LECTURE 4 CHM 151 ©slg TOPICS: 1. Atomic Symbols, Mass 2. Isotopes 3. Periodic Table 4. Molecules and Compounds Kotz, ;

THE “NUCLEAR” ATOM Nucleus, C atom Electron cloud.

ATOMIC SYMBOLS Each atom of an element can be represented by a symbol that describes how many protons, neutrons and electrons are contained in this basic unit: X A Z Mass number, A: total #, p + n Atomic Number, Z: #p (equals #e) Elemental symbol

Accordingly, A, the mass number of the atom, represents both the total number of nuclear particles (p + n) and the approximate mass of the atom (in amu’s) Because atoms (and the subatomic particles) are so tiny, a relative mass scale was setup to describe atomic weights in a convenient numerical range. The atom of Carbon which contains 6 protons and 6 neutrons in its nucleus is assigned the weight of amu (atomic weight units), which essentially makes the mass of each proton and neutron 1.00 amu. The Mass Number, A

Z, The Atomic Number All known elements are listed in the familiar “Periodic Table of the Elements” in order of increasing atomic number, found usually in the upper right hand corner above each element’s symbol. The atomic number represents the number of protons in the nucleus of every atom of that element. Since every atom is electrically neutral, the number of protons in the nucleus represents the total positive charge of the nucleus, which is exactly balanced by the total number of negatively charged electrons outside the nucleus.

Examples, Atomic symbols F U X A Z Atomic Mass #, p + n Atomic #, p (or e) 9p, 9e p = 10n p = 146n 92p, 92e Element

Group Work: Atomic Symbols

Isotopes of the Elements For a given element, the number of protons and electrons is a fixed value and determines which element is being described. The number of neutrons in the atom of a given element is not fixed, and several different atoms of a given element are generally found, varying in atomic mass due to differing numbers of neutrons. The different atoms of a given element which vary by neutron count and by mass are described as “isotopes” of that element.

Isotopic Symbols Isotopic symbols represent specific isotopic forms of an element. Consider hydrogen:

Atomic Mass, revisited: The atomic mass of any isotope of an element can be approximated by a simple addition of the number of p’s and n’s in the nucleus. However, naturally occurring samples of any element generally include several different isotopes of different atomic masses. The atomic mass value given for each element (as found in your Periodic Table) is a weighted average of all the isotopes.

The given atomic mass for any element will be closest to the most abundant element in most cases. Consider below the calculation of the atomic mass of magnesium, amu: Average Mass Element, amu = (mass, isotope A X % abundance A) + (mass, isotope B X % abundance B) + (mass, isotope C X % abundance C) General Method of Calculation:

Calculation of average atomic mass of Magnesium: ( x.7899) + ( x.1000) + ( x.1101) = (18.95) + (2.499) + (2.861) = amu

You may detect a discrepancy between the mass, in amu’s, of individual protons and neutrons (as given in the first table presented, lecture 3) and the given masses of the Mg isotopes: Note: Mass, proton: amu Mass, neutron, amu Mass, 24 Mg, amu amu X 12 = amu X 12=

Why Don’t the Numbers Add Up? The mass of the individual particles, p and n, were determined by methods which measured individual particles not bound up in the nucleus. When these particles are packed into the nucleus of an atom, a mass loss (or “mass defect”) always occurs. This loss is thought to be the result of a matter to energy conversion which holds together the nucleus, called the “binding energy”... (See Chapter 24, p 1099) In the nucleus of atoms, p’s and n’s show a mass very close to 1.00 amu rather than or

The Periodic Table Lists all known elements in order of increasing atomic number, left to right and top to bottom Arranged so that elements of similar chemical properties fall into the same column (called a group or family) Each horizontal line(called a period) represents elements of a complete range of chemical properties.

The beginning of each “period” features a very active metal The middle of the period includes increasingly less active metals The right hand side of the period includes elements becoming increasingly non-metallic The end of the period features an inert, unreactive element found only in the gas state

Each line of the table features a complete swing from reactive metal to non metal to inert gas. The action is repeated in each successive period, named after the action of a pendulum, in which each revolution or swing is also called a period or “repeated occurrence, from beginning to end and then back again”

metals metalloids Non metals Noble gases

Main Group or “Representative Elements” Alkali metals 1A (except H) Alkaline earth metals 2A Halogens 7A Noble gases inert gases rare gases 8A (0)

Transition Metals 3 columns 8B: more alike “across” than “down” Non- naturally occurring, newly found in lab

The inner Transition Metals: Found beneath the main body of the PT, but belonging to Period 6 and 7: #58 #90 #103 #71 “Lanthanides” “Actinides”

solids liquidsgases Note the seven common elements found in nature as “diatomic molecules”

Molecules and Compounds, Chapter 3 Atoms of the elements come together to form molecules and compounds. Molecules are made by the bonding of two or more atoms together into a independent, uncharged unit. The simplest molecules are the “diatomic elements”: H 2 N 2 O 2 F 2 Cl 2 Br 2 I 2

Compounds are composed of two or more elements, and can be broken down into these elements OR smaller compounds by chemical means: electric current 2 NaCl 2 Na + Cl 2 heat CaCO 3 CaO + CO 2

Formulas for Molecules and Compounds All compounds and molecules are represented by a formula which summarizes number of atoms of each element present: H 2 O H 2 SO 4 CH 3 CH 2 OH NaCl C 12 H 22 O 11 F 2 Fe 2 (SO 4 ) 3 (NH 4 ) 2 CO 3

TYPES OF FORMULAS CONDENSED FORMULAS: summation of all atoms in formula, alcohol: C 2 H 6 O STRUCTURAL FORMULAS: clue to connectivity in compound, alcohol: CH 3 CH 2 OH

TYPES OF COMPOUNDS MOLECULAR: all atoms in formula bonded together in a single unit called a “molecule” IONIC: formula represents the simplest ratio of positive to negative ions found in a sample of the compound

Difficult chore: Molecular compounds are named differently than ionic compounds... To distinguish between the types and to name each correctly, try these guidelines: If the compound formula starts with a nonmetal or metalloid,it is a molecular type compound and is named as such. If the formula starts with a metal, consider it an ionic compound, name as such. If it starts with H, it is an acid and has its own set of naming rules ! NAMING MOLECULES AND COMPOUNDS

NAMING IONIC COMPOUNDS, A PREVIEW: Cations with invariable positive charges are named by using their element name. Cations with a variable charge are named by stating the element name followed by a Roman numeral to state its charge in the compound. Ionic compounds are composed of a balanced set of cations (+ ion), and anions (- ion) in correct proportion to be electrically neutral. To name these compounds, one states the name of the cation, then the name of the anion.

Cations with a variable charge are named by stating the element name followed by a Roman numeral to state its charge in the compound. Fe 2+ “Iron(II)” Fe 3+ “Iron(III) Cu 1+ “Copper(I)” Cu 2+ “Copper(II)” Cations with invariable positive charges are named by using their element name. Na + “Sodium” Mg 2+ “Magnesium” CATION NOMENCLATURE

Polyatomic anions need to be memorized! (Table 3.1, p. 110) Ones containing O generally end in “ate”, although when several forms are encountered for same element, one will end in “ite” (less O)... SO 3 2- sulfite SO 4 2- sulfate NO 2 - nitrite NO 3 - nitrate Monoatomic anions are named by changing their elemental name to end in “ide” F - fluoride O 2- oxide S 2- sulfide H - hydride N 3- nitride P 3- phosphide ANION NOMENCLATURE