Covalent Bonding and Molecules When compounds form between elements with similar properties, electrons are not transferred from one element to another but instead are shared in order to give each atom a noble gas configuration. Lewis theory depicts bond formation in H 2 as H∙ + ∙H → H:H This type of arrangement, where two atoms share a pair of electrons, is known as covalent bonding, and the shared pair of electrons constitutes a covalent bond.

Covalent Bonding and Molecules A molecule is a combination of at least two atoms in a specific arrangement held together by chemical forces (chemical bonds). A molecule may be an element or a compound. Different samples of a given compound always contain the same elements in the same ratio. This is known as the law of definite proportions. SampleMass of O (g) Mass of C (g) Ratio (g O:g C) 123 g carbon dioxide : g carbon dioxide : g carbon dioxide :2

Covalent Bonding and Molecules If two elements can form two or more different compounds, the law of multiple proportions tells us that the ratio of masses of one element that combine with a fixed mass of the other element can be expressed in small whole numbers. In addition to carbon dioxide, carbon also combines with oxygen to form carbon monoxide. SampleMass of O (g) Mass of C (g) Ratio (g O:g C) 16.3 g carbon monoxide : g carbon monoxide : g carbon monoxide :2

The mass ratio of oxygen to carbon in carbon dioxide is 2.66:1, and the ratio of oxygen to carbon in carbon monoxide is 1.33:1. The ratio of two such mass ratios can be expressed as small whole numbers. Covalent Bonding and Molecules mass ratio of O to C in carbon dioxide mass ratio of O to C in carbon monoxide = =2:1

Covalent Bonding and Molecules Diatomic molecules contain two atoms and may be either heteronuclear or homonuclear. Polyatomic molecules contain more than two atoms.

Classify Each of the Following as Either an Atomic Element, Molecular Element, Molecular Compound, or Ionic Compound Aluminum, Al Aluminum chloride, AlCl 3 Chlorine, Cl 2 Acetone, C 3 H 6 O Carbon monoxide, CO Cobalt, Co atomic element ionic compound molecular element molecular compound atomic element

A chemical formula denotes the composition of the substance. A molecular formula shows the exact number of atoms of each element in a molecule. Some elements have two or more distinct forms known as allotropes.  For example, oxygen (O 2 ) and ozone (O 3 ) are allotropes of oxygen. A structural formula shows not only the elemental composition, but also the general arrangements. Covalent Bonding and Molecules

An empirical formulas, gives the whole-number simplest ratio of elements in a compound. A molecular formula gives the actual number of atoms of each element in a molecule of a compound. The formulas for ionic compounds are empirical – they can be referred as unit formula or formula unit

Covalent Bonding and Molecules Molecular substances can also be represented using empirical formulas, the whole-number simplest ratio of elements. Molecular formula: N 2 H 4 Empirical formula: NH 2 The molecular and empirical formulas are often the same.

Covalent Bonding and Molecules A molecular formula gives the actual number of atoms of each element in a molecule of a compound it does not describe the order of attachment, or the shape The molecular formula is C 2 H 2 O 4. This does not tell you that the carbon atoms are attached together in the center of the molecule, and that each is attached to two oxygen atoms.

Representing Compounds: Molecular Models Models show the 3-dimensional structure along with all the other information given in the structural formula A structural formula uses lines to represent covalent bonds and shows how atoms in a molecule are connected or b Ball-and-stick models use balls to represent the atoms and sticks to represent the attachments between them Space-filling models use interconnected spheres to show the electron clouds of atoms connecting together

Find the empirical formula for each of the following The ionic compound that has two aluminum ions for every three oxide ions arabinose, C 5 H 10 O 5 pyrimidine ethylene glycol Al 2 O 3 CH 2 O C2H2NC2H2N CH 3 O

Covalent Bonding and Molecules