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 More like a Ulted bookshelf…