Atoms & Molecules

Table of Contents

• Teaching  Tips: Atoms; Bonding & Molecules; Atomic Models
• Noteworthy Scientist: John Dalton (1766-1844)
• Fabulous Facts
• The Scientific Speaker
• Science Links

Teaching  Tip: Atoms

"What is everything really made of?" Questions like this have been asked for centuries, and scientists are continually trying to find the answer. At one time scientists believed that the smallest "building block" of matter was the atom, a name that comes from the Greek word meaning "incapable of being cut." Later it was discovered that though an atom is the smallest unit that has the chemical properties of an element, even atoms are made up of smaller particles. Elements are the simplest substances found in nature and they cannot be broken down further through ordinary chemical means. At least 92 naturally occurring elements have been discovered so far. The elements are organized in a periodic table based on their different properties.

To demonstrate the idea of an atom being the smallest unit of an element, take a pile of paper clips and divide it into two piles, and then divide those in half again. Continue dividing until you have one paper clip in a pile. The original pile represented matter, and you have just divided matter down to its smallest unit that still functions--one paper clip still holds loose papers together. Cut the paper clip in half. Does it still do the same job as a whole paper clip? No. In the same way, the atom is the smallest piece of an element that still functions as an element.

The dense central part of an atom, called the nucleus, is made up of protons and neutrons. Protons are small particles with a positive electrical charge. The number of protons in an atom, called the atomic number, determines the "identity" of the atom, or what element it is. For example, all copper atoms have 29 protons. (If you take a look at a periodic table, you'll see that the elements are put in order by atomic number.) Neutrons, as their name implies, have no electrical charge, but they add significantly to the mass of an atom. In fact, the approximate atomic mass of an atom is the sum of the mass of the protons and neutrons added together. (The atomic mass is listed right under the element symbol on the periodic table.) Though all atoms of a particular element will always have the same number of protons, sometimes the atoms of that element can have a different number of neutrons. In this case, those atoms are called isotopes.

An atom also contains other particles, called electrons, which orbit the nucleus. These have so little mass that they are ignored when calculating the atomic mass. Electrons have a negative electrical charge that balances with the positive proton charge to create a neutral atom. Given enough energy, however, electrons can sometimes jump away from an atom, ruining the electrical balance and giving the atom a positive charge. Likewise, sometimes an atom can gain an extra electron, giving it a negative charge. Atoms with unbalanced electrical charges, either positive or negative, are called ions. Positive ions -- atoms that have lost electrons -- are slightly smaller than the original atom, while negative ions -- which have gained electrons-- are slightly larger.

Teaching  Tip: Bonding & Molecules

All matter is made up of tiny atoms, so how do we get larger substance, like water, sugar, or iron? These very small atoms can bond together into bigger compounds, either ions or molecules.

Based on different relationships between elements, there are different types of bonds. When metals and nonmetals join, the bond type is ionic. An electron from one element is transferred to the outer electron level, or valence, of another element. The compounds formed in this way are ions, rather than molecules, because the bonded atoms change their amount of electrons and thus become electrically unbalanced.

Molecules consisting of nonmetals are joined by covalent bonds; their electrons are shared by pairs of atoms, not transferred, so the bond between them tends to be very tight.

In molecules consisting of metals, the bond type is called metallic. The name scientists use to explain the electron relationship in these molecules is called the electron-sea theory. Like in molecules with covalent bonds, the electrons are shared; but they are shared with all of the atoms together, not between individuals. The valence electrons (those that are in the outer electron level) become "free" and mobile in the middle of the compound, hemmed in by the positive charges of the protons of the joined atoms.

Molecules have different shapes, depending on the types of atoms bonded together. The Valence Shell Electron Pair Repulsion (VSEPR) theory explains this relationship as, molecules will form whatever shape will keep the valence electrons in the central atom as far apart from each other as possible.

Compounds of atoms can exist in three different states. Solids are formed by slow-moving molecules. Liquids are formed by faster-moving molecules; the attracting forces between atoms are partly overcome by the motion. In gases, molecules are moving very quickly, and the attracting forces are completely overcome. Heat causes molecules to move faster, which is why ice, a solid, will melt into water, a liquid, when heated. If you boil the water over the stove, it will evaporate as it gets hotter, turning into a gas. Usually liquids made of molecules that have a high atomic weight take longer to boil, because the molecules take longer to start moving.

To help you visualize how atoms bond together into molecules, experiment with our molecular model set.

Teaching  Tip: Atomic Models

The ancients thought that everything was made from four elements: earth, fire, water, and air. Later scientists came to believe that all matter is made from tiny unseen particles. They called these particles atoms, from a Greek word meaning "incapable of being cut." But how do you learn about something you cannot see? Scientists have long had the difficult task of understanding the structure of atoms by gathering evidence about them indirectly, since they can not see the atoms themselves. The theories that they have put forward are called atomic models.

The foundation for the modern atomic models was laid by John Dalton in the early 1800s. His model explained that elements are made up of minute particles called atoms, that atoms of different elements have different sizes and properties, that atoms of one element cannot be changed into atoms of another element, and that atoms form compounds by combining with each other. Working with very little information, Dalton began trying to determine the masses of the atoms of different elements.

Dalton and others had long believed that the atom was indivisible, but in the late 19th century scientists such as J.J. Thompson, experimenting with electricity, discovered the existence of negatively charged particles within the atom -- electrons. Since Dalton's model could not explain electrons, Thompson created a new atomic model. He claimed that electrons existed in a positively charged material that surrounds them and balances the charges making the atom neutral. Under certain circumstances, Thompson suggested, electrons could be removed from an atom. This atomic model is sometimes referred to as the "plum pudding" model, since the picture of electrons in positively charged material is rather like plums in pudding.

Thompson's model soon did not hold up under the evidence discovered in 1911 by Ernest Rutherford, who worked with radioactive elements and positively charged ions called "alpha particles" (consisting of two protons and two neutrons) that the elements emitted. Rutherford experimented with aiming alpha particles at gold foil. Most of the alpha particles went straight through the foil, as one would expect with Thompson's atomic model. Some of the particles, however, were reflected. Rutherford concluded that atoms must have some dense central portion that was strong enough to reflect the alpha particles directed at them. He called this the nucleus, and determined that it had positively charged particles called protons that balanced the negative charges of the electrons.

The electrons and protons could not account for the entire mass of an atom, however, and in 1932 James Chadwick identified neutral particles that are found in the nucleus: neutrons. Thus, building on the discoveries of others, scientists came to recognize that "indivisible" atoms are really composed of smaller particles.

Later atomic models, such as Bohr's model and the Quantum model accept the presence of electrons, protons, and neutrons as fact. These models focus on the placement and behavior of electrons, and they try to determine how electrons move in different levels of the atom.

Noteworthy Scientist: John Dalton (1766-1844)

John Dalton, the British chemist and physicist, was born in the Lake District of England in 1766, to a Quaker family. His education was a conglomeration of sources: he was taught by his father, attended a Quaker school, was taught mathematics by a relative, and studied science on his own. At the age of 12, he taught school for a short time. Later, he taught in schools with his cousin and his brother. In 1793 Dalton moved to Manchester and taught mathematics and natural philosophy at New College, a university which, unlike Oxford and Cambridge, was open to students of other denominations than Church of England. Later, when the college relocated, he remained in Manchester and gave private math and chemistry lessons.

In 1787 Dalton begin taking meteorological (weather-related) observations. Over the course of his life, he wrote down around 200,000 weather observations and measurements, and in 1793 he published his Meteorological Observations and Essays. Around this time, he wrote a paper about color blindness (originally known as Daltonism, in his honor). His paper was the first description of color blindness, a condition which he himself had.

Dalton is most famous for his atomic theory, first put forward in 1803. Although Dalton's theory did not explain everything correctly, it was still the basis for understanding some important properties of atoms. Dalton's theory held that chemical elements are made of atoms; that all atoms in one kind of element have the same mass and atoms of different elements have different masses; and that compounds of elements are made up in set ratios--atoms join in certain combinations. He made up his own symbols for notation of different elements and also worked to estimate atomic masses.

Beginning in 1794, Dalton was a member of the Manchester Literary and Philosophical Society (he became its president in 1817). He remained a bachelor for the whole of his life.

To find out more about John Dalton's system of notation and see what his symbols for various elements looked like, visit http://www.uh.edu/engines/epi1411.htm.

Fabulous Facts

What weight? Protons weigh much more than electrons do. Did you know that if an electron weighed as much as a dime, a proton would weigh as much as a gallon of milk?

Boiling when? Methane, which has a molecular weight similar to that of water, boils at -259 degrees Fahrenheit and at warmer temperatures exists in a gaseous state. So why does water have to be 212 degrees Fahrenheit before it will boil? Water molecules have a strong electrical attraction to each other, so their bonds require much more energy to break than methane's do.

The Scientific Speaker

*The word fission comes from the Latin noun fissio, meaning "a cleaving" and the Latin verb findere, meaning "to split." It is used to describe the splitting, or cleaving, of the nucleus of an atom.

*The word atom comes from the Greek for indivisible: the prefix a- means not, and the root means to cut. The Latin word, which came from the Greek, is very similar: atomus. In Middle English (spoken from about AD 1100-1500), the word was attome.

*A quark is a small particle that is found inside protons and neutrons. There are three quarks in each proton and each neutron, and the quarks are held together by other particles called gluons.

Science Links

For an online periodic table with information about each element, visit http://education.jlab.org/itselemental/index.html.

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