93 Dmitri's Table-Mid

Dmitri's Table FOCUS Question How does the periodic table help us understand what makes up matter?

Organizing the Elements Are you sitting in a chair? You are probably wearing shoes. Maybe you just had lunch. That chair, your shoes, and your food are types of matter. In fact, everything that takes up space and has mass—even air—is made of matter. All matter is made of tiny components called elements— substances that can't be broken down into smaller substances. There are about 120 known elements on Earth, and researchers continue to discover new ones. To organize and keep track of them all, scientists rely on a grid called the Periodic Table of the Elements.

By the mid-1800s, scientists had identified sixty elements. Researchers working in different parts of the world needed a way to discuss these elements and classify them by how they behave. These groupings would eventually be shared among all scientists. A few scientists devised their own charts, or periodic tables, to organize this important data. A Russian chemist named Dmitri Mendeleev published his periodic table in 1869. His table became the most popular one, and a version of it is still in use today.

Dmitri's Details Figuring out how to group sixty elements was not easy, but Mendeleev invented a way to describe each of the elements and organize them in a meaningful way. Dmitri's periodic table contained two details for each element—chemical symbol and atomic weight. A chemical symbol represents the element's name with one or two letters. This symbol often comes from the Latin word for the element. For example, hydrogen is H and gold is Au. Mendeleev then listed the elements by increasing atomic weight. At the time, scientists didn't know exactly how much atoms weighed, but they were able to estimate on the basis of results from chemical reactions. We now know that the tiny atoms of each element are made up of even smaller particles. Word Wise The Latin word for gold is aurum, so the chemical symbol is Au.

Inside an atom is a central core called a nucleus, which holds particles called protons and neutrons. Circling the nucleus are fast-moving particles called electrons. The more particles there are inside an atom, the more weight, or mass, it has. The atomic weight of an atom is the average of the total number of protons plus neutrons in all versions, or isotopes, of that atom. An isotope of an element varies in its number of neutrons, but the number of protons stays the same. A helium atom has two protons, two neutrons, and two electrons. It has an atomic weight of about four. A gold atom has 79 protons, 118 neutrons, and 79 electrons. It has an atomic weight of about 197. It is easy to see why helium is so light and gold is so heavy. Do You Know? The particles in an atom are too small to see, even with a microscope. Scientists used electric fields and radiation the movement of energy—to discover these tiny particles.

Setting the Table Mendeleev listed the elements by increasing atomic weight because the elements with similar atomic weights had similar properties. Hydrogen (H) is the lightest element on Earth, so it is first on the list. In 1869, lead (Pb) had the highest atomic weight of all the known elements, so it was last on his list. Using this method, Mendeleev predicted that new elements would be discovered He was right! Today's periodic table lists the elements by atomic number instead of atomic weight. Why? The atomic number equals the number of protons in an element, and each element has a unique number of protons. Hydrogen has 1 proton, and lead has 82. Current periodic tables continue to an element called ununoctium, which has an atomic number of 118. The atomic number appears above the chemical symbol on a periodic table square. The atomic weight is displayed below the chemical symbol. Math Moment Atoms of the element bromine have 35 protons. Half of its isotopes have 44 neutrons, and half have 46 neutrons. What is the average atomic weight of bromine?