Chapter 4 Notes

Description of an atom

Inside the nucleus

The nucleus contains ______and ______

protons

◦

neutron

◦

Outside the nucleus

The electron cloud contains the ______

◦

◦

Particle / Charge / Mass # / Location
Electron
Proton
Neutron

Then number of protons in an atom of a given element is the same as the atomic number (Z).

◦found on the Periodic Table, whole # for each element

carbon

phosphorus

gold

Mass number = protons + neutrons; always a whole number.

# of Neutrons = mass number - # of protons

# electrons in a neutral atom = # protons

Atomic mass – larger number in each element’s box on the periodic table. If you round the atomic mass of an element to the closest whole number you generally get the mass # for that element.

Example

Atomic Number / Mass Number / # protons / # electrons / # neutrons / Symbol
Mercury
Neon

Isotopes: atoms with the same number of protons but different number of neutrons

Examples:

Iron

Iron – 54

Iron – 56

Uranium

Uranium – 236

Uranium – 238

Average Atomic Mass for an Element

  1. Multiple the percentage (percent abundance) of each isotope of the element by its mass number.
  2. Add the products of the multiplications together.
  3. Divide by 100.
  4. Your answer should be very close to the atomic mass of the element for that element

Determine the average atomic mass of each element using the mass number of each isotope and its percent abundance in nature.

  • Lithium: 7.5% Li – 6 and 92.5% Li – 7
  • Chromium: 83.79% 52Cr, 9.50% 53Cr, 2.37% 54Cr and 4.34% 55Cr

Nuclear Chemistry

  1. Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a change in the atom itself.
  2. All elements heavier than bismuth (Bi) (and some lighter) exhibit natural radioactivity and thus can "decay" into lighter elements.
  3. Unlike normal chemical reactions that form molecules, nuclear reactions result in the transmutation of one element into a different isotope or a different element altogether (remember that the number of protons in an atom defines the element, so a change in protons results in a change in the atom).

Parts of a Reaction

Reactants  Products

Emission =

Capture =

Nuclear Particles

Alpha= 42HeBeta or electron = 0-1e

Positron = 0-1eneutron = 10n

Balancing Nuclear Reactions

Mass # and the atomic # totals must be the same for reactants and the products.

  1. 3919K 3517Cl + ___
  1. 20682Pb 0-1e + ___
  1. 23894Pu + ___42He + 23592U
  1. 23592U + 10n ___ + 14156Ba + 310n
  1. ___ +10n 14458Ce + 9436Kr + 210n

Types of nuclear reactions

  1. Alpha emission
  • 23892U 
  1. Beta emission
  • 23490Th 
  1. Positron emission
  • 2211Na 
  1. Electron capture –
  • 20180Hg 
  1. Neutron emission
  • 20984Po 

Writing Balanced Nuclear Reactions

  1. Alpha decay of Cu-68
  1. Positron emission of P-18
  1. Astatine-210 releasing 3 neutrons
  1. Electron capture of Ti-45

Half Life

  1. Radioactive isotopes or nuclides all decay because they are unstable, some just breakdown much faster than others
  2. Half-life – amount of time for half of the original sample to decay
  3. For two samples of the same isotope, regardless of the sample size, after one half-life, only half of the original amount of sample remains.

IsotopesHalf-Live

Carbon – 145730 years

Sodium – 2415 hours

Bismuth – 21260.5 seconds

Polonium – 215 0.0018 seconds

Thorium – 23075400 years

Thorium – 23424.1 days

Uranium – 2357.0 x 108 years

Uranium – 2384.46 x 109 years

Example Problems

  1. Barium – 139 has a half-life of 86 minutes. If you originally have a 10 gram sample of Barium-139, how much will be left after 258 minutes?
  1. How many days will it take 50 grams of Radon – 222 (half-life of 3.82 days) to decay to 3.125 grams?
  1. If a sample of Cesium-135 decays from 10 grams to 2.5 grams over a period of 84 days, what is the half-life of Cesium-135?

Fusion – combining two smaller nuclei into one heavier, more stable nucleus.

32He + 11H 42He + 01e

Fission – splitting a large unstable nucleus into two nuclei with smaller mass numbers.

20984Po  12552Te + 8432Ge

  1. Predict the number of valence electrons for each element based on its location in the Periodic Table of Elements.

Magnesium =

Carbon =

Phosphorus =

Lithium =

  1. At room temperature, what state of matter would the following elements be in?

Mercury-

Fluorine –

oxygen –

Sodium –

  1. Classify each as a metal, non-metal, or metalloid.

iron-

antimony –

nitrogen –

Sodium –

  1. In what family would each of these elements be classified?

Strontium –

Fluorine –

lead –

Sodium –

How many valence electrons must atoms have to become stable?

Most atoms do not have 8 valence electrons. How do they get 8 valence electrons?

Which family on the periodic table already has 8 valence electrons?

What is the exception to the 8 valence electron rule?

Sodium (Na)

# electrons

# valence electrons

What does Na do to get 8 valence electrons? Draw a picture.

Reaction

Why would an atom become an ion?

•Because it is more stable as an ion

•As an ion, it follows the “______”

•Atoms will ______to follow the octet rule

•Atoms want to have outer electron arrangements like that of noble gases

•Becoming an ion accomplishes this.

Lithium (Li)

How many electrons does Li have?

How many valence electrons does Li have?

What does Li do to become stable? Draw a picture

Reaction:

•Remember: If there is less than 6 total electrons, it is stable with just 2 valence electrons

Nitrogen

How many electrons does Nitrogen have?

How many valence electrons does nitrogen have?

Nitrogen can either gain three electrons or lose 5. Draw a picture

Reaction:

Think about it… Becoming ions

What will alkali metals do to become ions?

What will alkaline earth metals do to become ions?

What will boron family do to become ions?

What will carbon family do to become ions?

What will Nitrogen family do to become ions?

What will Oxygen family do to become ions?

What will halogens do to become ions?

Ions are NOT neutral

If an atom gives away electrons,

If an atoms accepts electrons,

Cations

______form cations

Cations are named the same as the metal

sodiumNa→

calciumCa→

Anions

______form anions

Anions are named by changing the ending of the name to ______

fluorine

phosphorus

Finding # of particles and charge

subtract that charge from the # of protons to get the number of electrons

2713Al+3=

3517Cl-1=

13756Ba+2=

Aluminum and Phosphorus

How to write ionic compounds

Step 1

Use the periodic table to find the charge of the cation and anion

Step 2

Step 3

Remove all (+, -, and 1’s)

Reduce the formula to the lowest whole number ratio

1