a - the charge of the substance b - the number of atoms in a naturally occurring
sample of the element
SYMBOL - the chemical symbol for the element
e.g. an element helium has an atomic # of 2 and a mass # of 4, with a charge of 0 and only contains 1 atom in a naturally occurring sample of the element, therefore to write all that information in chemical notation is as follows:
24 He 01
Isotopes - atoms of the same element with different mass numbers i.e. different numbers of neutrons
e.g. hydrogen has 3 isotopes, one with mass # 1, another with mass # 2 and the last with mass # 3
11 H - protium 12 H - deuterium 13 H - tritium (all isotopes of
hydrogen)
e.g. carbon has several isotopes, one with mass #12, another with mass #14
612 C 614 C (isotopes of carbon)
The arrangement of electrons in atoms
Electrons exist in shells which orbit the nucleus (which contains the protons and neutrons) of an atom as concentric circles. Each shell can hold a maximum number of electrons. The arrangement of electrons in an atom is called the electronic configuration.
The 1st shell fills up before any electrons enter the 2nd shell, the 2nd shell fills up any electrons enter the 3rd shell. However when 8 electrons enter the 3rd shell, it becomes temporarily “full” and then up to 2 electrons enter the 4th shell. If there are any more electrons, they will reenter the 3rd shell until it has been filled with 18 electrons. The last shell is called the valence shell and the electrons within that shell are called the valence electrons.
The 1st shell can hold a maximum of 2 electrons, the 2nd shell can hold a maximum of 8 electrons, the 3rd shell can hold a maximum of 18 electrons. *The 4th shell will not be discussed at this level*
The 1st 20 elements with their respective sub-atomic particles
Key
A - mass # # of e-s - number of electrons
Z - atomic # # of ns - number of neutrons
E.C. - electronic configuration
A general diagram of an atom
Points to consider
- always show the number of protons and neutrons (once you are given the mass number)
- draw the shells as either dotted circles around the nucleus or as solid lines
- electrons can be drawn as circles or as crosses.
e.g. a helium atom
Relative atomic mass - the average mass of all of the isotopes in a naturally occurring sample of the element
Calculation
A natural sample of chlorine contains 2 isotopes in the following proportions:
75% 3517 Cl and 25% 3717 Cl
R.A.M = (75/100 x 35) + (25/100 x 37) = 26.25 + 9.25 = 35.5
In other words, you find the sum of the percentages of the mass numbers of all the isotopes involved.
35.5 is the R.A.M of chlorine as shown in the Periodic Table
The Periodic Table
The elements in the table are arranged in increasing atomic number.
I II III IV V VI VII VIII
All the elements to the left of the staircase are metals, all the elements to the right of the staircase are non-metals and the shaded elements are called semi-metals or metalloids.
The elements enclosed by the heavy border are called the Transition Metals.
The Roman Numerals above each column represent the group number. For example the elements Li, Na, K etc are in Group I. The group number tells how many electrons are in the outermost shell. For e.g. Mg is in Group II there it has 2 outermost electrons. Each row is called a period. The period number tells how many shells an atom of the element has. For example the element Si is in row 3, therefore it has 3 shells. The elements in group VIII are called the inert or noble gases.
The element H (hydrogen) can be considered to be in either Group I or Group VII. This is why it is located in such a peculiar position.
There are 2 types of isotopes
Unstable isotopes are radioactive, meaning they emit energy and particles from their nuclei until they become stable. This emission of energy and particles have several uses, some such uses are:
- radiocarbon dating
- agriculture
- research and industry
- providing energy
- diagnosing and treating diseases
Summary
- an atom is the smallest part of an element that still possess the properties of that element
- an atom contains protons, neutrons and electrons
- an isotope is elements with different mass numbers
-
each shell can hold only a certain number of electrons: 1st - 2, 2nd - 8, 3rd - 18
- there are stable and unstable isotopes
- unstable isotopes are very useful to Man
