Chapter 3 Section 2

Review of Section 1 What are the 3 particles that make up an atom of an element? 1. Protons 2. Neutrons 3. Electrons Elements in the same group have similar properties. Electrons in the outer energy level are called valance electrons. Elements with the same amount of valance electrons have similar chemical properties.

Objectives Describe the chemical bonds that unite atoms to form compounds Relate the nature of chemical bonds that hold compounds together to the physical structures of compounds. Distinguish among different types of mixtures and solutions.

COMPOUNDS A compound is a substance that is composed of atoms of two or more elements that are chemically combined. Example: the two elements sodium and chlorine combine to form the compound: table salt. Water is another example. What 2 elements combine to form water? Remember: an element is chemically stable when its outer energy level is full.

When an elements outer energy level is NOT full, stability can be achieved by other elements through chemical bonds. Chemical bonds: forces that hold the elements together in a compound.

COVALENT BONDS One way atoms can fill their outermost level is by sharing electrons. Look at Hydrogen. Only has one electron in its outer level When it shares this electron with another Hydrogen atom, they both become more stable. How do the two atoms stay together? Hydrogen’s nucleus is made of one proton (and one neutron), making its overall charge positive. This positive charge attracts the negatively charged electrons.

A covalent bond is the attraction of two atoms for a shared pair of electrons that holds the atoms together.

Molecules A molecule is composed of two or more atoms held together by covalent bonds. Molecules overall have no charge. The total number of electrons equal the total number of protons Molecules are represented by chemical formulas that include the chemical symbol and subscript that stands for the number of atoms of that element in the molecule H2O= 2 Hydrogen atoms and 1 Oxygen atom

POLAR MOLECULES We just learned that water is made up of covalent bonds (sharing electrons) BUT the atoms do not share those electrons equally. With H2O, shared electrons are attracted more strongly to oxygen. As a result, electrons spend more time near the oxygen atom When atoms in covalent bonds do not share atoms equally, they form polar bonds.

A Polar bond has a positive end and a negative end A Polar bond has a positive end and a negative end. The overall shape of a molecule indicates whether it is polar or not.

IONS NOTE: Not all atoms bond by sharing electrons Sometimes, atoms gain or lose electrons from their outermost energy level. Ion: atom that gains or loses an electron, making it a charged particle In general: An atom with an outer energy level that is LESS than half full (has less than 4 electrons) LOSES an electron(s) An atom that has an outer energy level that has more than half of its electrons (more than 4) GAINS an electron(s)

When an atom loses an electron, it becomes positively charged Why?? It now has more protons than electrons When this happens the ion is written with a positive superscript. Example: Na+ If the atom loses 2 electrons it is written: Mg2+

When an atom GAINS an electron, its overall charge becomes negative. Example: O2- an Cl- If the atom’s outer shell is exactly half full (has exactly 4 electrons) it can form either a positive or a negative ion

NOTE: Some compounds contain ions made up of covalently bonded atoms Examples of these include: Silicate SiO44- and carbonate CO32-

IONIC BONDS OPPOSITES ATTRACT! Ionic bond: attractive force between two ions of opposite charge Example: Na+ and Cl- combine to form NaCl Note: Positive Ion is always written first Net charge on NaCl is zero because the positive and negative cancel. Problem Solving Lab pg 63

Metallic Bonds So far we know that most compounds on earth are held together by ionic or covalent bonds, or a combination of the both BUT there are other types of bonds: metallic bonds

When metals bond together, the valence electrons are shared by all the atoms Think of it as a group of positive ions floating in a sea of negative electrons. The positive ions are held together by the negative electrons This allows metals to conduct electricity because the electrons can move freely throughout the entire solid metal.

CHEMICAL REACTIONS Recap: Atoms gain, lose, and share electrons to become more stable, forming compounds. Sometimes compounds break down into simpler substances. Chemical reaction: change of one or more substances into other substances. Chemical reactions are described in chemical equations Example: water is formed by the chemical reaction between Hydrogen (gas) and Oxygen (gas) written as: 2H2 + O2  2H2O

2H2 + O2  2H2O Read as: 2 molecules of hydrogen and 1 molecule of oxygen react to yeild 2 molecules of water Hydrogen and Oxygen are the reactants. Water is the product

Mixtures and Solutions A mixture: combination of two or more components that retain their identities Heterogeneous mixture: when a mixtures components are easily recognizable Example: soil Homogenous mixture: component particles cannot be distinguished, even though they retain original properties. Example: brewed coffee Also called a solution

A solution: a homogenous mixture whose components cannot be distinguished and can be classified as liquid, gaseous, solid, or a combination.

Acids and Bases Acid: solution containing a substance that produces hydrogen ions (H+) in water Base: produces hydroxide atoms (OH-) in a solution A base can neutralize an acid by combining with hydrogen ions of the acid to form water H+ + OH-  H2O

pH The pH scale measures hydrogen and hydroxide ions in solutions, with 7 being neutral pH below 7= acidic pH above 7= basic