1. The Atom & Subatomic Particles

2. What was going on in the world of chemistry in the 1800’s?
Lots of new elements were being discovered.
Atomic weights were getting figured out. Atoms differ in their mass.
NOT AS EASY AS IT SOUNDS!

3. Timeline for Discovery of Elements

4. Meanwhile …
Right at the end of the 1800’s …

5. Roentgen discovered X-rays: 1895
Roentgen was working with cathode ray tubes when he discovered that they emitted x-rays.
Called them x-rays because he didn’t know what they were.

6. Natural Radioactivity - 1896
1896 Becquerel discovered that uranium ores emit radiation.
1898 Marie Curie discovered that thorium also gave off radiation.

7. Natural Radioactivity
Both Becquerel and Curie believed that more than 1 kind of radiation was emitted by these substances.
Curie called them “radioactive.”
1899 Rutherford discovered alpha rays and beta rays.
1900 Villard discovered gamma rays.

8. Radioactivity – Rutherford again!

9. Radioactivity
The 3 kinds of radiation behave differently in magnetic & electric fields.
They have different penetration power.
 or alpha rays stopped by paper.
 or beta rays stopped by thin metal foil.
 or gamma rays only partially blocked by lead and concrete.

10. See Table O in the Chemistry Reference Tables
Type of Radiation
Symbol
Mass
Charge
Alpha
4He or 4 4 +2
Beta 0
1/1836
Or .0005 -1
Gamma 0 2 2 -1

11. Henry Mosley
Roentgen’s x-rays were produced when cathode rays hit the glass of the tube. (The K.E. of the electron is converted into electromagnetic radiation.)
Mosley: Stop the cathode rays with something denser than glass and get even higher energy X-rays.

12. He tried 30 different metal targets!

13. Atomic Number - 1913 Mosley tried 30 different elements as “STOPPERS.”
Found that the greater the atomic weight of the “stopper,” the shorter the wavelength of the x-rays.

14. What does any good scientist do?
PLOTS THE DATA!
Tried plotting the wavelength of the x-ray vs. the atomic mass of the stopper. Not so nice.
Then tried correlating the wavelenth of the x-ray with an integer, n. Got a very pretty graph.

15. Mosley’s integer was the atomic number!

16. Atomic Number Atomic number = size of nuclear charge.
Atomic number = number of protons in nucleus.
Atomic number determines the identity of the element.
Use atomic number to order the periodic table.
For neutral atoms, atomic number also = number of electrons.

17. Henry Mosley
Killed in WWI at the age of 28.

18. What we knew by 1932: Relative Charge Relative Mass Electron, 0e -1
amu
Or 0 amu
Proton, 1p or 1H +1
amu
Or 1 amu
Neutron, 1n
amu -1 1 1

19. Location of Subatomic Particles
Electrons located outside nucleus.
Protons & neutrons located inside nucleus.
Protons & neutrons = nucleons.

20. Isotopes
Atoms of the same element with a different number of neutrons. The protons are the same!
So Dalton’s theory has been modified in a number of important ways.
The atom is divisible.
Atoms of the same element can be different.

21. Mass Number
Mass number = # protons + # neutrons.

22. Notation 12C 6 The left superscript is the mass number.
The left subscript is the atomic number.
Sometimes both are given; sometimes only the mass number is given. (You can figure out the atomic number from the P.T.)

23. Notation 12C 6 How many neutrons in this atom?
12 = no. of protons + neutrons.
6 = no. of protons.
12C 6
How many neutrons in this atom?
12 – 6 = 6 neutrons

24. 17O
Mass number is ?
Atomic number is ?
# of neutrons is ?
# of electrons in atom is? 17 8 8
17 – 8 = 9 8

25. 15N
Mass number is ?
Atomic number is ?
# of neutrons is ?
# of electrons in atom is? 15 7 7
15 – 7 = 8 7

26. 19F
Mass number is ?
Atomic number is ?
# of neutrons is ?
# of electrons in atom is? 19 9
19 – 9 = 10 9

27. Notation
Might also see C-12. The number following the symbol is the mass number!
You have to look up the atomic number.

28. What’s the difference between H-1, H-2, and H-3?
All H’s have 1 proton.
H-1: mass # = 1. Has 1 proton, 0 neutrons.
H-2: mass # = 2. Has 1 proton, so it must have 1 neutron.
H-3: mass # = 3. Has 1 proton, so it must have 2 neutrons.

29. Consider U-234, U-235, & U-238 What’s the atomic number of U?
How many protons in U?
How many neutrons in U-234?
How many neutrons in U-235?
How many neutrons in U-238?
How many electrons in U? 92 92
234 – 92 = 142
235 – 92 = 143
238 – 92 = 146 92

30. How many neutrons in Po-217?
What’s the atomic number?
How many protons?
So the neutrons are … 84 84
217 – 84 = 133 neutrons!

31. IONS Atoms: electrically neutral. # of electrons = # of protons.
IONS are atoms that have gained or lost electrons.
The protons and electrons don’t balance out anymore, so ions carry a charge.

32. IONS Charge of ion = # protons - # electrons.
(We subtract the electrons since they’re negative.)

33. IONS
If the atom loses electrons, it’s going to have more positive charge than negative. What kind of ion is it?
If the atom gains electrons, it’s going to have more negative charge than positive. What kind of ion is it?
positive
negative

34. Charge of Ions
If we want to specify an ion, we write the charge as a right superscript.
Cl-1 a chloride ion with a charge of -1.
Na+1 a sodium ion with a charge of +1.
O-2 an oxide ion with a charge of -2.
If there’s no right superscript, it’s understood to be zero and a neutral atom.

35. Putting it all together
How many protons, neutrons, & electrons in each of the following:
23Na+1
Got 1
Got 2
Lost 1
34S-2
19F-1 16 9 11
16 p, 18 n, 18 e
9 p, 10 n, 10 e
11 p, 12 n, 10 e
Got 3
Lost 2
Lost 2
64Zn+2
13N-3
25Mg+2 30 7 12
7 p, 6 n, 10 e
30 p, 34 n, 28 e
12 p, 13 n, 10 e

36. What is an a.m.u.? atomic mass unit
amu  1/12 the mass of the C-12 atom.
C-12 is used as the reference for atomic masses.

37. Atomic Weight or Mass Atomic mass is relative.
1st H-1 was the standard. It’s the lowest. 1 atom of H was 1 amu.
Then O-16 was the standard. O combines with lots of elements. 1 atom of O was 16 atomic mass units.
Now C-12 is the standard. 1 atom of C-12 has a mass of exactly 12 atomic mass units.