Elements Atoms. The ultimate building blocks of all physical substances that we encounter whether naturally occurring or man-made are the elements.

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1 Elements Atoms INTRODUCTION The ultimate building blocks of all physical substances that we encounter whether naturally occurring or man-made are the elements. There are as of this counting 116 distinct chemical elements. It is their innumerable combinations that allows us to produce and classify the various substances that we encounter daily. The elements are the entities that will allow us to begin our systematic study of chemistry. OBJECTIVES 1. The student will identify or write the definition for the terms given in this unit. 2. The student will identify and name the elements used in this unit, writing their symbols and spelling their names correctly. 3. The student will write the complete symbol for an element including the Mass Number and Atomic Number. 4. The student will determine the number of protons, neutrons and electrons in an atom of an element, given the Mass Number and Atomic Number. DISCUSSION A. Elements An element is any of 116 unique substances that singly or in combination constitute all matter. Matter is anything that has mass and occupies space. Each element has a name and a symbol. The name oftentimes originates from the discoverer or place of discovery. Some of the names are Greek, some are Latin in origin. The symbols for the elements are generally the first letter or first two letters of the name. The first letter or single letter is always capitalized and the second is small. There is no period at the end of the symbol.

2 Following is a list of some of the most commonly occurring or abundant elements along with their symbols. Element Symbol Element Symbol Hydrogen H Neon Ne Helium He Sodium Na Lithium Li Magnesium Mg Beryllium Be Aluminum Al Boron B Silicon Si Carbon C Phosphorus P Nitrogen N Sulfur S Oxygen O Chlorine Cl Fluorine F Argon Ar B. Atoms Some of the elements should be familiar to you. The news reports cover the recent accident at Three Mile Island mentioned Hydrogen frequently and how many times have you used Aluminum foil or seen a Neon light? An atom is the smallest part of an element that still exhibits all the properties of that element. Atoms are as small as the universe is large. We cannot see atoms, but there is much scientific evidence that they exist. Today s chemistry is based on the existence of atoms and the Atomic Theory which is the basis for our dealings with substances in chemistry. The following are the postulates (rules) of the Atomic Theory: 1. All matter is made of atoms. 2. Atoms of an element are the same. 3. Atoms of different elements are different. 4. Atoms combine in simple whole number ratios to form compounds. 5. A compound can be broken down into the atoms that make it up and these atoms can be reassembled into the same or a new compound. From now on all our dealings with chemical substances will be based on this Atomic Theory. Now whenever we are concerned with an atom of an element we may simply write its symbol. That is, the symbol may represent the element or an atom of the element.

3 C. Atomic Weight The Atomic Weight (AW) of an atom of an element is the relative weight of the atom compared to a carbon atom with an atomic weight of exactly amu. An amu is the unit that is used to describe the relative mass of an atom. The atomic weight may be found in tables of the elements and on the Periodic Table which is the subject of another unit. Following are the symbols and Atomic Weights for the eighteen elements encountered earlier. Element AW Element AW H 1.01 Ne He 4.00 Na Li 6.94 Mg Be 9.01 Al B Si C P N S O Cl F Ar D. Protons, Neutrons and Electrons Atoms of elements are made of combinations of subatomic particles. Each distinct element has a different number of subatomic particles. These particles are protons, neutrons and electrons. An atom is spherical in nature with a nucleus or point at the center of the sphere. Protons and neutrons make up the nucleus with the electrons surrounding it. The nucleus of an atom is much, much smaller than the atom itself. The relative weights of protons, neutrons and electrons can be described using the atomic weight scale used for atoms. Protons and electrons have electrical charges as well. The following table lists these weights and charges, as well as their symbols. Subatomic Particle Symbol Weight Charge Proton P 1 amu +1 Neutron n 1 amu 0 Electron e- 1/ 1835 amu -1 Note that the proton and electron have greatly unequal masses yet they have electrical charges of the same magnitude but opposite signs.

4 E. Atomic Number The Atomic Number (Z) of an atom is equal to the number of protons in the nucleus of an atom. Each element has its distinct Atomic Number. There are as many Atomic Numbers as there are elements, 116. In neutral atoms (atoms with no net charge) the number of electrons equals the number of protons or the Atomic Number. The Atomic Number is used to determine the number of protons and electrons in an atom as well as the number of neutrons. F. Mass Number The Mass Number (A) of an atom is the simple sum of the number of protons and neutrons in the nucleus of an atom. The Mass Number for an element may be determined by rounding off the listed Atomic Weight of an atom to the nearest whole number. With the Atomic Number and Mass Number the number of protons, neutrons and electrons in an atom my be determined. The following rules will be used: no. of protons = Atomic Number no. of electrons = Atomic Number (Neutral atoms only) no. of neutrons = Mass Number Atomic Number

5 The following table shows the Atomic Weight, Mass Number, Atomic Number, number of protons, neutrons and electrons and electrons for the first eighteen elements. Element Atomic Weight Mass Number Atomic Number Protons Neutrons Electrons H He Li Be B C N D F Ne Na Mg Al Si P S Cl Ar G. Complete Chemical Symbol The complete chemical symbol of an element includes the Mass Number and the Atomic Number. The Atomic Number (Z) is written as a lower subscript. The Mass Number (A) is written as an upper superscript. Both are written before the chemical symbol. The general form would be as follows: A Z X Where X is the chemical symbol, Z is the Atomic Number and A is the Mas Number. The eighteen elements would appear as: 1 1 H, 4 2 He, 7 3 Li, 9 4 Be, 11 5 B, 12 6 C, 14 7 N, 16 8 O, 19 9 F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar

6 H. Isotopes Isotopes are atoms of an element with different numbers of neutrons, and therefore different Atomic Weights and Mass Numbers. Isotopes are nature s variations of each atom. A sample of a pure element will contain the isotopes of that element. These isotopes differ only in the nucleus of the atom. Some atoms have only two isotopic forms, some as many as twenty-seven. Hydrogen, for example, has three isotopic forms. Hydrogen, Deuterium and Tritium they differ only in the number of neutrons and their relative abundances. The following table lists the make up and relative abundances of each of the three isotopes. Name Hydrogen Deuterium Tritium Complete Symbol Abundance Protons Neutrons Electrons 1 1 H % H.0149% H.0001% Using the complete chemical symbol it is easy to distinguish between the different isotopes of an element.

7 PROBLEMS 1. Define the following terms and give examples where appropriate a. Element f. Neutron b. Matter g. Electron c. Atom h. Atomic Number d. Atomic Weight i. Mass Number e. Proton j. Isotope 2. Write the symbols for the following elements: Hydrogen Lithium Boron Nitrogen Sodium Aluminum Phosphorus Chlorine Fluorine 3. Write the names for the following elements: He Be C O Mg Si S Ar Ne 4. Write the complete symbol for the following isotopes: Element Atomic Number Atomic Weight a. Ti b. Ac c. In d. Kr e. O Determine the number of protons, neutrons and electrons in the following isotopes C N 7 Po Ar 31Ga