1. Believing What You Cannot See
Atomic Structure
Believing What You Cannot See

2. Early Models of the Atom
Democritus (4th Century BC)
Was the first to propose the idea of atoms.
Not an experimental model.
Dalton’s Atomic Theory
All matter is composed of atoms
Atoms of different elements differ
Chemical change involves a rearrangement of atoms
Atoms are indivisible

3. What is an Atom?
Atom= the smallest particle of an element that retains the properties of that element. The atom is the representative particle of any element.

4. Just How Small is an Atom?
If an apple was magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple.
In 27 grams of aluminum there are 6.02 X 1023 atoms of aluminum (602,000,000,000,000,000,000,000 atoms)
*This is equal to one mole of aluminum (avogodro’s number)!!

5. What are the component particles that scientists have discovered within atoms?

6. Electrons
Electrons = negatively charged subatomic particles with nearly zero mass.
Located in orbitals within the electron cloud around the nucleus
J.J. Thomson experiment
-cathode ray tube

7. Cathode ray tube (CRT)
Electrons were introduced to one end (cathode) and were flung to the opposite end (anode) producing the cathode ray (a beam of electons).
At first, many thought it was light, but the ray in the tube could be deflected by magnets disproving it was light.
Different metals and gases were used in the CRT, but the result was always the same; a cathode ray was produced.
Scientists inferred that the particles making up the cathode ray were a part of all matter. This is how electrons were discovered.

8. Cathode Ray tube

9. Protons
Protons= positively charged subatomic particles (1,840 X the mass of an electron)
Located in the nucleus
With the neutron, accounts for the mass of the atom
Has a mass of nearly 1 a.m.u.

10. Neutrons
Neutrons = subatomic particles with no charge (mass nearly equal to a proton which is nearly 1 a.mu.)
Located in the nucleus
With the proton, accounts for the mass of the atom
Chadwick confirmed the existence of the neutron (1932)

11. The Atomic Nucleus
Nucleus= the central core of the atom, is composed of protons and neutrons.
Ernest Rutherford discovered the nucleus in his famous gold foil experiment. (video link)

12. Rutherford’s Gold Foil Experiment
Positively charged radioactive particles (alpha radiation) were shot at a thin piece of gold foil.
Why did some of the alpha particles bounce back?
Why did most of the alpha particles go through?
What did these findings prove about the atom?

13. The size of a nucleus to the electron cloud of an atom is similar to the size of one blade of grass in reliant stadium to the size of the stadium

14. Distinguishing Between Atoms
All atoms are composed of electrons, protons, and neutrons. Therefore, how do you differentiate between atoms?...

15. Atomic Number
Each element is different due to varying numbers of protons
Atomic Number = the number of protons in the nucleus of an atom.
Example – because Oxygen has eight protons, the atomic number is eight
(If it has eight protons, how many electrons are there? Why?)

16. Atoms (with the exception of ions) do not have a charge, thus if you have 8 positively charged protons then you must have 8 negatively charged electrons
8+ protons
8- electrons
0 charge

17. Ion
Charged particle = atom that has lost or gained electrons * When an atom loses an electron a positive ion (cation) is formed * When an atom gains an electron a negative ion (anion) is formed

18. Practice
Boron has an atomic number of 5, how many protons does it have? Electrons?
Nitrogen has 7 electrons, how many protons does it have? What about neutrons? What is the net charge (total charge)?
What is the net charge of an ion with 15 protons and 18 electrons?
What is the charge of an ion with that had a -3 charge and then lost 2 electrons?

19. Mass Number
Most of the mass of an atom is in the nucleus where the protons and neutrons are located.
Mass Number = total number of protons + total number of neutrons
If you are given the number of protons and the mass number you can determine the number of neutrons
- Ex. Determine the number of protons and neutrons in an atom of C-13

20. Not all atoms of the same element have the same number of neutrons!
Isotopes
Not all atoms of the same element have the same number of neutrons!
(These are called isotopes)

21. Isotopes
Isotopes = atoms that have the same number of protons but different numbers of neutrons (thus they have different mass numbers)
Note: sometimes they refer to an isotope by it’s name and mass number
-Example: Carbon-14 (how many neutrons does this particular isotope have?)

22. Atomic Mass
Atomic Mass = average mass of the atoms in a naturally occurring sample of the element
(The average mass includes the mass and the natural percent abundance of the different isotopes).

23. Calculating Average atomic mass
Knowing that the atomic mass is an average of the masses of it’s isotopes, you can calculate the atomic mass based on its abundance.
Nitrogen-14 has an exact atomic mass of and has an abundance of 99.63%. Nitrogen-15 has an exact atomic mass of and has an abundance of 0.37%. Find its average atomic mass.

24. Answer
( x ) + ( x ) =
On the periodic table, what is the average atomic mass of Nitrogen?