ARRANGING THE ELEMENTS CHAPTER 7 - 1

DISCOVERING A PATTERN Dmitri Mendeleev, a Russian chemist, discovered a pattern to the elements in 1869. After trying several ways, he finally arranged the known elements by the increasing atomic mass.

Periodic Properties of the Elements When arranged by atomic mass, the elements fell into a repeating pattern of properties called “periodic.” Mendeleev found that the elements’ properties followed a pattern that repeated every 7 elements. His table became known as the periodic table of the elements.

Predicting Properties of Missing Elements His arrangement had gaps between the known elements. Mendeleev predicted that elements would be found to fill the gaps. He used the pattern to predict the missing element’s properties.

CHANGING THE ARRANGEMENT All the elements’ properties didn’t fit. In 1914, Henry Moseley, a British scientist, determined the number of protons, the atomic number, in an atom. When arranged by atomic number, all the elements fit the pattern.

PERIODIC TABLE AND CLASSES OF ELEMENTS Elements are classified according to their properties. The classes are metals, metalloids, and nonmetals. GALLIUM SILICON PHOSPHORUS

METALS Most elements are metals. They have few electrons in their outer energy level. Metals are shiny, malleable, ductile, and are good conductors of electricity and heat.

NONMETALS Atoms of most nonmetals have an almost complete set of electrons in their outer level. More than half of the nonmetals are gases at room temperature. Nonmetals are not malleable or ductile, and if solid, they can be brittle.

METALLOIDS Metalloids border the zigzag line on the periodic table. They have half a complete set of electrons in their outer level. They have some properties of metals and some of nonmetals.

Chemical Symbols Each square on the periodic table includes an element’s name, chemical symbol, atomic number, and atomic mass. The chemical symbol has one or two letters. Example: hydrogen is H and helium is He.

Periods Each horizontal row of elements on the periodic table is called a period. The elements in each row follows a repeating pattern of properties.

Groups Each vertical column of elements on the periodic table is called a group. These often have the same chemical and physical properties. They are sometimes called a family because they share similar properties.

Atomic Number The atomic number is the number of protons or electrons in the element. The atomic number is above the chemical symbol.

LET’S TRY IT! Lab: Create a Periodic Table