Periodic Table Notes
Development of the Periodic Table:
Early civilizations were familiar with a few of the substances we now call elements. They made coins and jewelry from gold and silver. They also made tools and weapons from copper, tin, and iron. In the 19th century, chemists began to search for new elements. By 1830, they had isolated and named 55 different elements. The list of elements continues to grow today.
A Russian chemist, Dmitri Mendeleev, published the first version of a periodic table in 1869. Mendeleev noticed that when he arranged the elements in order of increasing atomic mass, he began to see a pattern. Elements with similar properties fell into groups on the table. At that time, not all of the elements were known. To make his table work, Mendeleev had to leave gaps for three missing elements. Based on the groupings in his table, he predicted the properties for the missing elements. Mendeleev’s predictions spurred other chemists to look for the missing elements. Within 15 years, all three elements- gallium, scandium, and germanium- were found.
Mendeleev’s table correctly organized most of the elements; however, a few elements seemed out of place. In the early 20th century, the English physicist, Henry Moseley improved Mendeleev’s table by arranging the elements in order by their atomic number rather than by increasing atomic mass. When organized by the number of protons in the nucleus (atomic number), it was clear that many elements were still undiscovered.
Today’s Periodic Table:
Today’s periodic table is called the Modern Periodic Table, and it is still organized by increasing atomic number. It is arranged into a system of 18 columns and 7 rows. A period is a row of elements on the periodic table whose properties change gradually and predictably. Each column is called a group or family because the elements in each column have similar physical or chemical properties.
Each element is represented on the periodic table by a box called the element key. An element’s “box” shows you the name of the element, its symbol, its atomic number, and its average atomic mass. These items may be listed in any order depending on who publishes the periodic table you are using.
Another number that is important is the element’s mass number. It is not listed in an element’s “box”, but you can use the average atomic mass to calculate it. All you do is round the average atomic mass to the nearest whole number. I’ll explain later in the notes what this number tells you about the atom.
______atomic number (always the # with the lowest value in the box)
______symbol
______element name
______average atomic mass (always the # with the largest value in box)
______mass number (atomic mass rounded to the nearest whole number)
What can you do with an element’s key?
An element’s key can be used to tell you about the number of protons, electrons, and neutrons in an atom of the element. An element’s atomic number is the number of protons in the nucleus of the atom and it is used to identify each element. Since the number of protons is equal to the number of electrons in an atom, the atomic number can also be used to find the number of electrons. Simple right?
It is a little harder to find the number of neutrons in an atom. The mass number of an atom is the sum of the protons plus neutrons in the atom. So, to find neutrons, use the following formula: mass number minus atomic number.
Example: The mass number of chlorine is 35. The atomic number for chlorine is 17.
So 35 minus 17 is 18. An atom of chlorine has 18 neutrons.
Once you know how to use the periodic table to find the number of protons, electrons, and neutrons, you can make models of atoms and draw Bohr diagrams without me having to give you the number of each particle.
Complete the following items in your notebook BEFORE you leave class today. Finish for homework if you do not complete the assignment in class.
1. Complete the Response side to the Cornell Notes on the Promethean Board.
2. Write definitions for the Key Terms in the vocabulary section of your notebook.
3. Draw and label the “box” (element key) for any element on the periodic table that you choose.
4. Find the number of protons, electrons, and neutrons in an atom of the element you chose and
list them in your notebook beside the “box”.
5. Write these questions in your notebook and answer them:
A. Explain why the elements in a column might be called a “family”.
B. The element with 13 protons is ______.
C. There are ______(how many) neutrons in an atom of sodium.
D. Gold atoms have ______(how many) electrons.
E. The atomic number for potassium is ______.
F. The atomic mass of carbon is ______.
G. The mass number for tungsten is ______.
H. The atomic number is the number of ______in an atom.
I. The ______is the sum of protons plus neutrons in an atom.
J. The # of electrons is equal to the # of ______.