Atoms, Molecules and Ions. In This Chapter

Transcription

1 Atoms, Molecules and Ions Chapter 2 In This Chapter History of atoms. Subatomic particles. Atomic numbers, mass numbers. Isotopes and Atomic weights. Compounds, Molecules and Ions. Nomenclature. 2 1

2 Elements An element is a pure substance that can t be changed into a simpler form of matter by a chemical reaction. Elements consist of atoms; the smallest structural unit of an element that retains the chemical properties of that element. Atoms are considered as the building blocks of every chemical around us. 3 The Origin of Elements After a very short time (matter of seconds) that the universe was produced by the Big Bang the only elements present were hydrogen and helium. After millions of years the cooling of the universe caused the hydrogen and helium to collect together and form large clouds, due to gravity. These clouds eventually contracted, became hotter and hotter and burst to become stars. Intense heat within the stars made the hydrogen and helium atoms to smash together and produce other heavier elements. This merging releases even more heat which is responsible for starlight. 4 2

3 Names of Elements Names come from various sources including early Greek, Latin or German names describing some property of the element, names of places, names of people. Iodine from Greek word iodes violet like. Bismuth from German weisse masse white mass, miners called it wismat. Germanium and Francium discovered in Element number 110 was recently named Darmstadtium to honor the German city in which it was discovered. Einsteinium and Curium to honor 5 Symbols of Elements An universally accepted abbreviation for the name of the element. Elemental symbols are unique. 14 elements have single letter symbols, while the rest have 2. The following rules are typically followed: Symbols have either one or two letters. If only one letter is used it is capitalized, C, H, O. If 2 letters are used only the first letter is capitalized, Na, Ca, Hg. 6 3

4 Early Atomic Theories Democritus, a Greek philosopher ( BC) proposed the idea of an atom as an indivisible component of matter, using the word atomos which means indivisible. John Dalton, in 1808, published A New System of Chemical Philosophy in which he presented his theory of atoms. Dalton s theory was the first one to provide a systematic description of the relationship between atoms and matter. 7 Dalton s Atomic Theory Dalton s basic postulates (ideas) were: All matter is composed of atoms which are indivisible and indestructible. Atoms are considered as the ultimate chemical particles. An element is composed of identical atoms with fixed, identical properties and masses. Compounds are formed by the combination of atoms of 2 or more different elements in a fixed whole number ratio. A chemical reaction involves a combination, separation or rearrangement of atoms. Atoms are neither created nor destroyed. 8 4

5 Problems with Dalton s Theory Atoms can be divided further. Elements can have more than one mass. Here is what we know about the atom now: 9 Regions of an Atom All atoms consist of 2 distinct regions. The nucleus: A small dense region at the center which contains positively charged protons and neutral (with no charge) neutrons. Surrounding the nucleus is a diffuse region which contains the negatively charged electrons. Atoms of different elements have different # of protons, electrons and neutrons. The # of protons decides the identity of the element. 10 5

6 Properties of Subatomic Particles Proton (p) Charge = 1 Mass = x g Neutron (n) Electron(e - ) Charge = 0 Charge = -1 Mass = x g Mass = x g Protons and neutrons have masses that are similar to each other. The electron is about 1800 times lighter than the other The Neutral Atom Neutral = No charge # of protons (positive charges) = # of electrons (negative charges) Atoms of different elements have different numbers of protons, neutrons and electrons. The number of protons determine the identity of the element. 12 6

7 Atomic Symbol Atomic Mass Number (A) = The sum of # of protons and the # of neutrons Atomic number (Z) = The # of protons 12 6 C In any neutral atom, the atomic number = # of electrons in one atom of that element Atomic Mass Number Atomic number = Number of neutrons. 13 Isotopes Dalton, had proposed that all atoms of the same element must have the same mass. One of the 2 flaws in Dalton s theory was, atoms of the same element can have DIFFERENT masses. The mass of an atom is due to the mass of the protons and the neutrons in that atom. Isotopes are atoms of the same element which have the same number of protons but different number of neutrons. The word isotopes is from the Greek words for equal place. 14 7

8 Isotopes Isotopes have the exact same physical properties but different chemical properties. (Some are RADIOACTIVE and can emit particles and energy). All naturally occurring elements have 2 or more isotopes. Isotopes exist in different amounts (called isotopic abundance) and have different lifetimes. A mass spectrometer is the most direct and accurate means of determining atomic weights and the existence of isotopes. 15 Example of Isotopes 13 6 C C 12 C 6 C 14 C

9 Atomic Mass The mass of an atom is decided by the number of protons and the number of neutrons present in the atomic nucleus. Since atoms are so tiny, using an ordinary unit such as gram or kilogram is inconvenient and the atomic masses are usually reported in a special unit called atomic mass unit (amu). One amu = x g 17 How and Why of the amu Dalton had proposed an atomic scale based on the mass of one Hydrogen atom, which he had assigned the mass of 1. Several of his reported masses were later proven wrong. The Hydrogen based scale was then replaced with an Oxygen based scale and then by the now accepted Carbon 12 based scale. One amu = 1/12 th of the mass of one carbon atom. The mass of one proton = 1 amu. The mass of one neutron = 1 amu. 18 9

10 Atomic Masses and Isotopes The sum of the atomic masses of the protons and the neutrons present in an atom should add up to give an exact round number and be the same as the atomic mass number A. The presence of isotopes makes matters slightly complicated. Each isotope of an element is present in certain percent abundance, which has to be accounted for when it comes to calculating the atomic masses. 19 Using % abundance The % abundance is factored in the calculation for finding the atomic mass of any element. Sample Exercise 2.4 shows how to make use of this idea. Consider the following example: 20 10

11 Isotopes of Neon Neon has 3 naturally occurring isotopes. The masses and natural abundances of these isotopes are as follows. Calculate the atomic mass of Neon. 1. Change each % value to a fraction amu amu amu % 0.27 % 9.25 % 2. Multiply the isotopic masses by these fractions and then add the results together. Atomic mass of Neon = (Mass of isotope #1 x fraction of #1) + (Mass of isotope #2 x fraction of #2) + (Mass of isotope #3 x fraction of #3) 21 Thus, the average atomic mass of Neon is amu 22 11

12 Compounds, Molecules & Ions Compounds Molecular Ionic Molecules composed of 2 or more elements Cations with positive charges Anions with negative charges There are more than 19 million registered compounds with no end in sight as to how many more will be prepared. Each compound can be identified and distinguished by its characteristic properties. 23 The Periodic Table A systematic arrangement of elements according to increasing atomic numbers. Comic book periodic table (

13 The Periodic Table A systematic arrangement of elements according to increasing atomic numbers. The vertical columns are known as groups. 25 The Periodic Table The horizontal rows are known as periods

14 Metals, Metalloids and Non-metals Metals: Solids at room temperature (except Hg). Hard, shiny. Good conductors of electricity and heat. Malleable (from Latin for a hammer). Ductile. High melting point and density. Typically located on the left side of the periodic table. Metals will typically combine with non-metals to form ionic compounds. 27 Metalloids Have properties that are in between the metals and non-metals. Certain metalloids are used as the raw materials for the manufacture of semiconductor devices

15 Non-metals Solids, liquids (Br) or gases. Dull, non-malleable and non-ductile. Low melting points and densities. Poor conductors of heat and electricity. Located on the right of the periodic table. 29 Elements that exist as diatomic molecules There are 7 known elements that exist as diatomic molecules: H 2, F 2, O 2, N 2, Cl 2, Br 2, I 2 When any of these elements are mentioned it is assumed that they exist as diatomic molecules unless specified otherwise. 2 other elements that are commonly polyatomic are Phosphorus (P 4 ) and Sulfur (S 8 ) 30 15

16 Chemical Formulas Show the symbols and the ratios in which the atoms of the elements in a compound combine. Do not show the arrangement of atoms in the compound (structural formulas). If the formula contains one atom of an element the symbol of the element represents the atom. The number 1 is not used as a subscript. The formula of water is H 2 O, not H 2 O 1. When the formula contains more than one atom of an element the number of atoms is indicated by a subscript written to the right of the atom. The number 2 to the right of the H indicates 2 atoms of hydrogen present in H 2 O. 31 Chemical Formulas (continued) When the formula contains more than one of a group of atoms that occurs as a unit, parentheses are place around the (group) and the number of units of the group are indicated by a subscript written to the right of the parentheses

17 Ions Ions are formed by the loss (cations) or gain (anions) of electrons by neutral atoms. Cations: Positively charged ions that are formed an element loses one or more electrons. (Table 2.4 in the book provides some examples) Atom K Potassium Ca Calcium Al Aluminum Cation K + Potassium ion Ca 2+ Calcium ion Al 3+ Aluminum ion 33 Ions (continued) Anions: Negatively charged ions that are formed when a neutral atom gains one or more electrons. Anions are named by using the stem of the parent element and changing the ending to -ide. (Table 2.5 in the book provides some examples) Atom Cl Chlorine Br Bromine O Oxygen N Nitrogen Anion Cl - Chloride ion Br - Bromide ion O 2- Oxide ion N 3- Nitride ion 34 17

18 Ionic charges and the periodic table With groups 1A and 2A the charge on the cations = group number. With groups 5A, 6A and 7A the charge on the anions = 8 group number 35 Name to Formula: Ionic Compounds Ionic compounds = Simplest possible combination of cations and anions. All compounds have a net electric charge = zero. The charges on the cations and the anions should cancel each other out. Consider sodium chloride: Made of the sodium ion (Na + ) and the chloride ion (Cl - ) The formula for this compound would be NaCl. The following formulas will be wrong: Na 2 Cl 2, NaCl 2, Na 2 Cl

19 More Ionic Compounds Write the formula for Calcium fluoride: Calcium ion is Ca 2+, fluoride is F -, only possible combination is CaF 2. Aluminum oxide: Aluminum:Al 3+, oxide O 2- : Al 2 O 3 (The least common multiple of 2 and 3 is 6, where 2(3+) + 3(2-) = 0) 37 Even More Ionic Compounds Name Ions Lowest common multiple Sum of charges on the ions Formula Potassium iodide K + & I - (+1) + (-1) = 0 Potassium sulfide K + & S 2-2(+1) + (-2) = 0 Copper sulfate Cu 2+ & SO 4 2- (+2) + (-2) = 0 Ammonium phosphate NH 4+ & PO 4 3-3(+1) + (-3) = 0 Aluminum chromate Al 3+ & CrO 4 2-2(3+) + 3(2-) =

20 Metal cation - monatomic nonmetal anion General formula M a X b. Always write the name of the metal ion followed by the name of the anion. Need to indicate the charge on the cation except when metals from groups 1A and 2A are present or the ammonium ion is present. NaCl MgO Fe 2 O 3 CuCl 2 39 Metal cation - polyatomic nonmetal anion General formula: M a X b Same rules as before. Only difference is the anion contains more than one elements. Na 2 SO 4 Ca 3 (PO 4 ) 2 K 2 Cr 2 O 7 (NH 4 ) 3 PO 4 ZnCO

21 Molecular compounds Nonmetal-nonmetal compounds. Look for the absence of common metals. General formula: A x B y Write the numerical prefix for nonmetal A. If there is only one atom of A present there is no need to use mono. If A is hydrogen then a numerical prefix is not used. Write the full name of A. Write the numerical prefix for y, if y = 1, no need to write mono. Add the stem of the name of the nonmetal B. Complete the name by adding the suffix ide to the stem of B. CO 2, N 2 O 5, CCl 4, SO

22 Acids An important class of compounds that can be considered as molecules in which one or more H + ions are attached to an anion. If the anion does not contain oxygen the acid is named by the prefix hydro- and the suffix ic. HCl: Hydrochloric acid & HF: Hydrofluoric acid If the anion ends with an -ate the suffix ic is added to the root name. H 2 SO 4 : Sulfuric acid (SO 4 2- sulfate ion), H 3 PO 4 : Phosphoric acid. If the anion ends with an ite, the suffix ous is added to the root name. H 2 SO 3 : Sulfurous acid (SO 3 2- sulfite ion), HNO 2 : Nitrous acid (NO 2- nitrite ion). 43 Organic compounds Contain primarily C and H but may also have N, O, S and P. Alkanes: Simplest kind, contain only C and H in the ratio C n H 2n+2. Methane(CH 4 ), ethane (C 2 H 6 ), Propane (C 3 H 8 ) and butane (C 4 H 10 )