Year 9 Chemistry, Mr. Blaurock – Worksheet 6
2. Atomic structure and the periodic table
The experiments of Ernest Rutherford showed that the mass of an atom is concentrated in a small, positively charged atomic nucleus. This nucleus is very small compared to the overall size of an atom though, only about a 1
100 000 th of the entire size. The bulk of an atom’s volume is made up by the negatively charged electron shell.
This directly contradicts Dalton’s theory of solid, spherical atoms that are the smallest possible building blocks of matter. It rather proves the existence and location of subatomic particles: the protons and neutrons in the nucleus and the electrons in the surrounding electron shell.
Characteristics of subatomic particles
Name Symbol Mass [u]
Relative charge [in e]
(e is the elementary charge, defined as the
charge of a proton) proton
neutron electron
Number of subatomic particles in an atom
We are familiar with the notation of elements in the periodic table:
ZX
A A = mass number
X = element symbol Z = atomic number
The mass and atomic numbers are directly linked to the number of protons, electrons as well as neutrons.
Task:
a) Show how the number of protons, electrons and neutrons can be calculated (for a uncharged atom) using A and Z.
number of protons = number of electrons = number of neutrons =
b) Calculate these numbers for the following elements
Element Carbon (C) Oxygen (O) Helium (He) Mercury (Hg) Plutonium (Pu) number of
protons number of electrons number of neutrons
Year 9 Chemistry, Mr. Blaurock – Worksheet 6
Isotopes and atomic masses in the periodic table
Isotopes are atoms that contain the same number of protons, but different numbers of neutrons. It is important to note, that all elements are defined by the number of protons in the nucleus. Any atom with 6 protons in the nucleus belongs to the element carbon, no matter how many electrons or protons there are.
BUT most of these isotopes are not stable. Each element typically has only one or two combinations of protons and neutrons that will form a stable nucleus. Other combinations will undergo radioactive decay and thus disintegrate (but that is the concern of physics, not chemistry!).
When differentiating between different isotopes of the same element the mass number is given.
Example: ❑U
238 and ❑U
235 are two isotopes of the element uranium.
Task: How many protons and neutrons does each isotope’s nucleus contain?
The atomic mass of many elements in the periodic table is given in a fraction (Example: 35,517Cl ) This seems strange, as the atomic masses should be integers following the assumption that each proton and neutron has a mass of approximately 1 u (and we can’t have halves of protons and neutrons)!
The solution to this is the isotopes! The table below explains this phenomenon for chlorine.
naturally
occuring chlorine Cl-isotope
35Cl Cl-isotope
36
Cl
Cl-isotope
37
Cl Atomic
mass [u]: 35,48 35 36 37
Percentage: 100 % 75,76 % Traces 24,24 %
Task:
a) Explain why chlorine has a fraction for its atomic mass in your own words.
b) Explain how to calculate the atomic mass of the naturally occurring chlorine using the atomic masses and percentages of the isotopes.
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
