21 LAB 2 – Molecules, Water & pH Overview In this laboratory you will be learning about several important concepts essential to understanding the fundamental life processes you will investigate later in this course. First you will look at how atoms, the building blocks of all matter (including living things such as yourself) connect with each other to form molecules. Second, you will investigate the properties of water, the medium in which all processes of life occur. And third, you will examine the concept of pH in aqueous (water-based) solutions, and how aqueous solutions of all types (e.g., biological fluids, beverages, medicines) resist changes in pH due to chemicals that act as pH buffers. Part 1: BUILDING MOLECULES You most likely have already covered the nature of atoms and molecules to some degree in the lecture portion of this course. Nevertheless, let’s remind ourselves of some key points with regard to atoms and how they form connections with each other to create molecules. All atoms are constructed of positively charged protons, uncharged neutrons and negatively charged electrons, and the nucleus of an atom is comprised of the protons and neutrons: Electrons move around the nucleus at incredible speeds in regions referred to as orbitals, and electrons tend to occupy orbitals in pairs. Most atoms in their neutral state (same number of protons and electrons) have at least one unpaired electron, and atoms do whatever they can to avoid having any unpaired electrons. The 3 ways in which atoms avoid having unpaired electrons are to: 1) donate (give up) unpaired electrons to other atoms 2) accept unpaired electrons from other atoms 3) share unpaired electrons with other atoms22 Atoms that donate (give up) or accept (gain) electrons end up with a number of electrons (negative charges) that does not equal the number of protons (positive charges). As a consequence the atom will have a net charge and is therefore referred to as an ion. Atoms that gain extra electrons become negatively charged ions (anions), whereas atoms that give up electrons become positively charged ions (cations). Most elements, however, avoid having unpaired electrons by sharing them with other atoms that also have unpaired electrons. The sharing of unpaired electrons provides a partner for each and avoids the instability associated with unpaired electrons. The sharing of such a pair of electrons between 2 atoms constitutes a type of chemical bond referred to as a covalent bond. As a general rule, atoms form one covalent bond for every unpaired electron. Below is a summary of the number of unpaired electrons and covalent bonds formed in several elements of biological importance: unpaired covalent element electrons bonds hydrogen (H) 1 1 oxygen (O) 2 2 nitrogen (N) 3 3 carbon (C) 4 4 Covalent bonds are the connections that hold atoms together in molecules, and we can describe a molecule as simply 2 or more atoms connected by covalent bonds. Two atoms can be connected by 1 covalent bond (single bond), 2 covalent bonds (double bond) or even 3 covalent bonds (triple bond), depending on the elements and the number of unpaired electrons each contains. The electrons shared in covalent bonds are not always shared equally. If the sharing of electrons is unequal, the bond is said to be polar since the distribution of negatively charged electrons across the bond is uneven. If the sharing of electrons across the bond is equal, the bond is said to be non-polar. The polarity of covalent bonds in molecules is very important as we shall see when we look at the properties of water. Now let’s concern ourselves with how covalent bonds connect atoms together by looking at several small molecules…23 Referring to the number of unpaired electrons for each element listed on the previous page (hence the number of covalent bonds each tends to form), you’re going to build some small molecules containing these elements using your molecular model kit. A couple things you need to understand before doing so are the concepts of a molecular formula and a structural formula. Molecular formulas contain the chemical symbols for each element in the molecule, with subscripts to indicate the number of atoms of that element. A chemical symbol with no subscript indicates only 1 atom of that element. For example, the molecular formula for a molecule of water is: H2O A water molecule therefore contains 2 hydrogen atoms and one oxygen atom. The 3 atoms in a water molecule are interconnected in some way by covalent bonds, however the molecular formula doesn’t indicate how these bonds are arranged. Structural formulas indicate the arrangement of covalent bonds in a molecule using lines to represent each covalent bond. Since hydrogen atoms have 1 unpaired electron (and thus can participate in only 1 covalent bond), and oxygen atoms have 2 unpaired electrons (and thus can participate in 2 covalent bonds), we should come up with a structural formula for water such that each unpaired electron is involved in a covalent bond. In other words, each hydrogen atom should be involved in one covalent bond, and each oxygen atom should be involved in two covalent bonds: H–O–H The structural formula above satisfies this requirement and is, in fact, the correct structural formula for water. Each hydrogen atom is involved in 1 covalent bond, and the oxygen atom is involved in 2 covalent bonds. There are no longer any unpaired electrons in these atoms (each originally unpaired electron now has a partner), thus the molecule and the atoms it contains are relatively stable. Let’s look at the molecular and structural formulas of two more molecules, remembering that atoms can be joined by as many as 3 covalent bonds: CO2 (carbon dioxide) O=C=O N2 (nitrogen gas) N N A correct structural formula for carbon dioxide requires that each oxygen atom be involved in 2 covalent bonds, and the carbon atom be involved in 4 covalent bonds. The only way to satisfy this requirement is with the