Properties of the Elements • As alchemists and chemists explored the world around them, they discovered a large number of elements. It became clear that these elements needed to be organized. In the late 1960’s, a Russian chemist and teacher named Dmitri Mendeleyev created his own organizational scheme by putting elements in groups based on similarities in their properties. Some of these properties are listed below. • Reactivity • Ratios in compounds • Atomic mass
Reactivity is a property that describes how easily an element will combine with other substances to form new compounds. An element that is highly reactive combines rapidly with other substances. For example, when metallic sodium comes into contact with water, it reacts vigorously.
Mendeleyev also paid attention to which elements combine with which, and he noted the ratios in which their atoms combine. For example, magnesium combines with chlorine in a 1:2 ratio, while sodium combines with chlorine in a 1:1 ratio. This means that magnesium and sodium must have similar reactivity (since both can combine with chlorine), but not exactly the same. Mendeleyev used another property--atomic mass--to sort the elements. Atomic mass is the mass of an atom, and it is measured in atomic mass units, or amu. The elements can be placed in order of their atomic masses. However, this alone does not tell you which elements are similar by properties.
Mendeleyev put the elements with similar reactivity and chemical formulas of compounds into columns. He also sorted them by atomic mass, from smallest to largest. This way, the elements in each column have similar physical properties and reactivity, and they tend to form compounds with other elements in the same ratios. This table became the foundation of the modern periodic table of elements.
The Periodic Table • Scientists have detected around 114 elements on the planet (there are about 118 total). Each element is unique, but groups of elements have similar properties.
Each element has a square on the periodic table. Within each square is information about that element including its name and symbol. The whole number (usually at the top of each square) is called the atomic number. The decimal number in each square on the periodic table is the average atomic mass in amu.
Most modern periodic tables have 18 vertical columns and 7 horizontal rows. The vertical columns are also called groups or families. The horizontal rows of the table are called periods because patterns repeat periodically (over and over again) in each row.
Chemists also have names for the sections of the periodic table. • The main group elements include groups 1A (alkali metals), 2A (alkaline earth metals), and 3A-8A (group 7A are the halogens, and group 8A are the noble gases). • The transition metals are in the middle of the periodic table. • The inner transition metals are below the periodic table. This section consists of the lanthanides and actinides.
Solids, liquids, and gases: most of the elements are solid at room temperature. Several are gaseous at room temperature; only three (gallium, bromine, and mercury) are liquid at room temperature.
Metals, metalloids, and nonmetals • The majority of the elements are metals. On most periodic tables there is a stair-step line that divides the table. Metals are found to the left and nonmetals are found to the right. The elements found along the stair-step are called metalloids (similar properties to both metals and nonmetals).
Reactivity: elements in the lower left and upper right of the periodic table (excluding noble gases) are the most reactive. On the other hand, elements in the middle of the periodic table, such as copper, silver, and gold, are not very reactive.
Fun fact It is illegal to purchase francium in the United States due to its violently explosive reaction with water.
Fun fact Gold is very nonreactive, but it can dissolve in aqua regia, a concentrated 1:3 mixture of nitric acid and hydrochloric acid. This reaction was used to keep two Nobel Prize medals out of Nazi hands in the 1940’s (the metals were dissolved; the dissolved gold was stored in common glassware; and after the war, the Nobel Foundation recast the medals using the dissolved gold).