Wednesday,  October  10   •  Lecture  10  (Tuesday)  

 

–  Clickers  are  Almost  Working!!    Need  to  Re-­‐register   –  Elements  and  Symbols  (3.1)   –  The  Periodic  Table  (3.2)   –  The  Atom  (3.3)  

•  Lecture  11  (Today)  

–  The  Atom  (3.3)   –  Atomic  Number  &  Mass  Number  (3.4)   –  Isotopes  and  Atomic  Mass  (3.5)   –  Electron  Arrangement  in  Atoms  (3.6)    

Select  the  Statement  that  is  True   The  Nucleus  of  an  Atom:   A.  B.  C.  D. 

contains  protons  and  electrons   takes  up  a  lot  of  space  in  the  atom   contains  protons  and  neutrons   contains  electrons  

Atomic Number The atomic number §  is specific for each element. §  is the same for all atoms of an element. §  is equal to the number of protons in an atom. §  appears above the symbol of an element in the periodic table. periodic  table  entry  for  sodium:   Atomic Number Symbol

11 Na 22.99

Atomic Mass (we’ll learn this later)

Atomic Number and Protons Each element has a unique atomic number equal to the number of protons: §  Sodium has atomic number 11 §  every Na atom has 11 protons. §  If we give an atom of Na 1 more proton it is no longer Na!

+

= 1 proton

11 protons 12 neutrons

12 protons 12 neutrons

Na atom

Mg atom

•  The  atomic  number   defines  the  idenUty  of   the  element  

Number of Electrons in an Atom All atoms in a pure sample of an element are electrically neutral; they have §  a net charge of zero. §  an equal number of protons and electrons. Number of protons = Number of electrons Example: Aluminum atoms have 13 protons and 13 electrons; the net charge is zero.

So,  for  neutral  atoms:   Atomic  number  =  number  of  protons  =  number  of  electrons  

Mass Number •  The mass number represents the number of subatomic particles in the nucleus, which is equal to the sum of the number of protons + number of neutrons. •  Since protons and neutrons have most of the mass in an atom, we call this the mass number. •  The mass number can be used to find the # of neutrons in an atom

Goal:  find  the  #  of  neutrons  in  a  Na  atom   •  mass  number  =  (#  protons  +  #  neutrons)   •  atomic  number  =  #  protons   (#  of  protons  +  #  of  neutrons)   − #  of  protons   #  of  neutrons  

For  a  Sodium  Atom:   Mass  #  =  23   Atomic  #  =  11  

mass  number   − atomic  number   #  of  neutrons  

12  neutrons  in  the  Na  atom!  

23   − 11   12  

Using the Atomic Number and Mass Number atomic number = # protons mass number = # protons + # neutrons for neutral atoms, protons = electrons, so atomic number = # electrons A  neutral  atom  has:   atomic  #  =  6   mass  #  =  12     •  How  many  protons  does  this  atom  have?   •  How  many  neutrons?   •  How  many  electrons?   •  What  element  is  this?  

A  neutral  atom  has:   atomic  #  =  6   mass  #  =  14     •  How  many  protons?   •  How  many  neutrons?   •  How  many  electrons?   •  What  element  is  this?  

Using the Atomic Number and Mass Number atomic number = # protons mass number = # protons + # neutrons for neutral atoms, protons = electrons, so atomic number = # electrons A  neutral  atom  has:   atomic  #  =  6   mass  #  =  12   12  C   6     •  6  protons   •  6  neutrons   •  6  electrons   •  Carbon  

A  neutral  atom  has:   atomic  #  =  6   14   mass  #  =  14   6  C     •  6  protons   •  8  neutrons   •  6  electrons   •  Carbon  (14C,  used  in  radiocarbon  daUng)  

Isotopes Isotopes §  are atoms of the same element that have different mass numbers. §  have the same number of protons but different numbers of neutrons. §  can be distinguished by atomic symbols.

Isotopes and Mass and Atomic Symbols Since each isotope of an element has a different number of neutrons, each isotope’s mass number will be different. We write these as atomic symbols: §  Mass numbers are in the upper left corner. §  Atomic numbers are in the lower left corner.

Isotopes  of  Carbon:   mass  number   atomic  number   •  These  might   also  be  wriaen   simply  as  12C,   13C,  and  14C  

12   6  

C  

12  nuclear   parUcles   −   6  protons   6  neutrons  

13   6  

C  

13  nuclear   parUcles   −   6  protons   7  neutrons  

14   6  

C  

14  nuclear   parUcles   −   6  protons   8  neutrons  

Atomic Symbols For an atom, the atomic symbol gives the number of §  protons (p+), §  neutrons (n), and §  electrons (e–) 16 8

O

8  p+   16  n   8  e−  

31 15

P

15  p+   16  n   15  e−  

65 30

Zn

30  p+   35  n   30  e−  

Atomic Symbols For an atom, the atomic symbol gives the number of §  protons (p+), §  neutrons (n), and §  electrons (e–) 16 8

O

8  p+   8  n   8  e−  

31 15

P

15  p+   16  n   15  e−  

65 30

Zn

30  p+   35  n   30  e−  

Two Kinds of Atomic Symbols Isotope  symbol   describes  a  single  atom  

mass   number   •  idenUfies  the   isotope   •  gives  the   mass  of  the   atom  

12 6

symbols  found  in  periodic  table   describes  a  typical  sample   made  of  many  atoms  

6

C atomic  number  

C 12.01

atomic   mass  

Atomic Mass The atomic mass of an element §  is listed below the symbol of each element on the periodic table. §  gives the mass of an “average” atom of each element compared to C-12. §  is not the same as the mass number. §  is calculated using a weighted average.

6

C 12.01

Calculating Atomic Mass for C •  To calculate the atomic mass, we use a weighted average •  The contribution to atomic mass is based on the abundance and mass of each isotope isotope  

mass  (amu)  

abundance  

contribu>on  to  atomic  mass  

12C  

12  (exact)  

×  

0.989  

= 11.868  amu  

13C  

13.003  

×  

0.011  

= 0.1430  amu  

14C  

14.003  

×  

~0  

= 0  amu   12.0110  amu   12.0  amu  

•  The  abundance  values  add  up  to  1   •  To  convert  the  abundance  to  a  %  value,  mulUply  by  100  

Summary of Terms Mass  Number   •  Used  to   describe   single   atoms  

Used  to   describe   samples  made   of  many  atoms  

The  number  of  (protons  +  neutrons)   per  atom  

Atomic  Number   • 

Atomic  Mass   • 

The  number  of  protons  per  atom  

The  average  mass  of  an  atom  in  a   sample    made  of  many  atoms  

Don’t  get  the  mass  number  mixed  up  with  the  atomic  mass!  

Electrons and Reactivity Atoms contain §  a very small nucleus packed with neutrons and positively charged protons. §  a large volume of space around the nucleus that contains the negatively charged electrons. It is the electrons that determine the physical and chemical properties of atoms.

Electron Energy Levels §  Electrons surround the nucleus in specific energy levels. §  Each energy level has a principal quantum number (n). §  The lowest energy level, which is closest to the nucleus, is labeled n = 1. §  The second-lowest energy level is labeled n = 2, the third n = 3, and so on.

Electron Energy Levels Electron energy levels increase in energy and number as electrons get farther away from the nucleus. The higher the electron energy levels, §  the more electrons they hold. §  the more energy the electrons have.

Sublevels Within each energy level, we have sublevels that §  contain electrons with identical energy. §  are identified by the letters s, p, d, and f. The number of sublevels within a given energy level is equal to the value of the principal quantum number, n.

Energy of Sublevels Within any energy level, §  the s sublevel has the lowest energy. §  the p sublevel follows and is slightly higher in energy. §  the d sublevel follows the p and is slightly higher in energy than the p. §  the f sublevel follows the d and is slightly higher in energy than the d.

Energy Levels and Sublevels n  =  7  

n  =  6   n  =  5  

n  =  4  

n  =  3  

n  =  2   n  =  1  

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