CHAPTER 2: ATOMS, MOLECULES AND

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1 CHAPTER 2: ATOMS, MOLECULES AND IONS Atom: Basic unit of matter. Smallest particle of the element which retain the properties of that element. Eg: Oxygen, Hydrogen Molecules: Two or more atoms (same or different type of atoms) join together by chemical force to form molecules. Eg: H 2, H 2 O Ions: Atoms or group of atoms (molecules) with positive or negative charge. Eg: Na +, Cl - +, NH 4 Fundamental Chemical Laws Law of Conversion of mass: In a chemical reaction, mass (weight) is neither created nor destroyed. Avogadro s Hypothesis At the same temperature and pressure, equal volumes of different gases contain the same number of particles. 5 liters of oxygen and 5 liters of nitrogen contain same number of particles! - But it is not true (because different atoms have different size and properties). To understand about the atoms, we need to know about what atoms made of, and how do the various atoms differ? Early experiments to characterize the atom 1) Thomson model He postulated that an atom consisted of a diffuse cloud of positive charge with the negative electrons embedded randomly in it. This model is called as palm-pudding model. Mass of reactant is equal to mass of product. Law of Definite Proportion: A given compound always contains exactly the same proportion of elements by mass. Law of Multiple Proportions: When chemical elements combine to form a compound, they do so in a ratio of small whole numbers. Dalton s atomic theory 1) Each element is made up of atoms. 2) Element are formed when same type of atoms join together. 3) Compounds are formed when atoms of different elements combine. 4) Chemical reactions Involve reorganization of atoms. During reaction, only the way atoms bound together changes, but there is no change in atom.. 2) Millikan experiment Millikan found out that the mass of electron is 9.11 X kg. 3) Rutherford experiment He explained that atom has A center of positive charge (nucleus) Electrons moving around the nucleus. It is called as nuclear model. General Chemistry Handout_Ch-2 (Page: 1)

2 ATOMIC STRUCTURE Element Symbol: One or two letter abbreviation. It is based on current name or original (old) name. Current Name Original Name Symbol Hydrogen Carbon Calcium Oxygen - H - C - Ca - O Iron Ferrum Fe Atom consist of Copper Cuprum Cu 1) Protons present in the nucleus. It has positive chare. 2) Neutrons present in the nucleus. It has no charge. It also has almost the same mass as a proton. 3) Electrons move around the nucleus. It has negative charge. Protons determine the identity of an atom (atoms identified depending on the number of protons present in it). Protons and neutrons are present in the tiny nucleus at high density. Both of them accounts for almost all the atom s mass (atomic mass). Electrons occupy most of atom s volume. Electrons form the outer shell of the atom, so that it allows one atom to interact with another. Thus, it is responsible for the chemical properties of the atom. Atoms are neutral in charge, because the number of protons is equal to number of electrons. ATOMIC SYMBOLS Silver Argentum Ag Sodium Natrium Na Atomic number (Z) = Number of protons (or electrons) Mass number (A) = Number of protons + Number of neutrons The atomic number (Z, number of protons) is written as a subscript to the left of the abbreviation, while the Mass number is written as a superscript on the left. Mass number (A) Element Symbol Atomic number (Z) Carbon Abbreviation: C Atomic number is in subscript: 6 C Mass number is in superscript: 12 C To identify the different elements (atoms), we have a representing each element. standard way of The element (carbon) is written as: 12 6C Element symbol with atomic number and mass number. General Chemistry Handout_Ch-2 (Page: 2)

3 Surprisingly, in periodic table only atomic number (Z) is mentioned along with the abbreviation of the element (no mass number). Carbon is written as 6 C Uses of atomic symbol You can find out atomic number and mass number from atomic symbol. Example Na Atomic number (Z) = 11 Mass number (A) = 23 From the atomic symbol, you can also calculate the number of electron, proton and neutron. Example Na Number of proton = 11 (because Z = number of proton) Number of electron = 11 (because it is equal to number of protons) Number of neutrons = Mass number atomic number Uses of periodic table = = 12 All the elements are arranged in periodic table depending on the atomic number (Z). From the periodic table you can find out number of protons and number of electrons present in an atom. 2 He Atomic number (Z) = 2 Number of protons = 2 Number of electrons = 2 If the element name (abbreviation) and mass number is given, you can write the atomic symbol. Na has mass number 23. Write the atomic symbol. Atomic number of Na = 11 (find out from the periodic table) Mass number = 23 Atomic symbol = 23 11Na General Chemistry Handout_Ch-2 (Page: 3)

4 ISOTOPES Atoms with the same atomic number but different mass numbers are called isotopes. Isotopes have same number of proton and electron, but different number of neutron. Because the chemical property of the atom depends isotopes have same chemical property. Sodium ( 11 Na) has 11 electrons and 11 protons. IONS If sodium has 12 neutrons it is written as 23 11Na.. If sodium has 13 neutrons it is written as 24 11Na Na and 24 11Na are called as isotopes. Atoms or group of atoms (molecules) with a net positive or called ions. on electrons, the negative charge is Chlorine ( 34 17Cl) has 17 protons (+ve charge) and 17 electrons (-ve charge). So it is neutral ( = 0). Chlorine ( 34 17Cl - ) has 17 protons (positive charge) and 18 electrons (negative charge). So it has net negative charge ( = -1). SIMPLE INTRODUCTION TO PERIODIC TABLE As the different elements were discovered, they are arranged in a table depending on the atomic number and chemical properties. This is called periodic table. 1. From left to right (row of periodic table) the atomic number (Number of protons) increases. 2. From top to bottom (vertical column of periodic table), the elements have similar chemical properties. It is very important to know the metals and non-metals in periodic table (very helpful in naming simple compounds. Example. Na + (net positive charge), Cl - (net negative charge) Cations: Positive ions are called cations. They are produced by the removal of one or more electrons. Example Na Removal of one electron Na + Sodium ( 23 11Na) has 11 protons (+ve charge) and 11 electrons (-ve charge). So it is neutral (no charge). Sodium ion ( 23 11Na + ) has 11 protons (+ve charge) and 10 electrons (-ve charge). So it has net positive charge ( = +1). Anions: Negative ions are called anions. They are produced by the addition of one or more electrons. Example Cl Addition of one electron Cl - General Chemistry Handout_Ch-2 (Page: 4)

5 Metals Most of the elements are classed as metals because they conduct heat and electricity. Metals frequently lose electrons to form positive ions. It can form ionic compound with a negatively charged non-metal. Metals are in the left side and middle of the periodic table. Non-metals Non-metals frequently gain electrons to become negatively charged. They can form ionic compound with a metal ion. They also can form covalent compound by bonds with each other. Non-metals are in the upper right side of the periodic table (Diagonal from B to At) and H. COMPOUNDS Two or more different chemical elements join together by chemical force (chemical bond) to form compounds. Eg. Na + + Cl - NaCl Classification of Ions (depending on number of atoms): Ions are classified into monoatomic ions number of atoms present. Monoatomic ion: An atom which has net positive or negative charge. H +, Na +, Cl - Polyatomic ion: Group of atoms (more than 1 atom) which have net positive or negative charge NH + - 4, NO 3 Classification of Ions (depending on number of atoms): and polyatomic ions depend on the Ions also classified based on the capacity of the metal to form one or more cations, Type I: Metals forms only one cation. These metals present in the left side of the periodic table (and Ag +, Zn 2+, Cd 2+, Al 3+, Ga 3+ ). Example. Na form only Na + (not Na 2+ or Na 3+ ). Type II: Metal forms more than one cation. These metals present in the middle of the periodic table. Example. Fe can form Fe 2+ and Fe 3+ NH NO 3 - NH 4 NO 3 Classification of compounds 1. Covalent compounds are formed by sharing electrons between non-metals. H + H H 2 Ga Ionic compounds - Consist of positive (metal cation) and negative (non-metal anion) charged ions were attracted together because of opposite charge. Compounds made in this manner are also called as ionic solids or salts. Na + + Cl - NaCl General Chemistry Handout_Ch-2 (Page: 5)

6 Mass number (A) Mass number = Number of Protons + Number of Neutrons It is indicated in the top left side of the element symbol. Example Atomic mass (atomic weight) 12 6 C: Mass number of C (carbon) = 12 Atomic mass is the mass of an atom in atomic mass units (amu). Atomic mass = Mass number in amu (unit) 1 amu = Mass of 1 proton or 1 neutron Average atomic mass Mass of 12 6C = 12 amu Elements present in nature as mixtures of isotopes (same numbers of protons but different numbers of neutrons). Carbon 98.89% present as 12 C 1.11% present as 13 C < 0.01 % present as 14 C Atomic mass of C = (12 x 98.89) + (13 x 1.11) + (14 x 0.01) 100 = ( )/100 = = amu Atomic mass can be determined using an instrument called Mass Spectrometer. MOLE (also written as mol) 1 mole of a substance = x atoms (or) units x is Avogadro's number. 1 mole of carbon = x carbon atoms 1 mole of hydrogen = x hydrogen atoms 1 mole of CH 4 = x CH 4 molecules Solution 1: How many moles of oxygen atoms are there in 10 moles of KClO 3? 1 KClO 3 molecule contains 3 oxygen atoms. 1 mol of KClO 3 contains 3 mol O Therefore, Measuring moles 10 mol of KClO 3 contains 30 mol O 1 mole of element = Mass number in grams 1 mole of 12 C (carbon) = 12 grams of 12 C Thus, 12 gram of 12 C (carbon) = x carbon atoms 1 gram of 1 H (hydrogen) = x hydrogen atoms We can find out the relation between atomic mass unit (amu) and gram (g) x carbon atoms = 12 grams 1 carbon atom = 12 amu (6.022 x atoms) x (12 amu / 1 atom) = 12 g = x amu = 1 g General Chemistry Handout_Ch-2 (Page: 6)

7 MOLAR MASS Molar mass = Sum of the masses of 1 mole of constituent atoms (in grams). CO 2 contain one C and two O atoms (or) 1 mol of CO 2 contains 1 mol C and 2 mol O. Mass of 1 mol C = grams Mass of 1 mol O = grams Molar mass of CO 2 = (1 x 12.01) + (2 x 16.00) grams = grams Solution 1: Calculate the molar mass of CaCO 3 1 mol of CaCO 3 contains 1 mol Ca, 1 mol C and 3 mole O. 1 mol Ca = g 1 mol C = g 3 mol O = 3 x 16 = 48 g 1 mol of CaCO 3 = = g Solution 2: Calculate the molar mass of (NH 4 ) 2 Cr 2 O 7 1 mol of (NH 4 ) 2 Cr 2 O 7 contains 2 mol N, 8 mol H, 2 mol Cr, and 7 mole O. 2 mol N = 2 x = g 8 mol H = 8 x = g 2 mol Cr = 2 x 52 = 104 g 7 mol O = 7 x 16 = 112 g Solution 3: How many H atoms are in 72.5 g of C 3 H 8 O? 1 mol C 3 H 8 O = (3 x 12) + (8 x 1) + 16 = 60 g C 3 H 8 O Convert g to mol (Mol = Mass / Molar mass) Moles of C 3 H 8 O = 72.5 g / 60 g = 1.21 mol 1 mol C 3 H 8 O molecules contain 8 mol H atoms 1.21 mol C 3 H 8 O molecules contain = 1.21 x 8 mol H atoms 1 mol = x atoms/units 9.68 mol H = 9.68 x x H atoms = 5.83 x l10 24 H atoms = 9.68 mol H atom Solution 4: How many grams do 8.50 x molecules of NH 3 represent? x NH 3 atoms/units = 1 mol Convert number of molecules to moles 1 mol of NH 3 = (3 x 1.008) = g 1 mol NH 3 = x NH 3 molecules 8.50x10 22 NH 3 molecules x 1 mol NH 3 x grams 6.022x10 23 molecules of NH 3 1 mol NH 3 = grams 8.50 x molecules of NH 3 = 2.40 grams 1 mol of (NH 4 ) 2 Cr 2 O 7 = = g General Chemistry Handout_Ch-2 (Page: 7)

8 Sample Questions: 1) Which of the following is true? a) An individual atom is a solid b) An individual atom is a liquid c) An individual atom is a gas d) An individual atom cannot be considered to be a solid, liquid or gas 2) The weight of a container with some chemicals is 250 g. If the chemicals are burned in a closed container, which one of the following is true? a) Weight decrease less than 250 g b) Weight do not change (250 g) c) Weight increase more than 250 g d) Weight change depends on the chemical 3) In a combustion reaction, 46 g of ethanol reacts with 96 g of oxygen to produce water and carbon dioxide. If 54 g of water is produced, what mass of carbon dioxide is produced? Mass of Ethanol + Oxygen = Mass of Water + Carbon dioxide 46 g + 96 g = 54 g + Mass of carbon dioxide Mass of carbon dioxide = (46+96) 54g = 88 g 4) Which one of the following has no charge? a) Electron b) Proton c) Neutron d) All the above 5) Which one of the following determines the identity of an atom? a) Electron b) Proton 6) Which one of the following is highly responsible for the chemical properties of an atom? a) Electron b) Proton c) Neutron d) None of the above 7) How is an ion formed from a neutral atom? a) Adding or subtracting protons b) Adding or subtracting neutrons c) Adding or subtracting electrons d) All the above 8) Elements were arranged in periodic table depending on a) Atomic mass b) Number of neutrons c) Alphabetical order d) Atomic number 9) How many protons, neutrons and electrons are in U? Atomic number = 92 Atomic mass = 235 Number of protons = 92 Number of electrons = 92 Number of neutrons = = 143 Calculate the number of protons, electrons and neutrons for the following atoms. Atom No. of Protons No. of Electrons No. of Neutrons Pb = C = F = Ca = U = 143 c) Neutron d) All the above General Chemistry Handout_Ch-2 (Page: 8)

9 10) How many protons and electrons in He? Atomic number of He (see the periodic table) = 2 Number of protons = 2 Number of electrons = 2 Calculate the number of protons and electrons for the following atoms. Atom Atomic number (from the periodic table) No. of Protons No. of Electrons Mg Zn ) How many protons, neutrons and electrons are in 79 Br? Atomic mass (A) = 79 Atomic number (see the periodic table) (Z) = 35 Number of protons = 35 Number of electrons = 35 Number of neutrons = = 44 Calculate the number of protons, electrons and neutrons for the following atoms. 13) What is the atomic symbol ( A ZX) for Z=8, and number of neutron = 9. If Z = 8, Element name = O (see the periodic table) Number of protons = 8 (from Z, atomic number) Number of neutrons = 9 Mass number (A) = = 17 Atomic symbol = 17 8O 14) How many numbers of protons and electrons present in Ba 2+ ion. Atomic number of Ba (see the periodic table) (Z) = 56 Number of protons in Ba 2+ = 56 Number of electrons in Ba 2+ = 56-2 = 54 For each of the following ions, indicate the number of protons and electrons. Atom Z No. of Protons No. of Electrons Zn = 28 N = 10 Br = 36 Atom A Z No. of Protons No. of Electrons No. of Neutrons 133 Cs = Br = Fe = 30 12) What is the atomic symbol ( A ZX) for Z=27 and A = 60. If Z = 27, Element name = Co (see the periodic table) Atomic symbol = 60 27Co General Chemistry Handout_Ch-2 (Page: 9)