Section 8.1

Are electrons only transferred? While studying ionic and metallic bonding, electrons have been moving between atoms. Atoms have lost or gained electrons as they formed ionic bonds, or atoms have so loosely held their valence electrons that they were free to travel from atom to atom – essentially forming a “sea of electrons” in which metal cations existed. + -

What else is possible? Instead of transferring electrons, atoms may stay neutral (not form ions) yet still bond together by sharing electrons – this is known as a covalent bond. This frequently happens when two nonmetals form a bond, such as hydrogen and chlorine. That pair of electrons is shared between the H and Cl atoms.

What’s it called? When the bond(s) holding the atoms together in a substance are covalent bonds, the result is a molecule (which results in a neutral group of atoms). Sometimes atoms of the same element will bond together:

What’s it called? The molecules shown below have two atoms of the same type forming a molecule – they are called diatomic molecules. It will be very important in the future to remember the diatomic elements – N, O, F, Cl, Br, I and H (upside down L, plus hydrogen).

Compounds M0lecules are frequently formed from different atoms covalently bonding together: These examples shown above are called molecular compounds – the compound arises from covalent bonding (not ionic bonding).

Compounds Note that nitrogen monoxide is a diatomic molecule, but the rest consist of 3 or more atoms – they are said to be polyatomic. Every molecule of water is the same: it has 2 hydrogen atoms bonded to an oxygen atom. This gives rise to a molecular formula: H 2 O

Formulas Compounds can be either ionic or molecular – it depends upon the type of bonds they contain. Remember that the formula for an ionic compound is called a formula unit (the simplest ratio of the ions in the compound). Molecular formulas accurately represent how many of each type of atom is bonded together to form the molecule – you do not simplify the ratio.

Why the difference in Formulas? Notice that in an ionic compound (such as salt) the ions are attracted to all oppositely- charged ions that are nearby. However, molecules (such as water) exist as discreet units. It is easy to tell where one molecule of water ends and another begins.

Molecular Formulas Remember that a subscript written after an element symbol in a chemical formula tells how many atoms of the preceding element will be found in one unit of the substance. Water is not the only substance composed of just oxygen and hydrogen atoms. Hydrogen peroxide is too:

Molecular Formulas Although a molecular formula tells you what atoms make up a molecule, they don’t tell you how they are connected. For that you need structural formulas.

Telling the Difference Note that both ethanol and dimethyl ether have 2 carbons, 1 oxygen and 6 hydrogens (C 2 H 6 O). However, they are very different compounds due to the differences in the arrangement of the atoms in the molecules.

Depicting a Molecule Molecules can be represented with different types of models to help you to understand their basic structure. Here are some of the types of models shown for a molecule of ammonia:

Is it ionic or molecular? How do we know whether a bond is covalent or ionic? We compare the electronegativities. If the difference in electronegativity for the two bonded atoms is 2.0 or greater, then the bond is classified as an ionic one. If the difference is less than 2.0, then the bond will be considered to be covalent. These cutoff values were arbitrarily determined by scientists.

Property Comparison Because of the strong electrostatic attraction between ions, ionic bonds are very strong and form in multiple directions. This makes ionic compounds have very high melting and boiling points. Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds. (We will discuss why this is the case in section 4 of chapter 8.) Realplayer\8.1 Covalent bonds overview.flv

References ew/compounds_molecules.jpg ew/compounds_molecules.jpg