Isotopes

1
IsotopesThe Arrangement of Electrons in an
Atom

• Mr. Perez
• Grade 6 Science

2
Isotopes Different Atomic Masses

• How can you tell one element from another?
• All atoms of the same element have the same
  atomic number
• This means that atoms of the same element have
  the SAME number of protons in the nucleus AND
  electrons in the electron cloud
• Can the same element have a different atomic
  mass? Remember, the atomic mass is the number of
  protons and neutrons in the nucleus.
• Yes. Atoms of the same element can have a
  DIFFERENT number of neutrons in the nucleus (but
  the number of protons will ALWAYS stay the same)
• Why? If the number of neutrons change, the
  atomic mass will change too.

3
Isotopes

• An isotope is an atom of the same element that
  has a different atomic mass. The number of
  neutrons is different. Isotopes can be stable or
  radioactive.
• Each element will always have the same number of
  protons (and electrons), but it will have a
  DIFFERENT number of neutrons
• Some Periodic Tables will give the most common
  isotope of an element. In this case, the atomic
  mass will be written as a whole with no decimal
• Other Periodic Tables will give an average of the
  atomic masses of ALL of the isotopes of an
  element.
• Most elements have 2 or more stable isotopes.
• Radioactive elements decay into nonradioactive
  forms of other elements

4
Isotopes of Common Elements

• An example of an element with three isotopes is
  Hydrogen H-1, H-2, and H-3
• The number 1, 2, and 3 represent the atomic mass
• Protium (H-1) has one proton and no neutrons
• Deuterium (H-2) has one proton and one neutron
• Tritium (H-3) has one proton and two neutrons

5
Isotopes of Common Elements

• There are three natural isotopes of Carbon (
  C-12, C-13 and C-14)
• Plants take in C-12 during photosynthesis, which
  is passed on to animals that eat the plants.
  When animals die, C-14 decays without being
  replaced. The half-life of C-14 is 5,730 years.
  Carbon-14 testing is used to date dead organisms
  as it decays from Carbon-14 into Nitrogen-14.
• Nitrogen-15 (or N-15) is an isotope of nitrogen
  used in agricultural and medical research
• Two important isotopes of uranium are U-235 and
  U-238 because they are used in nuclear reactions
  during fission (decaying) as they decay into
  Barium and Krypton

6
Electron Cloud Model

• For many years, scientists thought that the
  electrons circled the nucleus like planets
  orbiting around the sun. This was Bohrs model.
• Scientists now know that the path of electrons DO
  NOT follow an exact path as they orbit the
  nucleus
• The electrons are found in an electron cloud
• Scientists cannot predict the exact location of
  the electrons in the electron cloud
• An electron cloud is sometimes compared to bees
  buzzing around a bee hive because the bees
  behaviors and locations are not predictable

7
Electrons - Energy Levels

• Scientists know that electrons are arranged in
  energy levels
• An energy level is the place in an electron cloud
  where an electron is most likely to be found
• Each energy is a different distance from the
  nucleus
• The first energy level is closest to the nucleus
  AND has low energy electrons
• As you move farther away from the nucleus, the
  electrons have a higher amount of energy
• Each energy level can hold a different number of
  electrons
• The 1st energy level has 2 electrons, the 2nd
  energy level has 8 electrons, and the 3rd energy
  level and beyond can have 8 (or more) electrons

8
Electrons - Energy Levels

• For the first 20 elements, the electrons in the
  atom of an element fill up the energy levels in
  order, beginning with the 1st energy level and
  then work outwards away from the nucleus
• Two hydrogen atoms are usually found together to
  form H2 because they need 2 electrons to fill the
  first energy level.
• An atom of Helium has 2 electrons, so it will
  only fill up the first energy level
• At atom of Lithium has 3 electrons, so two of
  these electrons fill up the 1st energy level, and
  then the third electron is alone in the 2nd
  energy level

9
Electrons - Energy Levels
Valence

• How can we find out which element is shown in the
  diagram?
• First, find the atomic number by counting the
  number of protons OR electrons
• The atomic number is 18

• Then find the atomic mass. Count the number of
  neutrons in the nucleus, then add that number to
  the atomic number.
• This element is Argon. Look at the Periodic
  Table.
• Argon has an atomic of 18 and an atomic mass of
  40

10
Electrons - Energy Levels

• Electrons have the capability to move from one
  energy level to another
• If an electron gains energy it jumps to a
  higher energy level (away from the nucleus)
• If an electron loses energy it drops back to a
  lower energy level (closer to the nucleus)

11
Positive and Negative Ions

• A valence energy level is the atoms outer energy
  level
• An ion is an atom that gains or loses electrons
  in its valence energy level (or valence shell).
• Look at the elements in the first two groups (or
  columns) on the left side of the Periodic Table.
• These elements have nearly empty valence shells
  (with only 1 or 2 electrons) so they give up
  their electrons to become positively charged.
  Positive ions have protons gt electrons
• These elements are called positively charged
  cations
• Look at the elements in the 16th and 17th groups
  on the right side of the Periodic Table.
• 1) These elements have nearly full valence
  shells, so they add electrons to become
  negatively charged. They have a negative
  valence. Negative ions have electrons gt protons.
• 2) These elements are called negatively
  charged anions

12
Positive and Negative Ions

• Group 1 Alkali Metals Valence 1 cations

• Group 17 Halogens Valence -1 anions

Li
Na
F
Cl

• Group 2Alkaline Earth Metals Valence 2 cations

• Group 16 Non Metals Valence -2 anions

Be
Mg
O
S

• The valence electrons are called the oxidation
  on the P.T.