Chemistry Background Review reading
Metals are elements, compounds or alloys characterized by properties such as high electrical and thermal conductivity, shiny luster, relatively high densities, metallic bonds, and form cations and anions with non-metals. Most of the elements on the Periodic Table are metals. The luster of most metals are opaque and shiny. Due to the tightly packed crystal lattice of the metallic structure, metals are not categorized as transparent and readily reflect light off their surface. Metals are good conductors of heat and electricity. Heat, energy and electricity travel very quickly through metals. The highest conductivity occurs in metals with only one electron occupying a state in the outermost orbital shell. Silver, copper and gold are examples of high-conductivity metals. Malleability is the ability of a metal to bend or be hammered into thin sheets or other shapes without breaking. Examples of metals highly malleable are gold, iron, lead, aluminum and silver.
In general, non-metals differ from metals in the following ways: poor conductors of heat and electricity, have lower densities, negative valences, significantly lower melting and boiling points than metals, and are dull and brittle. Only 18 elements on the Periodic Table are considered non-metal and examples are hydrogen, carbon, nitrogen, oxygen, Group 17 halogens, and Group 18 noble gases. The luster of non-metals are usually dull, non-metallic, glassy, earthy, or resinous. Non-metals are poor thermal and electrical conductors. Solid non-metals are brittle and break without significantly changing shape under stress (not malleable or ductile).
Metalloids have properties of both metals and non-metals. They are located along the stair-step line that distinguishes metals from non-metals on the Periodic Table. Some metalloids include Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium and Polonium. Silicon and Germanium are semi-conductors, meaning they carry an electrical charge under certain conditions. Some metalloids have a metallic luster, such as silicon. Silicon, however, is also brittle; therefore, it has characteristics of both metals and non-metals.
Atoms are the basic building components of all matter and cannot be subdivided by ordinary chemical means. Atoms are composed of three types of particles: protons, neutrons and electrons. Protons and neutrons reside in the dense, central, positively charged nucleus of an atom. Electrons are located orbiting the region outside the nucleus. Protons and neutrons are similar in mass and each weighs approximately 1,836 times greater than a single electron. The mass of a single proton, for example, is approximately 1.67 x 10-27 kg. The mass of an electron is approximately 9.11 x 10-31 kg. Since the mass of an electron is insignificant when determining atomic mass, only the sum of the protons and neutrons are used when determining atomic mass.
Protons have a positive charge (+) and neutrons are neutral with no charge. Electrons have a negative electric charge (-). In a neutral atom, the number of protons always equals the number of electrons. Only when an atom takes an electron from another atom does it become charged. This charged form of an atom is called an ion.
Electrons spin as they circle outside the nucleus billions of times every second. Due to the extremely fast speed they are moving, the path they travel is not the same each time. They appear to look like a cloud around the nucleus instead of traveling in a fixed path. Through quantum mechanics, the exact location of an electron at given point cannot be determined. According to this model, electrons exist in atomic orbitals, which are a set of quantum states of the negatively charged electrons trapped in the electrical field generated by the positively charged nucleus. Electrons are arranged in energy levels around the nucleus categorized by quantum numbers. The names of the orbitals indicate the orbital shape and are used to describe electron configurations. When they gain or lose energy, they jump between energy levels as they are rotating around the nucleus. Only a certain number of electrons are in an energy level at the same time.
The atomic number of an element is the number of protons in the nucleus of an atom of that element. Each element is identified by its atomic number. For example, the atomic number of carbon is 6. Every atom of carbon contains 6 protons in its nucleus. If an atom does not have 6 protons, then it is not an atom of carbon. Adding or removing protons from the nucleus of an atom creates a different element. For example, adding a proton to an atom of carbon creates an atom of nitrogen (atomic number 7). Thus, the number of protons determines an element’s identity.
The valence electrons are the electrons in the last energy level of an atom. These are the electrons an atom gains or loses in a chemical reaction. They combine with valence electrons of other atoms to form chemical bonds. Therefore, valence electrons are important in determining the chemical properties of an element. The number of valence electrons of an element is determined by its placement on the Periodic Table. Elements in the same group generally have the same number of valence electrons. Elements in group 1 have 1 valence electron. Hydrogen, Lithium, and Sodium each have 1 valence electron. Elements in group 14 have 4 valence electrons. Carbon and Silicon each have 4 valence electrons. The number of valence electrons increases moving from left to right across the Periodic Table (groups 1 and 2 and 13-18). An element’s valence shell is full and most stable when it contains 8 electrons, which corresponds to the same electron configuration as a noble gas. This stability is the reason noble gases are so unreactive. The Octet Rule is a general rule stating atoms tend to combine in a way that leaves 8 electrons in their valence shells. As a result, atoms generally react by gaining, losing or sharing electrons in order to achieve a complete octet of valence electrons. Reactions occur either ionically or covalently. For example, elements in group 1 are highly reactive with elements in group 17. Sodium, with one valence electron, reacts with chlorine which has 7 valence electrons and forms the stable compound sodium chloride (1 + 7 = 8). The Periodic Table is used to predict the properties of an element based on its location on the table. Elements are arranged by increasing atomic number. Elements are located within a group (column) and a period (row) on the table based on their properties. Periodicity refers to the fact the Periodic Table is made up of repeating and predictable patterns that conform to certain rules.
Elements within the same group have the same electron configuration in their valence shell. This is an important factor determining their similar properties. Elements tend to gain or lose valence electrons to achieve stable octet formation. Electrons are added one at a time moving from left to right across a period. As this happens, the electrons of the outermost shell experience increasingly strong nuclear attraction, so the electrons become closer to the nucleus. Moving down a column, the outermost electrons become less tightly bound to the nucleus. This is because the number of filled principal energy levels increases downward within each group. These trends help explain the periodicity seen in other elemental properties. In moving from top to bottom down a group, the atomic radius increases, ionization energy decreases and electronegativity also decreases. In moving from left to right across periods, atomic radius decreases, ionization energy increases and electronegativity also increases.
Elements are also classified based on shared characteristics. In general, most metals are located to the left, bottom, and center of the table. Most non-metallic elements are to the right and to the top. The metalloids are adjacent to a diagonal line from Boron to Astatine. Most of the gases are located to the upper right section of the Periodic Table.
The Periodic Table is divided into nine families. The Alkali Earth Metals are found in Group 1. They are highly reactive and have one valence electron. They are malleable, ductile, good thermal and electric conductors, and are softer than most other metals. Group 2 elements are Alkaline Earth metals. All of these elements have two valence electrons. The transition metals are located in Groups 3-12. They are ductile, malleable, and good thermal and electric conductors. Their valence electrons are present in more than one shell and have several common oxidation states. The “other metals” are located in Groups 13-15. All of these metals are solid and do not exhibit variable oxidation states. Their valence electrons are only present in their outer shell. The metalloids have properties of both metals and non-metals and are located along the boundary separating the metals from the non-metals. Non-metals are the elements in Groups 14-16, are not good conductors and can be very brittle. These elements are Carbon, Nitrogen, Oxygen, Phosphorus, Sulfur, and Selenium. The Halogens are in Group 17. All of the halogens have 7 valence electrons and are highly reactive. The Noble Gases are found in Group 18. All noble gases have 8 valence electrons and are nonreactive. There are 30 rare Earth elements composed of the lanthanide (elements 58-71) series and the actinide (elements 90-103) series. Most of the actinides are synthetic and are found in Group 3 of the Periodic Table, and the 6th and 7th periods.