2. Subatomic Particles
After his work with gold foil, Rutherford and his student, Henry Moseley, began working with other metals.
Moseley was able to show that different elements contained different numbers of positive charges in the nucleus.
These positive charges came to be known as protons.

5. Atomic Number
In a neutral atom, the number of protons in the nucleus determines the number of electrons.
The number of protons is called the atomic number.
Each element has a unique number of protons

6. The atomic number is prominently displayed on all periodic tables

9. The Neutron
In the 1920’s physicists believed that atoms were composed of protons and electrons.
By 1930 physicists realized that there had to be some sort of neutral particle in the nucleus.
In 1932, Rutherford associate James Chadwick isolated the neutron, with a mass nearly identical to the proton.

10. Comparing Subatomic Particle Properties

11. Mass Number
The number of protons in an atom of a particular element is always the same.
The number of neutrons can vary, however:
The number of protons and neutrons in the nucleus is called the mass number.

12. # neutrons = mass number – atomic number
How many neutrons?
The composition of an atom of any element can be determined if you know the atomic number (Z) and the mass number:
# neutrons = mass number – atomic number
Atoms with the same number of protons, but different numbers of neutrons are called isotopes.

13. Representing Atoms
The atoms’ mass number is written in several ways: after the element
carbon-14
using superscripts and the chemical symbol
14C
The atomic number is usually written as a subscript, or left out (because it never varies).

14. Take out your worksheet:

17. Try These Problems
How many neutrons in an atom of potassium-40? _______
How many protons in an atom of 206Pb? ______ How many neutrons? _______
A neutral atom has 14 electrons and 14 neutrons. Write the chemical symbol for this isotope. _____________
An atom of chlorine has 20 neutrons. Write the chemical symbol for this isotope. _________

18. Draw a model of an atom of neutral atom 4He in the box below
Draw a model of an atom of neutral atom 4He in the box below. Include protons, neutrons, and electrons in your model.

19. Fill in the Table! Symbol He U Na Atomic number 42 Mass number 4 40
210 95
214 23
# protons 20 82 84 50
# neutrons
110
143 69
# electrons 74

21. Ions
Atoms do not have to have an equal number of protons and electrons.
Any atom where
number of protons ≠ number of electrons
is called an ion
Atoms that lose electrons become (+) ions
Atoms that gain electrons become (-) ions
Ionic charge is shown at the upper right of the element symbol.

22. Answer the following:
How many electrons does an atom of Ca2+ have? How many protons?
How many electrons does an atom of O2- have? How many protons?
How many electrons does an atom of Pb4+ have? What is its atomic number?

23. Atomic Synonyms Nucleon: a proton or a neutron Nuclide = isotope
AMU: atomic mass unit; approximately 1 for protons and neutrons

24. Homework
symbol 11 32 12 38 28 10 54
name
Zinc-65
Xenon-132

25. Nuclide Table #2 symbol Rn Mass # 198 35 210 222 79 17 38 18 nameRn
Mass #
198 35
210
222 79 17 38 18
name
Polonium-210