot diagrams are written, an X will be used to show a generic element. 1 valence electron X 3 valence electrons X 5 valence electrons X 8 valence electrons X

Electron Dot Diagrams Practice Draw Electron-Dot Diagrams for: Sodium Carbon Oxygen Argon Na C O Ar

Energy Levels and Elements Hydrogen contains only one electron and therefore one valence electron. This means that hydrogen’s outer energy level is not full and therefore not stable. In contrast, helium’s valence contain two electrons. Helium already has a full outer energy level by itself and is chemically stable.

Energy Levels and Elements When you look at the Main Group elements through the halogens, you see that each of the elements falls short of having a stable energy level. Each group contains too few electrons for a stable level of eight electrons.