Summary of the Atom

Summary of the Atom • atoms are the smallest particles that can be uniquely associated with an element • each element has unique atoms • atoms are composed of e-, p and n • atoms are electrically neutral (# of e- = # of p) • for a single element, isotopes differ only in number of n (neutrons) • atoms have characteristic masses (atomic weights) • atoms combine with one another in definite, whole number proportions to make compounds ~ 10-10 m electron Mass 9 x 10-31 kg nucleus Mass > 10-26 kg ~ 1 – 7 x 10-15 m (1 – 7 fermi) The Spacious Atom Microcosms of our solar system, atoms are dominantly empty space: electron orbits If an oxygen atom had a total radius of 100 km, the nucleus would be a ~1 m diameter sphere in the middle. Electrons in Orbit In a simplistic model, electrons float around the nucleus in energy levels called shells. electron orbits As the number of electrons increases, they start to fill shells farther out from the nucleus. In most cases, electrons are lost or gained only from the outermost shell. Atom Nucleus The Nuclear Model of the atom Subatomic Particles Particle Symbol Charge Relative Mass 0 1 Electron Proton ep+ 11+ Neutron n 0 1 Atomic Number Atomic Number 11 Symbol Na All atoms of an element have the same number of protons 11 protons 11 Sodium Na Number of Electrons  An atom is neutral  The net charge is zero  Number of protons = Number of electrons  Atomic number = Number of electrons in a neutral atom Mass Number Counts the number of protons and neutrons in an atom Atomic Symbols  Show the mass number and atomic number  Give the symbol of the element mass number 23 Na sodium-23 atomic number 11 Notation for Atoms 12C 13C only one isotope of carbon only one isotope of carbon C all isotopes of carbon Basic Definitions • “atomic number” = number of protons in the nucleus; • “atomic weight” = average mass of an atom calculated from the masses and natural abundances of all isotopes (use atomic weights to calculate the molecular weights of compounds from their constituent elements!) • “mass number” = sum of protons + neutrons in the nucleus • “isotopic mass” = mass of a single isotope More Atomic Symbols 16 O 8 8 p+ 8n 8 e- 31 P 15 15 p+ 16 n 15 e- 65 Zn 30 30 p+ 35 n 30 e- Isotopes  Atoms with the same number of protons, but different numbers of neutrons.  Atoms of the same element (same atomic number) with different mass numbers Isotopes of chlorine 35Cl 17 37Cl 17 chlorine - 35 chlorine - 37 Mass spectrometry Atomic weight measurements How was the atomic weight measured? • By mass spectrometry – This also measures % natural abundance for a given isotope Atomic weight calculation There are three naturally occuring isotopes of neon (Ne): 20Ne 21Ne 22Ne isotopic mass = 19.99244018 amu isotopic mass = 20.9938467 amu isotopic mass = 21.9913855 amu the atomic weight is reported in text as: 20.1797 amu Learning Check 1 Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 6 13C 6 14C 6 #P _______ _______ _______ #N _______ #E _______ _______ _______ _______ _______ Solution 12C 6 13C 6 14C 6 #P __6___ #N __6___ #E __6___ _ 6___ _ _7___ _ 6___ ___6___ ___8___ ___6___ Learning Check 2 An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 2) 35 3) 65 B. Number of neutrons in the zinc atom 1) 30 2) 35 3) 65 C. What is the mass number of a zinc isotope with 37 neutrons? 1) 37 2) 65 3) 67 Solution An atom of zinc has a mass number of 65. A. Number of protons in the zinc atom 1) 30 B. Number of neutrons in the zinc atom 2) 35 C. What is the mass number of a zinc isotope with 37 neutrons? 3) 67 Learning Check 3 Write the atomic symbols for atoms with the following: A. 8 p+, 8 n, 8 eB. 17p+, 20n, 17e___________ ___________ C. 47p+, 60 n, 47 e- ___________ Solution 16O A. 8 p+, 8 n, 8 e- 8 B. 17p+, 20n, 17eC. 47p+, 60 n, 47 e- 37Cl 17 107Ag 47 Learning Check 4 An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 14 2) 16 3) 34 B. Its mass number is 1) 14 2) 16 C. The element is 1) Si 2) Ca 3) 34 3) Se D. Another isotope of this element is 1) 34X 16 2) 34X 14 3) 36X 14 Solution An atom has 14 protons and 20 neutrons. A. It has atomic number 1) 14 B. It has a mass number of 3) 34 C. The element is 1) Si D. Another isotope of this element would be 3) 36X 14 Masses of Atoms  A scale designed for atoms gives their small atomic masses in atomic mass units (amu)  An atom of 12C was assigned an exact mass of 12.00 amu  Relative masses of all other atoms was determined by comparing each to the mass of 12C  An atom twice as heavy has a mass of 24.00 amu. An atom half as heavy is 6.00 amu. Atomic Mass Na 22.99  Gives the mass of “average” atom of each element compared to 12C  Average atom based on all the isotopes and their abundance %  Atomic mass is not a whole number Calculating Atomic Weight or Mass  Percent(%) abundance of isotopes  Mass of each isotope of that element  Weighted average = mass isotope1(%) + mass isotope2(%) + … 100 100 Atomic Mass of Magnesium Isotopes 24Mg 25Mg 26Mg Mass of Isotope Abundance = = 24.0 amu 25.0 amu 78.70% 10.13% = 26.0 amu 11.17% Atomic mass (average mass) Mg = 24.3 amu Mg 24.3 Atomic mass calculation How was the atomic mass calculated? • multiply each isotopic mass by the reported natural abundance for the isotope, then: • add these individual contributions for each isotope to get the average atomic mass for the element Atomic mass calculation There are three naturally occuring isotopes of neon (Ne): 20Ne mass # = 19.99244018 amu (90.51%) 21Ne mass # = 20.9938467 amu (0.27%) 22Ne mass # = 21.9913855 amu (9.22%) the atomic mass is reported in text as: 20.1797 amu 18.10 + 0.057 + 2.03 = 20.19 amu Learning Check 5 Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the atomic mass of gallium? Solution Ga-69 68.9 amu x Ga-71 (%/100) 60.2 100 = 41.5 amu for 69Ga 70.9 amu x 39.8 100 = 28.2 amu for 71Ga Atomic mass Ga = 69.7 amu Finding An Isotopic Mass A sample of boron consists of 10B (mass 10.0 amu) and 11B (mass 11.0 amu). If the average atomic mass of B is 10.8 amu, what is the % abundance of each boron isotope? Assign X and Y values: X = % 10B Y = % 11B Determine Y in terms of X X + Y = 100 Y = 100 - X Solve for X: X (10.0) + (100 - X )(11.0) 100 100 = 10.8 Multiply through by 100 10.0 X + 1100 - 11.0X = 1080 Collect X terms 10.0 X - 11.0 X = 1080 - 1100 - 1.0 X = -20 X = -20 - 1.0 = 20 % 10B Y = 100 - X % 11B = 100 - 20% = 80% 11B Learning Check 6 Copper has two isotopes 63Cu (62.9 amu) and 65Cu (64.9 amu). What is the % abundance of each isotope? (Hint: Check Zumdahl or any other chemistry text for atomic mass) 1) 30% 2) 70% 3) 100% Solution 2) 70% Solution 62.9X + 6490 = 64.9X = 6350 -2.0 X = -140 X = 70% Atomic Masses 13C 12C 13.00335 amu (1.11%) 12.0000 amu (98.89%) atomic weight of C = 12.01115 amu WHY? Calculating masses of atoms relative to 12C (mass of 12C atom) * 1.58320 = mass of F atom = 18.99840 reported atomic weight of F = 18.9984 Charged Atoms: Ions Left to their own devices, atoms are electrically neutral. That means that they have an equal number of protons and electrons. During the course of most natural events, protons are not gained or lost, but electrons may be. Atoms with more or fewer electrons than protons are electrically charged. They are called ions: an atom that loses electrons takes on a positive charge (cation); an atom that gains electrons takes on a negative charge (anion). An ISOTOPE is one of a set of nuclides with the same Z and consequently different A. (ie isotopes are the same chemical element but different masses). e.g. 12 13 14 6C 6C 6C An ISOTONE is one of a set of nuclides with the same N and consequently different A. e.g. 39 40 41 18 A 21 , 19 K 21, 20 A 21 An ISOBAR is one of a set of nuclides with the same A but different N and Z. e.g 14 14 6C 7 N 14 8O • More on atomic notation, which is based on the nuclear structure: – Isotope: same Z, different A and N – Isobar: same A, different Z and N – Isotone: same N, different Z and A Example: From the following list of atoms, which are isotopes, isobars, and isotones? 131 54 Xe 130 53 I 132 55 Cs 131 53 I Component Atom Xe A Z N I Cs I