Atoms and Molecules

Atoms and molecules are the fundamental units of all matter in the universe. Almost all of chemistry deals with interactions between molecules or individual atoms, so a thorough knowledge of these units is very important.

Atoms make up the smallest discernable units of matter; that is, atoms are the smallest units that can be separated into groups. While atoms are made up of even smaller particles, these smaller particles are all identical and cannot be distinguished. You have probably seen the conventional diagram of an atom, which is illustrated below. While this model is now obsolete, it is a good example of the basic structure.

Atoms are made up of three types of smaller particles, called the "elementary" particles: protons, neutrons, and electrons. The first two types are contained in a cluster at the center, called the nucleus. Protons are fairly massive particles that carry a positive electric charge. The number of protons in an atom determines several important characteristics, including to which element the atom belongs.

Neutrons are also massive particles (both protons and neutrons have masses of about 1 on the atomic mass scale) that reside in the nucleus, but they have no electric charge. Neutrons are though to be the "glue" that hold the positively-charged protons together (since like charges repel, several protons would otherwise fly apart). The number of neutrons is usually, but not always, equal to the number of protons in lighter elements; in heavier elements, such as uranium, the ratio is closer to 1.5 neutrons per proton.

Note that two atoms can have the same number of protons but different numbers of neutrons. For example, hydrogen usually has one proton and no neutrons, but one form of hydrogen called deuterium has one proton and one neutron. Yet another form, known as tritium, has one proton and two neutrons. Atoms with the same number of protons but different numbers of neutrons called isotopes.

A system exists for noting the number of protons and neutrons in an atom. A superscript number to the left of an atom gives its atomic mass: the total number of protons and neutrons present (the electrons do not contribute substantially to atomic mass). The subscript number below gives the atomic number, or the amount of protons in the atom. The number of neutrons can be found by subtracting the two numbers. In a more condensed version, the element symbol is followed by a number indicating the total weight.

Example Problem 1 Identify the number of neutrons in each atom below. A. C.   B. D. Answers: A. The diagram gives us a total mass of 238 and a proton count of 92, so the number of neutrons is 238 - 92 = 146. This is the primary form of uranium--notice the high ratio of neutrons to protons in this large element. B. Using the same method as above, the number of neutrons is 143. U-235 is an isotope of uranium with three fewer neutrons. It is used as nuclear reactor fuel and in atomic bombs. C. This form of carbon has six neutrons, as would be predicted (since it also has six protons). D. Carbon-14 is an isotope of carbon with two extra neutrons, for a total of eight. It slowly decays into C-12 over thousands of years, a process which can be used to date organic matter.

Electrons are the third type of elementary particle, and they are very different from the other two. Electrons are very light (about one-thousandth the mass of a proton) and instead of being contained in the nucleus, they orbit about it. The electrons orbit so rapidly that they blur into an "electron cloud" that surrounds the nucleus. Due to centrifugal force, the electrons want to fly out of the atom, but the attraction between the positively-charged protons in the nucleus and the negatively-charged electrons keeps them in stable orbits. However, atoms are mostly empty space, because the electrons orbit at a great distance from the nucleus. If the nucleus of an atom were a marble placed at the center of a sports stadium, the electrons would be the same distance from the nucleus as the walls of the stadium are from the marble!

Although electrons are tiny particles, they have the same amount of charge as a proton: a proton and an electron together are electrically neutral, as their positive and negative charges cancel each other out. Atoms usually have the same amount of electrons and protons, meaning they have no net charge. Atoms that gain or lose electrons have a negative or positive charge; such charged atoms are called ions. A positive ion is represented by the element's symbol (such as Li for lithium) with a superscript "plus" sign on the right. If an atom loses more electrons, a number is included before the plus to indicate how many electrons are lost. Conversely, a negative ion (an atom that has gained electrons) is shown by including a "minus" sign, using a preceding number if more than one electron is gained.

Example Problem 2 Identify the number of electrons gained or lost in each ion below: A. H+ B. O2- C. Mg2+ D. Al3+ E. Cl- Answers: A. Since one plus sign is indicated, the atom has lost one electron, giving it a positive charge. B. This atom has gained two electrons, giving it a negative charge. C. This positive ion has lost two electrons. D. This atom has lost three electrons, giving it a very positive charge. E. This negatively-charged ion has gained one electron.

When two atoms interact, only the outermost layers of these electron clouds actually touch, so the electrons of an atom determine most of its chemical properties. Thus, it makes sense that isotopes have almost identical properties: because they have the same number of protons, they have the same number of electrons. The extra or missing neutrons have little effect except on the density of the isotope.

Almost every type of atom combines with other atoms to form larger particles called molecules. For example, two hydrogen atoms and one oxygen atom combine to form a very familiar molecule, water. This molecule is written as H₂O; the subscript on the H means that two hydrogen atoms are present in the molecule. Other molecules follow the same pattern: C₂H₆, methane, has two carbon atoms and six hydrogen atoms in each molecule. Molecules vary in size from groups of only two atoms (such as H₂) to much larger structures: deoxyribonucleic acid (DNA), the substance that carries genetic information in our cells, has millions of component atoms.

Example Problem 3 Identify the number of each type of atom in the molecules below. A. H2SO4 (sulfuric acid) B. MgCl2 (magnesium chloride) C. NH3 (ammonia) D. KCrO4 (potassium dichromate) E. CH3COOH (acetic acid) Answers: A. Counting atoms gives 2 hydrogen, 1 sulfur, and 4 oxygen. If you didn't know S was sulfur, fear not! The next section will explain the Periodic Table, which lists all 100+ elements known. B. This molecule has 1 magnesium atom and 2 chlorine atoms. C. Ammonia contains 1 nitrogen atom and 3 hydrogen atoms. D. This diverse molecule has 1 potassium (K) atom, 1 chromium atom (Cr), and 4 oxygen atoms. E. Acetic acid, the common weak acid found in vinegar, has 2 carbon atoms, 4 hydrogen atoms, and 2 oxygen atoms.

Sometimes, larger molecules are written in a way that makes their structure clearer, as acetic acid was above. As another example, C₄H₁₀O (butanol) can be written as CH₃CH₂CH₂CH₂OH to emphasize the way the hydrogen atoms cluster around the carbon atoms and the OH group at the end of the molecule (the actual shape of the molecule will be discussed in later chapters). The expanded-notation practice is very common in organic chemistry, and you will often see "hybrid" notation using parts from both the condensed and expanded methods (such as C₄H₉OH for butanol). In any notation, just adding up all occurences of each atom will reduce it to the condensed notation.

Example Problem 4 Convert the following notations to the condensed form. A. HOH B. NaOSONa C. CH3CH2CH2CH3 Answers: A. This molecule simplifies to H2O, or familiar water. B. Condensing this molecule gives Na2SO3, sodium sulfite. C. Counting atoms gives 4 carbon and 10 hydrogen, for a condensed notation of C4H10, butane.

In our examples so far, we have usually used common elements like carbon and oxygen; however, some more exotic elements (like Cr, chromium) have been thrown in for interest. It would be a real inconvenience if every aspiring chemist had to learn the names, symbols, and other data for every one of the hundred-odd elements. Luckily, the Periodic Table of the Elements, one of the most important resources in science, provides all this information and more in one easy-to-use reference, making it one of the most convenient as well!

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