ATOMIC STRUCTURE
DRAW AN ATOM!
THE ATOM
Atomic Number (Z):
The number of protons (or electrons) in one atom
of a specific element
Mass Number (A):
The sum of all the protons and neutrons in the
nucleus of one atom of a specific element
How many protons, neutrons, and electrons does radium have?
BOHR-RUTHERFORD DIAGRAMS
• Which element is this?
• How do you know?
• How many protons should
be in the nucleus?
• How many neutrons?
1st shell: 2
2nd shell: 8
3rd shell: 8
4th shell: 18
5th shell: 18
SHORTCUT – LEWIS DOT DIAGRAMS
• Draw only the valence electrons
• Electrons in the outermost shell
• These are the electrons involved
in chemical bonding/reactions
• More loosely held by the nucleus
SIZE OF AN ATOM
• Atoms are extremely small – so tiny that scientists had to develop their own
unit of measurement to describe their size!
• https://www.youtube.com/watch?v=yQP4UJhNn0I
• Atomic Mass Unit (amu): 1/12 of the mass of a carbon-12 atom
• 1 amu = ~1.66 x 10-27 kg
• If the nucleus determines atomic mass, then why are there decimals in the mass
numbers on the Periodic Table?
ISOTOPES
• Isotopes: atoms of the same element that have different numbers of neutrons
https://www.youtube.com
/watch?v=sQNKKIVXRuE
ISOTOPIC ABUNDANCE
• Most elements have one very common isotope
• Scientists use mass spectrometers to identify
isotopes and their respective abundances
CALCULATING ATOMIC MASS
• Calculate the atomic mass
of copper.
Atomic mass =
[% abundance of isotope 1 x mass of isotope 1] + [%
abundance of isotope 2 x mass of isotope 2] + …
PRACTICE
Calculate the atomic mass of magnesium. Magnesium-24, magnesium-25, and
magnesium-26 have isotopic abundances of 78.7%, 10.1%, and 11.2%
respectively.
RADIOISOTOPES
• Sometimes the amount of neutrons in the
nucleus is too high or too low and the element
is unstable.
• The nucleus then splits to become more stable.
• This is radioactive decay, and unstable
isotopes are known as radioisotopes.