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The periodic table is the most important chemistry reference there is. It arranges all the known elements in an informative array. Elements are arranged left to right and top to bottom in order of increasing atomic number http://periodic.lanl.gov/use.shtml. Order generally coincides with increasing atomic mass http://periodic.lanl.gov/use.shtml.

The different rows of elements are called periods. The period number of an element signifies the highest energy level an electron in that element occupies (in the unexcited state). The number of electrons in a period increases as one traverses down the periodic table; therefore, as the energy level of the atom increases, the number of energy sub-levels per energy level increases.

Using the data in the table scientists, students, and others that are familiar with the periodic table can extract information concerning individual elements. For instance, a scientist can use carbon’s atomic mass http://periodic.lanl.gov/use.shtml to determine how many carbon atoms there are in a 1 kilogram block of carbon.

People also gain information from the periodic table by looking at how it is put together. By examining an element’s position on the periodic table, one can infer the electron configuration. Elements that lie in the same column on the periodic table (called a “group”) have identical valance electron configurations http://periodic.lanl.gov/use.shtml and consequently behave in a similar fashion chemically. For instance, all the group 18 elements are inert gases. The periodic table contains an enormous amount of important information. People familiar with how the table is put together can quickly determine a significant amount of information about an element.

 

How to use the Periodic Table

When you open any file of an element in the periodic table, you will find a small table with some basic information about that element. Here’s how you use that table.

1 – Atomic Number

http://periodic.lanl.gov/1.shtml– Atomic Symbol

1.008 – Atomic Weight

 

Atomic Number

The number of protons in an atom defines what element it is. For example carbon atoms have six protons, hydrogen atoms have one, and oxygen atoms have eight. The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behaviour of the element.

 

Atomic Symbol

The atomic symbol is one or two letters chosen to represent an element (“H” for “hydrogen,” etc.). These symbols are used internationally. Typically, a symbol is the truncated name of the element or the truncated Latin name of the element. Click here http://periodic.lanl.gov/list.shtml for a list of the elements and their symbols.

 

Standard Atomic Weight

The standard atomic weight is the average mass of an element in atomic mass units (“amu”). Though individual atoms always have an integer number of atomic mass units, the atomic mass on the periodic table is stated as a decimal number because it is an average of the various isotopes of an element. The average number of neutrons for an element can be found by subtracting the number of protons (atomic number) from the atomic mass.

Atomic weight for elements 93-118. For naturally-occurring elements, the atomic weight is calculated from averaging the weights of the natural abundances of the isotopes of that element. However, for man-made trans-uranium elements there is no “natural” abundance.

The IUPAC convention is to list the atomic weight of the longest-lived isotope in the periodic table. These atomic weights should be considered provisional since a new isotope with a longer half-life could be produced in the future.

 

Electron Configuration

The electron configuration is the orbital description of the locations of the electrons in an unexcited atom. Using principles of physics, chemists can predict how atoms will react based upon the electron configuration. They can predict properties such as stability, boiling point, and conductivity. Typically, only the outermost electron shells matter in chemistry, so we truncate the inner electron shell notation by replacing the long-hand orbital description with the symbol for a noble gas in brackets. This method of notation vastly simplifies the description for large molecules.

Example: The electron configuration for Be is 1s22s2, but we write [He]2s2 where [He] is equivalent to all the electron orbital’s in the helium atom. The Letters, s, p, d, and f designate the shape of the orbital’s and the superscript gives the number of electrons in that orbital.

 

Characterising the Elements

Elements can generally be described as either metals or nonmetals. Metal elements are usually good conductors of both electricity and heat.

The dividing line between metals and non-metals is not hard and fast, thus the distinction between “Post-transition metals” and “Metalloids” is represented differently on different versions of the Periodic Table. For example, in some tables, Group 12 is categorised with the post-transition metals, and in others, aluminium and tin are included characterized as Metalloids or poor metals. In our version of the table, we have chosen the most commonly accepted demarcations between these elements.

Alkali metals. The alkali metals make up group 1 of the Table, and comprise Li through Fr. They have very similar behaviour and characteristics. Hydrogen is group 1 but exhibits few characteristics of a metal and is often categorized with the non-metals.

Alkaline earth metals. The alkaline earth metals make up group 2 of the periodic table, from Be through Ra. The alkaline earth metals have very high melting points and oxides that have basic alkaline solutions. Their characteristics are well described and consistent down the group.

Transition metals. The transition elements are metals that have a partially filled d subshell (CRC Handbook of Chemistry and Physics) and comprise groups 3 through 12 and the lanthanides and actinides (see below).

Post-transition metals. The post-transition elements are Al, Ga, In, Tl, Sn, Pb and Bi. As their name implies, they have some of the characteristics of the transition elements. They tend to be softer and conduct more poorly than the transition metals.

Metalloid (or “semi-metal” or “poor metal”). The metalloids are B, Si, Ge, As, Sb, Te, and Po. They sometimes behave as semiconductors (B, Si, Ge) rather than as conductors.

Lanthanides. The lanthanides comprise elements 57 (lanthanum, hence the name of the set) through 71. They are grouped together because they have similar chemical properties. They, along with the actinides, are often called “the f-elements” because they have valence electrons in the f shell.

Actinides. The actinides comprise elements 89 through 103. They, along with the lanthanides, are often called “the f-elements” because they have valence electrons in the f shell. Only thorium and uranium are naturally occurring actinides with significant abundance. They are all radioactive.

Nonmetals. The term “nonmetals” is used to classify the elements H, C, N, P, O, S, and Se.

Halogens. The halogen elements are a subset of the nonmetals. They comprise group 17 of the periodic table, from F through At. They generally very chemically reactive and are present in the environment as compounds rather than as pure elements.

Noble gases. The noble gases comprise group 18. They are generally very stable chemically and exhibit similar properties of being colourless and odourless.

 

Chemical Properties

Atom

All macroscopic matter is made out of many tiny particles called atoms. The study of how these atoms interact is called chemistry.

 

Subatomic Particles

The three particles that make up atoms are protons, neutrons, and electrons. Protons and neutrons are heavier than electrons and reside in the “nucleus,” which is the centre of the atom.

Protons have a positive electrical charge, and neutrons have no electrical charge. Electrons are extremely lightweight and are negatively charged. They exist in a cloud that surrounds the atom. The electron cloud has a radius 10 000 times greater than the nucleus.

 

Nucleus

The nucleus of an atom is made up of protons and neutrons in a cluster. Virtually all the mass of the atom resides in the nucleus. The nucleus is held together by the tight pull of what is known to chemists and physicists as the “strong force.” This force between the protons and neutrons overcomes the repulsive electrical force that would, according to the rules of electricity, push the protons apart otherwise.

 

Electrons

The electron is the lightweight particle that “orbits” outside of the atomic nucleus. Chemical bonding is essentially the interaction of electrons from one atom with the electrons of another atom. The magnitude of the charge on an electron is equal to the charge on a proton. Electrons surround the atom in pathways called orbitals. The inner orbital’s surrounding the atom are spherical but the outer orbital’s are much more complicated.

 

Chemical Bonding

Chemically bonding occurs when two particles can exchange or combine their outer electrons in such a way that is energetically favourable. An energetically favourable state can be seen as analogous to the way a dropped rock has a natural tendency to fall to the floor.

When two atoms are close to each other and their electrons are of the correct type, it is more energetically favourable for them to come together and share electrons (become “bonded”) than it is for them to exist as individual, separate atoms.

When the bond occurs, the atoms become a compound. Like the rock falling to the floor, they “fall” together naturally — periodic.lanl.gov/index.shtml