Compounds Ionic Covalent

2.5 Molecules and Molecular Compounds

• A molecule consists of two or more atoms bound together.
• Each molecule has a chemical formula.
• The chemical formula indicates
1. Which atoms are found in the molecule, and
2. In what proportion they are found
• Compounds composed of molecules are molecular compounds
• These contain at least two types of atoms.
• Different forms of an element have different chemical formulas are known as allotropes. Allotropes differ in their chemical and physical properties. (Chapter 7 will give more information on allotropes of common elements)

Molecular and Empirical Formulas
• Molecular formulas
 Give the actual numbers and types of atoms in a molecule.
 Examples: H2O, CO2, CO, CH4, H2O2, O2, O3, and C2H4
• Empirical formulas
 Give the relative numbers and types of atoms in a molecule (they give the lowest whole-number ratio of atoms in a molecule).
 Examples: H2O, CO2, CO, CH4, HO, CH2

Picturing Molecules
• Molecules occupy three-dimensional space.
• However, we often represent them in two dimensions.
• The structural formula gives the connectivity between individual atoms in the molecule.
• The structural formula may or may not show the three-dimensional shape of the molecule.
• If the structural formula does not show the shape of the molecule, then either a perspective drawing, ball-and-stick model, or space-filling model is used.
 Perspective drawings use dashed lines and wedges to represent bonds receding and emerging from the plane of the paper.
 Ball-and-stick models show atoms as contracted spheres and the bonds as sticks. The angles in the ball-and-stick model are accurate.
 Space-filling models give an accurate representation of the relative sizes of the atoms and the 3D shape of the molecule.

2.6 Ions and Ionic Compounds

• If electrons are added or removed from a neutral atom, an ion is formed.
• When an atom or molecule loses electrons, it becomes positively charged.
 Positively charged ions are called cations.
• When an atom or molecule gains electrons, it becomes negatively charged.
 Negatively charged ions are called anions.
• In general, metal atoms tend to lose electrons, and nonmetal atoms gain electrons.
• When molecules lose electrons, polyatomic ions are formed (e.g. SO42-, NO31-)


Predicting Ionic Charges

• An atom or molecule can lose more than one electron.
• Many atoms gain or lose enough electrons to have the same number of electrons as the nearest noble gas (group 18 or 8A).
• The number of electrons an atom loses is related to its position on the periodic table.

Ionic Compounds

• A great deal of chemistry involves the transfer of electrons between species.
• Example:
 To form NaCl. the neutral sodium atom, Na, must lose an electron to become a cation: Na1+.
 The electron cannot be lost entirely, so it is transferred to a chlorine atom, Cl, which then becomes an anion: Cl1-.
 The Na1+ and Cl1- ions are attracted to form an ionic NaCl lattice, which crystallizes.
• NaCl is an example of an ionic compound – consisting of positively charged cations and negatively charged anions.
 Important: note that there are no easily identified NaCl molecules in the ionic lattice. Therefore we cannot use molecular formulas to describe ionic substances.
• In general, ionic compounds are usually combinations of metals and nonmetals, whereas molecular compounds are generally composed of nonmetals only.
• Writing empirical formulas for ionic compounds:
 You need to know the ions of which it is composed.
 The formula must reflect the electrical neutrality of the compound.
 You must combine cations and anions in a ratio so that the total positive charge is equal to the total negative charge.
 Example: consider the formation of Mg3N2:
• Mg loses two electrons to become Mg2+
• Nitrogen gains three electrons to become N3-
• For a neutral species, the number of electrons lost and gained must be equal.
• However, Mg can lose electrons only in twos, and N can accept electrons only in threes.
• Therefore, Mg needs to form 3 Mg2+ ions (total 3 x 2 positive charges) and 2 N atoms need to form 2 N3- ions (total 2 x 3 negative charges)
• Therefore, the formula is Mg3N2
•

Chemistry and Life: Elements Required by Living Organisms

• Of the 112 elements known, only about 26 are required for life.
• Water accounts for more than 70 percent of the mass of the cell.
• Carbon is the most common solid constituent of cells.
• The most important elements for life are H, C, N, O, P, and S (red).
• The next most important ions are Na1+, Mg2+, K1+, Ca2+, and Cl1- (blue)
• The other 15 elements are needed only in trace amounts (green).



CHEMISTRY The Central Science 8th Edition Brown, LeMay, Bursten Ch 2: Atoms, Molecules, and Ions