Water is a Molecule

Subatomic particles

Probably the first chemical formula you learned was waterThe universal solvent essential for life. which is H₂O.  H₂O is a molecule, which is a collection of atomsThe smallest units of matter that retain the properties of an element. of different identities.  Humans are always limited by their currentThe flow of electrical charge, as in ions moving across a neuron’s membrane. technology, and today, atoms seem to be the smallest packet of matter that we can possibly investigate.  However, we know that atoms are made of things called subatomic particles.  The subatomic particles are protons, which we can note as p+, electronsNegatively charged subatomic particles found in atoms. or e-, and neutronsElectrically neutral particles in an atom’s nucleus. n0.  Protons and neutrons reside in the nucleusThe control center of the cell that contains DNA and directs cellular activities. of an atom, which is the center.  If your protons and neutrons are not in your nucleiClusters of neurons in the CNS responsible for processing information., you should seek medical attention.  Or a physicist.  The electrons are flying about the nucleus.  They contain energyThe capacity to do work or cause change. because they are in motion.  The electrons are not flying about randomly, but are organized into things called shells.  Shells are not orbits, like the orbits that the planets use to circle the sun, but like the shell on a malted milk ball like Whoppers, or like any chocolate coated candy.  The first shell closest to the nucleus will hold only 2 electrons.  If the atom has more than 2, electrons will overflow into the second shell, which can hold 8 electrons.  If the second shell becomes full, electrons will overflow into the third which can also hold eight.  For this class, we can limit it at these three shells.  The shells don’t have to hold these numbers, but this is the maximum number of electrons they can hold.  There are some things you have to know about shells and electrons: shells like to have their maximum number of electrons and electrons like to be in pairs.

Atoms have predefined identities

The number of subatomic particles in an atom are determined by the atomic number, found from the periodic table.  Referring to the table, we see that an oxygen atom would have eight protons and the hydrogen would have only one. Since atoms have no charge, we can infer that an oxygen atom would have eight electrons and the hydrogen would have only one.  Unfortunately, since a hydrogen atom has only one proton, its lonely electron has trouble being attracted to its proton.

Looking at the oxygen atom we see that there are two electrons in the first shell and the balance of six electrons in the second shell.  This means that there are two spots in the second shell that it would like to fill.  It can do this by bonding with other atoms or moleculesGroups of atoms bonded together. by sharing, trading, or just standing next to some of the other atom or molecule’s electrons.

Water is a molecule

Think of the picture of a water molecule as the headRounded proximal end that fits into the acetabulum of the hip bone. of Mickey Mouse.  You have the face, the oxygen, and the ears, the hydrogens.   

One molecule of water has two covalent bonds that join each hydrogen atom to the oxygen atom. A covalent bondStrong chemical bonds formed by the sharing of electrons between atoms. consists of a pair of shared electrons, like hand holding instead of just standing next to each other.

An oxygen atom has a greater affinity or attraction for electrons than a hydrogen atom does. This makes sense considering that oxygen has more protons than hydrogen.  Therefore, when a hydrogen atom and an oxygen atom share a pair of electrons in a chemical bond, the electrons spend more time around the oxygen nucleus than they do around the hydrogen nucleus because there are more protons. We can think of this like an electron party around the oxygen nucleus; apparently the place to be…if you’re an electron.

Water is polar

This unequal sharing of electrons creates two separate areas of electrical charge. Around the oxygen atom, or Mickey Mouse’s chin, the molecule is partially negatively charged with more electrons, more of the time. Around the hydrogen atoms, or the ears, the molecule is partially positively charged because the electrons spend less time there.

This type of bond between the atoms of a water molecule is called a polar covalent bond and water is a polar molecule—a molecule with two “poles”—one slightly negative and the other slightly positive.  Many students find an association to polar molecules with their charged poles to the magnetically charged north and south poles of the Earth or the positive and negative poles of a magnet. 

A polar substance, with both positively and negatively charged poles is in contrast to a nonpolar molecule, which has no charge.

Nonpolar       vs        Polar

Students seems to have a common misconception here about polar and nonpolar.  They sometimes think polar means positive and nonpolar means negative.  Polar means charged and thus represents BOTH positive and negative.  This polarity results from that unequal sharing of the electrons among the atoms.  Nonpolar molecules share their electrons equally, and have no charge, like a block of wood or like Princess Leia here.  Kidding, this is carbon dioxide, which is a nonpolar molecule like oxygen.

Hydrogen Bonds

In nature, opposite charges attract. So the positive pole of one water molecule is attracted to the negative pole of another water. In liquid or solid water, the water molecules arrange themselves to take advantage of this attraction between opposite charges. This attraction has a name: hydrogen bondsWeak attractions between hydrogen and electronegative atoms like oxygen or nitrogen.. Hydrogen bonds tend to make water sticky, sticking to each other and to other molecules. It is this “stickiness” that underlies the versatile properties of water.