Group: A column of the periodic table; elements in it act alike
Period: A row of the periodic table
How many groups are in the standard periodic table?: 18
How many periods are in the standard periodic table?: 7
Alkali metals: Group 1 (except hydrogen): very reactive soft metals like sodium
Alkaline earth metals: Group 2: reactive metals like calcium and magnesium
Transition metals: The block in the middle, like iron, copper and gold
Halogens: Group 17: reactive nonmetals like chlorine and iodine
Noble gases: Group 18: very unreactive gases like helium and neon
Metalloids: Elements with properties of both metals and nonmetals, like silicon
Nonmetals: Elements that are usually dull, brittle and poor conductors
Metals: Shiny elements that conduct heat and electricity and can be shaped
Lanthanides: Row of 15 elements shown below the main table, from lanthanum
Actinides: Row of 15 elements shown below the main table, from actinium
Atomic number: The number of protons in an atom's nucleus
Atomic mass: The average mass of an element's atoms, in atomic mass units
Element symbol: One or two letters standing for an element; the first is capital
Who created the first widely used periodic table in 1869?: Dmitri Mendeleev
How did Mendeleev arrange the elements?: By atomic mass, leaving gaps for undiscovered elements
How is the modern table arranged?: By increasing atomic number
Valence electrons: Electrons in an atom's outermost shell
How many valence electrons do halogens have?: 7
How many valence electrons do alkali metals have?: 1
How many valence electrons do most noble gases have?: 8 (helium has 2)
Which group contains carbon?: Group 14
Which group contains oxygen?: Group 16
Which group contains nitrogen?: Group 15
Which element is placed alone at the top-left, and is not an alkali metal?: Hydrogen
Which side of the table holds mostly metals?: The left side and middle
Which side of the table holds the nonmetals?: The upper right
What is the zigzag line on the right side of the table for?: It separates metals from nonmetals; metalloids sit along it
Why are noble gases unreactive?: Their outer electron shell is full
Why do elements in a group act alike?: They have the same number of valence electrons
Which period 1 elements exist?: Hydrogen and helium
Which is the lightest metal, atomic number 3?: Lithium
Synthetic element: One made in a lab and not found in nature in large amounts
Radioactive element: An element whose atoms break down and give off energy
Which element has the lowest atomic number?: Hydrogen (1)
Isotope: Atoms of the same element with different numbers of neutrons

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