Elements make up the periodic table. Section 1.2D

Elements can be organized by similarities. Elements can be organized by the masses of their atoms. Not all the atoms of an element have the same atomic mass – elements have isotopes Atomic mass of an element is the average mass of all the element’s isotopes

Mendeleev’s Periodic Table 1800’s several scientists tried to organize the elements based on their properties 1860’s Russian chemist Dmitri Mendeleev – thought of how he could organize elements based on their physical and chemical properties Arranged cards with the element, its atomic mass, and properties

Mendeleev’s Periodic Table Chart – rows and columns Rows = elements showing similar chemical properties; arranged rows so the atomic masses increased as you move down Periodic Table – shows a periodic, or repeating, pattern of properties of the elements Mendeleev’s Notebook

Predicting New Elements Mendeleev left some empty spaces where no known elements fit the pattern. He predicted that new elements that would complete the chart would eventually be discovered. Many chemists found it hard to accept this, but six years later gallium was discovered and fit in the blank after aluminum In the next 20 years, 2 other elements Mendeleev predicted were discovered.

The periodic table organizes the atoms of the elements by properties and atomic number. Modern periodic table differs from Mendeleev’s Elements with similar properties are found in columns, not rows. The elements are not arranged by atomic mass but by atomic number.

Reading the Periodic Table Each square of the periodic table gives particular information of the atoms of an element. The number at the top of the square - atomic number= the number of protons in the nucleus of an atom of that element Chemical symbol – an abbreviation for the element’s name, which is written below the symbol The number below the name – atomic mass = avg. atomic mass of all the isotopes of the element

Reading the Periodic Table Color of the symbol = the physical state of the element at room temp. (not always these colors) White = gas Blue = liquid Black = solid The background colors of the squares indicate whether the element is a metal, nonmetal, or metalloid

Groups and Periods Group = elements in a vertical column of the periodic table that show similarities in their chemical and physical properties Sometimes a group is called a family of elements, because these elements seem to be related.

Groups and Periods Group 17 – the halogen group Tend to combine easily with many other elements and compounds, especially with the elements in Groups 1 and 2 Have some similarities, but their physical properties are not the same (ex. Flourine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids at room temp.) = related but not identical

Groups and Periods Period – each horizontal row on the periodic table Properties of elements change in a predictable way from one end of a period to the other Ex. Period 3 – goes from metals to metalloids to nonmetals

Trends in the Periodic Table An element’s position in the table can give information about the element. Atoms form ions by gaining or losing electrons Atoms of elements on the left side of the table: form positive ions easily (lose electrons) Group 1 atoms lose and electron to form ions with one positive charge (1+) and atoms in Group 2 can lose two electrons to form ions with a charge of 2+ Atoms of elements in Group 18 normally do not form ions Atoms of elements in Group 17 often gain one electron to form a negative ion (1-) and atoms in Group 16 can gain two electrons to form a 2_ ion Atoms of Groups 3 to 12 form positive ions, but the charge can vary

Trends of the Periodic Table Atomic size increases as you move down a group (column) and decreases as you move left to right across a period

Trends in the Periodic Table Density generally increases from the top of a group to the bottom, but within a period, the elements at the far left and far right sides of the table are the least dense, and the elements in the middle are the most dense (osmium (Os) has the highest known density and is located at the center of the table)