"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.
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.
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.