1. Atomic Theory Atoms are incredibly small!
What we know about them is based on indirect evidence.

2. History of atomic theory
500 BC Democritus proposed idea of atom-but he had no experimental data to support it.
1800 Dalton proposed an atomic theory supported by experimental data.

3. Dalton’s Atomic theory
Support for his theory came when he began looking at the masses of elements that combined with eachother.
He found that elements combine in simple ratios by mass.

4. Dalton’s atomic theory
all matter is composed of tiny, indivisible particles called atoms.
All atoms of the same element are identical.
Atoms can neither be created nor destroyed.
Atoms of two or more different elements can combine in small whole number ratios to form compounds.

5. Dalton’s atomic theory
2 problems with his theory:
Atoms of the same element have different masses
atoms have subatomic particles (electrons, protons and neutrons)
So Dalton’s theory is not completely correct.

6. The Subatomic particles
MASS
Charge

7. The subatomic particles
Electron discovered ~1900 by JJ Thomson using his cathode ray tube.
Electron charge found by American Robert Millikan ~1910. Oil drop experiment
Proton found using adaptation of cathode ray tube ~1900.
Neutron discovered in 1932 by Chadwick.

8. Rutherford’s Gold Foil Experiment
A consequence of the plum pudding model was that the atom would have a weak positive charge.
Rutherford tested this idea by firing positively charged particles at gold foil.
He expected particles to pass through.
His results surprised him.

9. Results of Gold Foil Experiment
Atom is mostly empty space.
Protons are found in a dense nucleus in the center of the atom.

10. Problems with Rutherford’s model
Could not explain why electrons would not spiral into the nucleus.

11. Atoms to Elements What makes elements different from one another?
The number of protons, electrons and neutrons in them.
C has 6 protons, 6 neutrons and 6 electrons, while H has 1 proton, 0 neutrons and 1electron.

12. Atomic number Atomic number: the number of protons in an atom.
Because atoms are neutral we know that protons must equal electrons and so atomic number is also equal to the number of electrons.
Atomic number also equals the number of electrons in the neutral atom

13. How many protons in these atoms? How many electrons?Li Be B F Ne

14. Mass number
Mass number- the total number of neutrons and protons in an atom.
If we know the mass number and the atomic number we can find the number of neutrons in an atom.
Mass number - atomic number = number of neutrons

15. How many neutrons? How many electrons
Atomic number
Mass number

16. Notation
Mass Number Cu 65 29
Atomic Number

17. Another notation
Cu-65
Element
Mass number

18. Isotopes
Isotopes- elements with same number of protons and electrons, but different numbers of neutrons.
For example: Cu 63 29 Cu 65 29
How many protons, neutrons and electrons does each Isotope have?

19. Cu Cu Isotopes 65 29 63 29 Mass # Atomic # protons = 29 electrons = 29
neutrons = = 36
Atomic #
neutrons = = 34

20. FAQ: Average Atomic Masses
“If protons and neutrons weigh 1amu, then how come the periodic table has atomic masses with decimals? How does this happen?”
for example: the 2 isotopes of copper weigh 65 and 63amu’s. In periodic table it says 63.55amu.

21. Average atomic masses
The average atomic mass is an average of the masses of the isotopes of an element.
The average atomic mass of Cu is an average of the two masses of the isotopes: 65amu and 63amu.
It’s a weighted average that takes into account the relative abundance of each isotope.

22. Atoms and ions
Atoms are electrically neutral because they have equal numbers of protons and electrons.
EXAMPLE: Hydrogen has 1proton and electron. They cancel eachother.
• Atoms can gain or lose electrons when they
form compounds.
• When atoms lose or gain electrons they become
charged.
• Atoms having a positive or negative charge are
called ions.

23. Element Symbol Atomic Mass Protons Neutrons Electrons
Number Number
Sodium
Iron Fe
Sn 28 Pd
Nickel 33

24. Na Atoms and ions Sodium, Na, loses an electron when it forms NaCl. 2311 23
How many electrons in Na?

25. Na Atoms and ions 23 11 Sodium has atomic number 11,
meaning it has 11 protons and if
it is neutral it should have 11
electrons.
11 protons + 11electrons = 0 charge
So how many electrons would
Sodium have if it lost an electron? Na 11

26. Na Atoms and ions 23 11 If Sodium lost one electron it
would still have 11 protons, but
now it would have 10 electrons.
11 protons + 10 electrons = +1 charge Na 11
We write the positively charged ion,
or cation, as Na+

27. Cl Atoms and ions 35 17 Chlorine gains an electron when it
forms compounds.
How many electrons does chlorine
have? 17

28. Cl Atoms and ions 35 17 Chlorine has atomic number 17,
meaning it has 17 protons and if
it is neutral it should have 17
electrons.
17 protons + 17electrons = 0 charge
So how many electrons would
Chlorine have if it gained an electron? 35 Cl 17

29. Cl Atoms and ions 35 17 If chlorine gained one electron it
would still have 17 protons, but
now it would have 18 electrons.
17 protons + 18 electrons = -1charge Cl 17
We write the negatively charged ion,
or anion, as Cl-

30. Write the symbol and name of the ion formed:
A. Strontium atom loses two electrons
B. Iodine atom gains one electron
C. Magnesium atom loses two electrons
D. Oxygen atom gains two electrons

31. Complete the table