Dmitri Menveleev In the last century, a scientist named Dmitri Mendeleev looked for ways to organize the current knowledge about newly discovered elements and their atoms. He invented a periodic table – an organized arrangement of elements that explained and predicted physical and chemical properties. Mendeleev’s table was key to understanding elements and discovering new elements and compounds.
Initially, Mendeleev organized elements based on their atomic mass (average weight of the atoms in the element). He found that this did not always work. So he decided to group elements based on their atomic number (the number of protons in the nucleus) instead. Using the periodic table, Mendeleev was able to actually predict the existance of certain elements even before they were discovered! Ex. Gallium was predicted in 1871 and not actually discovered until 1875.
Mendeleev’s decision to use the atomic number rather than atomic mass proved to be a good one. When subatomic particles (protons, neutrons, and electrons) were later discovered, it became clear that the atomic number (number of protons in the nucleus) held the key to the identity of an element.
Groups of Elements The periodic table groups elements with similar properties. You can quickly identify an element as a metal if it is on the left or in the centre of the table. Non-metals are generally found on the right side.
Chemical Groups The set of elements in the same column in the table is called a chemical group (having similar chemical and physical properties). These properties, however, vary from element to element in a column.
Periods The horizontal rows of elements are called periods. The first period contains two elements: hydrogen and helium. As you go from left to right within a row, the atomic number increases and the elements gradually change from metallic (lithium) to non-metallic (flourine), and then finally to noble gases (neon) at the far right.
Today, the modern periodic law states: If the elements are arranged according to their atomic number, a pattern can be seen in which similar properties occur regularly. Hand out periodic tables and colour groups to match the table at the back of the book.
Assignment Refer to your periodic table as the class reads pages 110-113 in the text. Use a colour code and colour your in the following sections on your periodic table: 1. Noble gases 2. Alkali Metals 3. Halogens 4. Group of One 5. Metalloids 6. Metals
Group of One Hydrogen is a unique element. It’s most common isotope has only a single proton and no neutron in its nucleus. Hydrogen doesn’t have much in common with the alkali metals. It’s a colourless, odourless, tasteless, highly flammable gas. Almost all of Earth’s hydrogen exists in combination with other elements. Its reactivity is too great for it to exist in the atmosphere as a free element.
Alkali Metals Occupy the far left column of the periodic table (green). Extremely reactive. Found in nature only as compounds, and are found everywhere on Earth. Most common are sodium compounds, which occur in plants, animals, soil and sea water. Lithium, sodium, patassuim, etc…shiny, silvery.
Metalloids Elements that possess both metallic and non-metallic properties. Found on both sides of the zigzag line that divides the metals from the non-metals. Ex. silicon, boron, germanium, arsenic, selenium, antimony, tellurium, polonium, and astatine.
Halogens Occupy the 17 th column of the periodic table (fluorine, chlorine, bromine…) Most reactive non-metals. Because of their reactivity, they almost always appear naturally as compounds, not as elements. Most common halogen compounds are chlorine compounds found in living things, ocean water and rocks.
Noble Gases Occupy the far right column of the periodic table (blue). All gases at room temperature, the noble gases are often called inert gases because they are so unreactive, almost never forming chemical compounds with other elements. Argon makes up 1% of every breath you inhale. Very useful elements. Ex.: neon, helium