ATOMS ELEMENTS, PERIODIC TABLE, MOLECULES, METALS, and NONMETALS

ATOMS  The present model of atoms is as follows. An atom has a central nucleus, which is very small compared with the rest of the atom and contains most of the atomic mass (or weight). The nucleus carries a positive electric charge and is surrounded by a diffuse shell, or cloud, of negatively charged particles called electrons. The size ratio of the atom to the nucleus is 10,000 to 1.  The simplest atom of all, hydrogen, has one particle—called a proton—in its nucleus. The mass of a proton is 1836 times the mass of an electron. A proton carries a positive electric charge with an assigned value of +1, and an electron carries a negative electric charge with an assigned value of -1. The atoms of other elements have more than one proton in their nucleus, and, in addition, other elements have another kind of nuclear particle called a neutron. The neutron has nearly the same mass as the proton, but the neutron has no electric charge. §The number of protons in the nucleus of an atom of a certain element determines the element’s atomic number. For example, an atom with a nuclear charge of +26 has 26 protons in its nucleus and must be iron. To be electrically neutral, an atom of iron must have 26 electrons surrounding the nucleus.

ATOMIC MODELS Experimental data has been the impetus behind the creation and dismissal of physical models of the atom. Rutherford's model, in which electrons move around a tightly packed, positively charged nucleus, successfully explained the results of scattering experiments, but was unable to explain discrete atomic emission—that is, why atoms emit only certain wavelengths of light.

BOHR MODEL Bohr began with Rutherford’s model, but then postulated further that electrons can only move in certain quantized orbits. This model was able to explain certain qualities of discrete emission for hydrogen, but failed completely for other elements.

SCHRODINGER MODEL Schrödinger’s model, in which electrons are described not by the paths they take but by the regions where they are most likely to be found, can explain certain qualities of emission spectra for all elements; Further refinements of the model, made throughout the 20th century, have been needed to explain all observable spectral phenomenon.

ATOMS and MOLECULES §An atom is the smallest unit of an element that has the properties of the element. §A molecule is the smallest unit of a compound or the form of an element in which atoms bind together that has the properties of the compound or element.

MOLECULES Molecules are compounds made up of specific combinations of atoms. Familiar substances may theoretically be divided into single molecules, as modeled here, but no further. Like a strict recipe in which atoms are the ingredients, each molecule has a chemical formula. If any ingredients are subtracted or changed, the molecule becomes something completely different.

ELEMENTS §Hydrogen, oxygen, chlorine, sodium, and iron are examples of elements. §Elements cannot be resolved into simpler substances by ordinary heat, light, electricity, or attack by other substances. §To say that elements can never be broken down would not be accurate, but breaking them down takes millions of times more energy than can be applied by ordinary means. It requires either special equipment, such as a particle accelerator, or temperatures like those in the interior of the sun.  An element can therefore be defined as a substance that cannot be broken down into simpler substances by ordinary means. §Ninety elements are known to occur in nature, and 22 more have been made artificially. Out of this limited number of elements, all the millions of known substances are made.

ISTOPES The atomic number of an atom represents the number of protons in its nucleus. This number remains constant for a given element. The number of neutrons may vary, however, creating isotopes that have the same chemical behavior, but different mass. The isotopes of hydrogen are protium (no additional neutrons), deuterium (one neutron), and tritium (two neutrons). Hydrogen always has one proton in its nucleus. These illustrations are schematic representations of the atom and are not to scale. Actually, the nucleus is approximately 10,000 times smaller than the average orbital radius of the electron, which defines the size of the atom.

ATOMIC MASS and ISOTOPES §The total number of protons and neutrons in a nucleus is called the mass number, since these particles account for almost the entire mass of an atom. §However, atoms of the same element can have the same number of protons but different numbers of neutrons, thus giving rise to varieties, or isotopes, of the same chemical element. The word isotope (Greek iso and topos) means “same place.” §Different isotopes of the same element occupy the same place in the periodic table of the chemical elements and have very nearly identical chemical properties. §Thus hydrogen, which has a mass number of 1, has an isotope, deuterium, which has one proton and one neutron in its nucleus and a mass number of 2. Deuterium accounts for 1.5 percent of naturally occurring hydrogen. Hydrogen and deuterium undergo the same chemical reactions, although not necessarily with equal ease.

ATOMIC WEIGHT §The term atomic weight means the average weight (more correctly, the mass) of an atom of an element, taking into account the masses of all its isotopes and the percentage of their occurrence in nature. §Isotopes are generally written as 12 C or carbon-12, with the number denoting the total number of protons and neutrons in the atom. Four out of every five elements occur in nature as mixtures of isotopes.

METALLIC BONDS Silver, a typical metal, consists of a regular array of silver atoms that have each lost an electron to form a silver ion. The negative electrons distribute themselves throughout the entire piece of metal and form non-directional bonds between the positive silver ions. This arrangement, known as metallic bonding, accounts for the characteristic properties of metals: They are good electrical conductors because the electrons are free to move from one place to another, and they are malleable (as shown here) because the positive ions are held together by non-directional forces.

COMPOUNDS §Salt, water, iron rust, and rubber are examples of compounds. §A compound is made up of elements, but it looks and behaves quite differently, as a rule, from any of its component elements. Iron rust, for example, does not look and feel like its components: oxygen gas and iron metal. Some synthetic fabrics, with fibers made from coal, air, and water, do not feel at all like any of the components that make them up. This individuality of properties, as well as other qualities, distinguishes a compound from a simple mixture of the elements it contains. §Another important characteristic of a compound is that the weight of each element in the compound always has a fixed, definite ratio to the weight of the other elements in the compound. For example, water always breaks down into parts of hydrogen by weight to parts of oxygen by weight, which is a ratio of about 1 to 8, regardless of whether the water came from the Mississippi River or the ice of Antarctica. In other words, a compound has a definite, invariable composition, always containing the same elements in the same proportions by weight; this is the law of definite proportions.

WATER IS A COMPOUND Water is an example of a compound. A water molecule consists of an oxygen atom and two hydrogen atoms.

METALS §The structure of the atom, in particular the configuration of the electron cloud, is responsible for the obvious physical differences between metals and nonmetals.  Metals have a characteristic luster, are opaque, can be hammered and drawn into various shapes, and conduct electricity. Nonmetal elements, on the other hand, are often gases, and, if solid, nonmetals are generally brittle, sometimes transparent, and do not conduct electricity. §The atoms of metals have outer shells that contain few electrons and are nowhere near filled (and therefore lack the stability of a noble gas). As a result, all metals tend to easily lose some of these outer electrons. §This means, chemically, that metals tend to form positively charged ions, or positively charged atoms or molecules, when they enter into chemical combination. §Physically, the fact that the outer shells of metal atoms are unfilled means that these “loose” electrons can flow and enable metals to conduct electricity; this fact also accounts for the mechanical properties of metals.

NON-METALS §Nonmetals, have outer shells that are nearly filled up to the stable grouping of eight electrons. §In their chemical reactions they tend to add electrons to achieve the state of a stable noble gas. By adding electrons, nonmetals form negatively charged ions. §They can also add electrons by sharing them with another atom and forming a covalent bond.  The noble gases, with exactly eight electrons in their outer shells (two in the case of helium), are nonmetals.