Atoms can attain a more stable arrangement of electrons in their outermost shell by interacting with one another. An ionic bond is formed when electrons are transferred from one atom to the other. A covalent bond is formed when electrons are shared between atoms.

Molecular Compounds and Ions 1. What does it mean to say that an atom is neutral? contains equal number of protons and electrons 2. What would happen to the charge on an atom if an electron were removed? If an electron were added? have a positive charge, have a negative charge 3. What is the name for an atom that has a charge? an ion 4. What is a molecule? A particle that contains more than one atom 5. How do the molecules of elements and compounds differ? the molecules of a compound contain atoms from more than one element

With the exception of the noble gases, which exist as isolated atoms, most elements found in nature form molecules Molecule – smallest electrically neutral unit of a substance that still has the properties of the substance. A molecule is made up of two or more atoms that act as a unit Examples: H2O O3 CO2

Compounds – Atoms of different elements can combine chemically to form compounds. Compounds composed of molecules are called molecular compounds; Compounds composed of ions are called ionic compounds Molecular Compounds tend to have relatively low melting and boiling points exist as gases or liquids at room temperature most molecules of molecular compounds are composed of the atoms of two or more nonmetals Ions – not all compounds are molecular, many are composed of particles called ions. Ions are atoms or groups of atoms that have a positive or negative charge. An ion forms when an atom or group of atoms loses or gains electrons

Ions EXAMPLE: NaCl net charge on a sodium atom is zero [11+(-11) = 0] in forming a chemical compound, an atom of sodium tends to lose one of its electrons, making its number of electrons no longer equal to the number of protons the atom of sodium becomes an ion, because there are more positive charges (protons) than negative charges (electrons), the sodium ion has a positive charge

NaCl

Cation – atoms or groups of atoms that have a positive charge atoms of metallic elements tend to form cations by losing one or more electrons cations have fewer electrons than the electrically neutral atom from which it is formed an ionic charge is written as a number followed by a sign What is the charge of a sodium cation? [1+ (11 protons, 10 electrons)] = Na+ or Na1+ What is the charge on a magnesium ion? [2+ 12 protons, 10 electrons)] = Mg2+ How many electrons does an Aluminum atom lose to form an aluminum cation? (3 electrons)

Ions vs. Atoms There are many differences between the metal and its respective cation Example: Sodium metal reacts violently with water, but sodium cations are quite unreactive

Anion – atoms or groups of atoms that have a negative charge atoms of nonmetallic elements tend to form anions by gaining one or more electrons anions have more electrons than the electrically neutral atom from which it is formed the name of an anion of a nonmetallic element is not the same as the element name. Anions typically end in “-ide”

EXAMPLE: NaCl net charge on the chlorine atom is zero [17+ (-17) = 0] in forming a chemical bond, a nonmetal tends to gain electrons to form an anion the chloride anion has 17 protons and 18 electrons, it has an ionic charge of 1- written as Cl- Common anions Oxide ion, oxygen atom gains 2 electrons, giving it an ionic charge of 2-, written as O2-

Examples Cations are givers (positive) Anions are takers (negative) sulfur forms a _________ anion Sulfide S2- bromine forms a __________ anion Bromide Br-

Cations & Anions Activity

Ionic Compounds – compounds composed of cations and anions ionic compounds are usually composed of metal cations and nonmetal anions Ionic compounds, even though they are composed of ions that are electrically charged, the total positive charge equals the total negative charge. The overall compound is electrically neutral Usually solid crystals at room temperature, melt at high temperatures

Examples NaCl - sodium cation & chloride anion CaO (lime) – calcium cations & oxide anions

Characteristics of Molecular and Ionic Compounds Molecular Compound Ionic Compound Representative unit Molecule Formula unit (balance of oppositely charge ions) Type of elements Nonmetallic Metallic combined with nonmetallic Physical state Solid, liquid, or gas Solid Melting point Low (usually below 300C) High (usually above 300C)

Review Questions- Molecular Compounds & Ions Give the name and symbol of the ion formed when: a sulfur atom gains two electrons sulfide ion S2- an aluminum atom loses three electrons aluminum ion Al3+ a calcium atom loses two electrons calcium ion Ca2+

Review Questions- Molecular Compounds & Ions How many electrons are lost or gained in forming each ion? Ba2+ 2 lost As3- 3 gained Cu2+ What is the symbol and name for the cation formed when: a potassium atom loses one electron K+ a zinc atom loses two electrons Zn2+

Review Questions- Molecular Compounds & Ions Write the symbol and name for the anion formed when: a fluorine atom gains one electron F- a sulfur atom gains two electrons S2-