NaCl Li 2 O

Chapter 22 – Chemical Bonds 22.1 Notes – Stability in bonding

Objectives Describe how the properties of compounds differ from those of their constituent atoms. Describe how a compound differs from its component elements. Explain what a chemical formula represents. Discuss why chemical bonding occurs.

Compounds When elements chemically combine Often have chemical and physical properties that aren’t anything like those of the individual elements

Example NaCl = Table Salt – Na = shiny, soft metal that reacts violently with water – Cl = poisonous greenish yellow gas How are Na and Cl different… – Physically? – Chemically?

Chemical Formula Tells what elements a compound contains and the exact number of the atoms of each element in a unit of that compound – Shows the kinds and numbers of atoms in the smallest representative unit – If only one atom of an element, you only write the symbol – If more than one atom, a number is used as a subscript H 2 O 2 Hydrogen atoms and 1 oxygen atom Superscript = written above

Atomic Stability Atoms form compounds because electric forces between electrons and protons attract and hold atoms and molecules together

Noble gases Unusually stable – 8 outer electrons = full valence shell Atoms are stable when they have a full valence shell Elements get full shells by forming compounds – Gain, lose, or share electrons

Electrons and Energy Levels Period = how many energy levels Group = how many valence electrons

How do elements fill outer energy levels? They do this by combining with other atoms that also have partially complete outer energy levels As a result, each achieves stability. Group 1 and 17 (7A) atoms combine easily. Why?

This illustration shows electron dot diagrams for sodium and chlorine. When they combine, sodium loses one electron and chlorine gains one electron. Na had only one electron in its outer energy level, which is lost to Cl as they combine to form sodium chloride. Na drops down to a new outer energy level, and it is stable with eight electrons. – Exceptions: H & He = only need 2 electrons to be stable

Chemical bond Attraction between atoms as they gain, lose, or share electrons, pulling them together to form a compound. A chemical bond is the force that holds atoms together in a compound

In-Class Assignment/Homework 22.1 WKT