This work was created by Dr Jamie Love and licensed under a Creative Commons Attribution-ShareAlike 4.0 International License Creative Commons Licence.

Arthur's notes for Water (Principles)


A molecule is a combination of two or more atoms bonded (bound) together.
If made from the same element it is an elemental molecule like H2.
If made from more than one element it is a compound molecule (or simply a compound) like H20.

Molecules can be monatomic (made of only one atom), diatomic (made of two atoms), triatomic (made of three atoms) and so on. Therefore, helium (He) is monatomic, molecular hydrogen (H2) is a diatomic elemental molecule and water (H20) is a triatomic compound molecule.

Molecular formulas (their abbreviations) list the elements found in the molecule, with a subscript (number slightly below and behind) each element found more than once in the molecule. For example, water is H20 - containing 2 hydrogens and one oxygen atom per molecule. It is pronounced "H two O".
Molecular formulas can also be written to take into account variations due to isotopes. Heavy water is written 2H2O, to show that the hydrogens are both made of two nucleons (instead of just one). The superscript (number to the upper left of the element's abbreviation) is the nucleon count (atomic weight, or more correctly atomic mass) of the atom(s).
The order in which the elements are listed is established by complex rules, but it doesn't really matter (at this stage).

Electrons are responsible for the formation of bonds. They are the glue of molecules, holding them together. All chemical reactions are simply the rearrangement of the bonds (electrons) among the molecules.

Electrons are not just randomly circling the nucleus. Bohr proved that electrons orbit in specific "shells" at specific positions around the nucleus. Each shell can hold only a certain number of electrons. The K-shell holds 2, the L-shell holds 8, and the M-shell holds another 8 electrons (but can hold up to 18 electrons in the larger atoms).
The electrons fill each shell in succession, first filling the K-shell, then L-shell and then the M-shell, until it has all the electrons needed (equal to the number of protons in a neutral atom). This arrangement of electrons around the atom is the atom's electronic configuration. All this behavior is explained by the physics of quantum mechanics.
Lewis discovered that atoms try to obtain the electronic configuration of a noble element, with a complete (outer) shell.

It is this "desire" (caused by the physics of quantum mechanics) which causes covalent and electrovalent molecules to form. Noble elements, "born" with their shells completely filled, have no need to form bonds so they don't undergo chemical reactions and thus are often called "inert" (meaning they don't do anything).

Covalent bonds are formed between two atoms when they SHARE TWO electrons between them, rapidly swapping them back and forth as they try to satisfy their desire for a complete (outer) shell. That is, a covalent bond is formed when two atoms share a PAIR of electrons between them.

Lewis structures are a way of keeping track of the outer shell electrons and figuring out how covalent bonds are made. Kind of like working out a puzzle. Each (abbreviated) atom is surrounded by x's or o's to represent the electrons in each atom's outer shell. Note: electrons of the inner shell(s) are not used in bonding and thus are not included in the Lewis structures.
(Lewis structures only apply to covalent molecules but they are sometimes used to illustrate ionic molecules too.)
Pairs of shared electrons are placed between the "sharing" atoms - each pair (xo) being a covalent bond. The goal is to arrange the electrons (x's and o's) such that all the atoms end up with the electronic configuration of an inert element (a complete outer shell).
The number of bonds that an atom can make depends upon the number of electrons in its outer shell. Atoms with only one atom to share (H and Li, for example) can only have a covalency of one. Atoms with two electrons to share (O) have a covalency of two, those with three electrons to share (N) have a covalency of three and those with four electrons to share (C) have a covalency of four. Carbon makes the most covalent bonds. It can make up to four single bonds, up to two double bonds, or a triple bond (with a single bond to something else). But carbon cannot make a "quad (4) bond".

The atmosphere is about 80% nitrogen (N2) and 20% oxygen (O2) with traces of carbon dioxide (CO2), methane (CH4), helium (He) and water (H20).

Electrovalent (ionic) bonds are formed when one atom gives away an electron and another atom accepts it, forming a pair of oppositely charged ions. The cation (the atom that donated the electron and thus has a positive charge) and the anion (the atom that accepted the electron and thus has a negative charge) are drawn together by "electrostatic forces". Opposite charges attract each other.

Electronegative elements are ready, willing and able to steal electrons from other atoms.
Electrovalent bonds are more likely to form when a very electronegative element (one that needs only one electron to complete its shell) is around a poorly electronegative element (one that needs only to lose an electron or two in order to have a complete outer shell). Both atoms are happy to give or take electrons, thus becoming ions.
We can write down these electron transfer reactions to help us figure out what is going on.
Li ------> Li+1 + e-
F + e- ------> F-1
Then the oppositely charged ions attract each other to form LiF.
Table salt (NaCl) is an electrovalent compound produced by the electrostatic attraction created when Cl (a very electronegative element) steals an electron from Na (a poorly electronegative element).

Electrovalent bonds are caused by the TRANSFER of electrons.
Covalent bonds are caused by the SHARING of PAIRS of electrons.
BUT, there is no clearly defined difference between them, just a range of conditions - from very covalent (always sharing) to very electrovalent (always transferring). It depends on the atoms and their neighboring atoms. Water is mostly a covalent molecule, however a few molecules at any one time will actually transfer an electron becoming electrovalent.

Metallic bonds are a kind of "super sharing" of electrons from (the outer shell of) the metal atoms. A piece of metal is a bunch of metal cations in a cloud of free electrons.

It is this unusual electronic "configuration" which gives metals the properties of being:
malleable (can be shaped by hammering),
ductile (can be drawn into wires),
shiny (has a luster) and
conduct electricity (allow electrons to flow over long distances).

These three bonds (covalent, electrovalent and metallic) are called "strong" because the electron sharing, transfers and supersharing form strong bonding between the atoms.

There are a series of "weak bonds" that use their electron shells in less powerful ways. These weak bonds are often used to hold one molecule to another (rather than to hold the individual atoms together in the molecule). They are very important in determining the states of matter (gas, liquid, etc.), how things dissolve, and are very important in the chemistry of life. Some disagreement exists among Alchemists as to whether these are bonds or "forces".

A polar molecule has at least one polar bond. That is, a bond with an uneven distribution of the electrons it shares. This gives a "direction" to the covalent bond. It is a "little negative" at the end of the bond whose atom tends to hold the electrons more closely, and a "little positive" at the other end where the atom is not very electron "greedy". This produces a polarized bond. We represent these partial charges using a Greek letter called "little delta" meaning "a little difference".

Apolar molecules have their electrons evenly (symmetrically) distributed.

Water is a very polar molecule. The oxygen tends to hold the electrons closer to it than the hydrogens do. Thus water's oxygen end has a "little delta negative" charge and the hydrogens have a "little delta positive" charge. The slightly negative oxygen atom can then attract a positive charge (either a fully positive charge of a complete cation, or the partial positive charge of a polar bonded atom from elsewhere). In a similar way, water's hydrogens can attract negative charges.


This makes water a "universal solvent" meaning it can dissolve almost anything. At least anything with a charge! Water forms a "solvation shield" around ions, by forming a sphere of water around it. This keeps that ion's oppositely charged ion from finding it. Molecules that would normally form electrovalent compounds (salts are a good example) are prevented from doing so by the water solvation shields. Therefore electrovalent compounds dissolve very well in water. At least until there are too many ions for the water to shield. If there are too many ions (or too little water, depending on how you look at it) the ions are able to meet and stay together, forming the electrovalent compound (salt!) and precipitating ("undissolving") out of the solution.
Polar solvents (like water) are good at dissolving other polar or ionic substances. These substances are described as "hydrophilic" ("water loving"). On the other hand, apolar solvents (like oil) are bad at dissolving substances with a charge, but good at dissolving "hydrophobic" ("water hating") substances. Like dissolves like!

Hydrogen bonds (or forces) are the electrostatic interaction produced by the molecules that have a hydrogen bonded to an electronegative atom (O, Cl, F, even N). These hydrogen "bonds" are NOT the bond between the hydrogen and the electronegative atom to which they are covalently bound. Instead, hydrogen bonds are caused by the electron withdrawing effect of the uneven covalent bond. (Hydrogen by itself cannot make hydrogen bonds.)
Hydrogen bonds seem to extend the -H bond to any negatively charged atom (whether they are complete ions or partial ions).

Water's very polar O-H bonds allow hydrogen bonds to form easily.

Van der Waals bonds (or forces) are very, very weak forces produced when (the atoms of) molecules come very close together. Like a key in a lock. Electrostatic repulsion causes the electron clouds (shells) to be slightly distorted (pushed away) from the nucleus of each atom. This causes a very, very slight positive charge from the nucleus to "shine through", attracting the electrons from the other atom to it. These van der Waals forces are very, very weak but extremely important in the chemistry of life.

Chemical properties describe the ability of one substance to change into another completely new substance.
Physical properties describe a substance as it is; like its hardness, color, even smell!
A substance may change its physical state as temperatures change, but it still keeps its chemical identity.
Strong bonds are responsible for linking ATOMS together into molecules.
The weak forces link the MOLECULES together and it is the linking together of these molecules that give substances most of their physical properties.

The strong bonds include electrovalent (ionic) bonds that hold salt grains together, and covalent bonds that hold hair together (for example). These are pretty hard to break up.
Metal bonds are slightly weaker than either electrovalent (ionic) or covalent bonds.
The weak forces include the hydrophobic and hydrophilic interactions, and hydrogen bonds. Both of these are only about 1/10th (10%) as strong as the strong bonds (covalent and electrovalent).
Van der Waals forces are the weakest of all forces, only 1/100th (1%) as strong as the strong bonds (and, thus, only 1/10th or 10%, as strong as hydrogen bonds or hydrophilic and hydrophobic interactions).

Temperature (heat) affects the most important of physical properties - the states of matter.
At extremely high temperatures, like on the sun or in a lightning bolt, electrons are stripped from atoms producing ions of gas called a plasma. Plasmas have unique and unusual properties. They glow and can be moved around with magnetic or electrical fields. Plasma, ionized gas, is considered the fourth state of matter. (Although you may find some folks who do not even know about it!) Plasma involves atoms not molecules.

Weak forces are easily disrupted by heat. An understanding of these weak interactions and how they are affected by temperature explains the (3) remaining states of matter.
The hotter it is the more energy there is in the molecules.
The more energy there is the more each molecule wiggles.
The more the molecules wiggle the harder it is for weak bonds to form between them.
It is these "inter-molecular" bonds (bonds between molecules) which determine if a substance is a gas, a liquid or a solid. Each molecule has its own boiling and melting temperatures, determined by the weak forces it uses at each temperature.

Gases have so much energy they not only wiggle, they go flying off and away from any other molecules they bump into. The energy in steam molecules is enough to overcome any tendency for two neighboring molecules to stick to each other by the weak forces. Steam has too much energy in it for the water molecules to stick to each other by van der Waals forces, or even hydrogen bonds. Molecules in a gaseous state (NOT "gas state") will fill a container and escape through any hole they find.

Liquids have enough wiggle in them to keep the molecules interacting, but they can still slip past each other and change places with neighboring molecules. Liquid water molecules have about the same amount of wiggle energy as bonding energy, so they cohere (stick) to each other, but only briefly. That's why liquid water behaves as a liquid, having a definite volume and taking the shape of the container.

Molecules in solid substances wiggle just a tiny bit (a slight vibration). The wiggle is not enough to tear weak bonds, just strain them a bit. As molecules cool (from a liquid state) they can take up definite positions with respect to each other and form permanent weak bonds.
Freezing water molecules line up their oxygens and hydrogens, forming hydrogen bonds between each molecule. Because of the shape of water molecules they form regular, flat six sided crystals (snow flakes).

Evaporation is NOT boiling. It is a "vapor state". Some of the water molecules at the surface of liquid water, will wiggle fast enough to leap completely away from their neighbors and fly off into the air. They don't have a chance to bond to another water molecule. The molecule behaves as if it were in a gaseous state, but it's below the temperatures needed to make it boil. This is called a vapor. All solids and liquids give off vapors - molecules or atoms that have evaporated from the "condensed" (solid or liquid) form.
Air can carry only so much water vapor (called humidity). Eventually air becomes saturated with water. At that point the water molecules in the air start to meet each other and hydrogen bond together. Tiny water droplets will be created and start to fall out of the air. That's called precipitation.

In covalent molecules, the outer (valence) electrons are controlled by more than one atom causing those atomic orbitals to overlap and form molecular orbitals. All electrons in the outer valence shell are involved in a making the bond but it is the subshells (orbitals) which determine the direction of the bonding and (thus) the shape of molecules.
The eight electrons used to draw Lewis structures are all from the same (outermost) shell - 2 from the s orbital and 6 from the p orbitals. Many of the bonds formed between atoms are produced by molecular orbitals that are a hybrid of the s and p orbitals.
The shape and direction of all covalent molecular orbitals (including the hybrid molecular orbitals) can be explained by Valence Shell Electron Pair Repulsion theory (VSEPR theory). Fundamental to understanding VSEPR is the fact that pairs of electrons (the pairs in each orbital) will arrange themselves to be as far away as possible from other pairs (other orbitals) due to electrostatic repulsion.

The Rules of VSEPR and the steps to predicting the arrangement of covalent molecular orbitals (and thus molecular shapes):

1) Draw a Lewis structure for the molecule and assign the electrons.
i) assign valence electrons to covalent bonds (as far as possible).
ii) arrange the non-bonded valence-shell electrons (those not involved in the bonds) into "lone pairs" (as far as possible).
iii) you may be left with one unpaired electron, called the odd electron.
2) Arrange the repulsion axes to be as far apart as possible.
i) note that each bond (single, double or triple) is a single repulsion axis.
ii) any loan pair(s) or odd electron is also a repulsion axis.
iii) two repulsion axes form a linear connection, three axes form a triangle and four repulsion axes form a tetrahedron.
3) Reassign the (non-equal) axes to take into account differences in electrostatic repulsion.
i) lone pairs are the most repulsive. Two lone pairs will repel each other more than a loan pair repels a bond pair.
ii) bond pairs are mildly repulsive. Triple bonds are the most repulsive and single bonds are the least.
iii) the odd electron is the least repulsive of all.

VSEPR predicts the shape of the repulsion axes (the molecular orbitals). BUT the shape of the molecule may not be the same as the shape of the axes because of "invisible" orbitals (axes) caused by lone pairs or an odd electron.

Ionic molecules are best described as charged spheres that form alternating three dimensional structures.
Metal bonds are similar and simpler.
Hydrogen bonds make straight (linear) bonds.
Van der Waals forces are controlled by the shape of their electron cloud created by the atoms involved in the molecule.


This work was created by Dr Jamie Love and licensed under a Creative Commons Attribution-ShareAlike 4.0 International License Creative Commons Licence.