Forming Compounds

Framing Questions What is a compound? Why do compounds form? How can atoms bond? What kinds of compounds do these form? What are the properties of ionic compounds? What are the properties of molecular compounds? Define ionic charge, oxidation number, diatomic molecule and electrolyte.

Why do Compounds Form? So we know that compounds are pure substances with two types of atoms bonded together. Bonds occur because of the “Happy Atom Syndrome” – trying to become stable An atom may acquire a full valence shell in three ways: An atom may give up electrons (becomes a cation +) An atom may gain electrons (becomes an anion -) An atom may share electrons

Why do Compounds Form? When atoms give up or take up electrons to form ions, they gain a charge (+ or -). This charge causes them to attract the opposite charge and form a bond. When atoms share electrons, they also form a bond.

How do Atoms Together? "Stick" Bonding involves either a transfer or a sharing of electrons to form “stable” ions. There are 2 main types of bonds that hold compounds together: 1. Ionic bonds (transfer electrons) 2. Covalent bonds (share electrons)

Ionic Bonds Because anions and cations are electrically attracted to each other (electronegativity), they form compounds called Ionic Compounds (ionic bond = ionic compound)

Animations to ponder…

Ionic Charge and Oxidation Numbers To figure out which elements will form ionic bonds together, you must understand their ionic charge. An element’s ionic charge is directly related to its number of valence electrons. If an element has less than 4 valence electrons, it will give them away and become positive. It’s ionic charge is the # of electrons it will give away and its charge (+). If an element has more than 4 valence electrons, it will take more and become negative. It’s ionic charge is the # of electrons it will take and its charge (-). Its oxidation number is the same as its ionic charge. Ionic Charge and Oxidation Numbers Element # of valence electrons Ionic charge Oxidation number Na 1 Na+ +1 Mg 2 Mg2+ +2 P 5 P3- -3 O 6 O2- -2

Covalent Bonds Covalent bonds are formed when atoms share electrons. A neutral (non-charged) particle that is made up of atoms joined by covalent bonds is called a molecule, so we have Molecular Compounds.

Animations to ponder…

Diatomic Molecules Molecules containing two atoms are called “diatomic”. This means they have paired with themselves, sharing electrons (covalent bonds). Seven elements exist as diatomic molecules in the gaseous state: Hydrogen Oxygen Nitrogen Flourine Chlorine Bromine Iodine

Make sure you understand these next properties for our upcoming lab…

Properties of Ionic Compounds Ionic compounds are usually bonds between metal cations and non-metal anions. These bonds form a regular, repeating pattern which forms a crystalline shape. Properties: Strong bonds because the attraction between oppositely charged ions is strong Tend to have relatively high melting points because a large amount of energy is needed to break the strong bonds Conduct electricity when absorbed in water (electrolyte = a substance that dissolves in water to produce a solution that conducts electricity) Not electrical conductors in a solid state

Properties of Molecular Compounds Molecular compounds are formed when atoms of non-metals are joined by covalent bonds. Properties: Strong bonds but weak forces of attraction between molecules (because no charges) Tend to have relatively low melting points because little energy is needed to break the forces of attraction between molecules Do not conduct electricity in solid or liquid state or when absorbed in water because they do not contain ions (no charges)

Homework Answer Think about it… Na can give away an electron and be happy, so it’s likely to form an ion and therefore an ionic bond If H gives away its electron, it doesn’t have any, so it’s more likely to form a covalent bond and share it instead!