When James Watt invented his steam engine, he was asked how many horses the engine could replace. To find out, Watt built a rope-and-pulley device with which he could measure the time it took a horse to lift various weights. After many experiments Watt concluded that a horse could produce 746 Watts of power for a sustained period of time. (Watt used English units of measurement in his experiments. The metric unit of power was later named in his honor). Watt's definition of horsepower is still used today, even though the power output of an average horse during the course of a working day is only about two-thirds of a standard horsepower

Up of elements with the same number of valence electrons. vertical column in the periodic table of elements such as alkali metals or halogens. a horizontal row of elements in the periodic table. this is a characteristic of the elements in the periodic table that shows a pattern. it may increase or decrease across or down the table. periodic trend these are the highly reactive elements located in group 1 of the periodic table. these elements have one electron in their outer energy level which makes them highly reactive with water and halogens. these are the reactive elements located in group 2 of the periodic table. these elements have two electrons in their outer energy level which makes them reactive with water and halogens. alkaline earth metals these are the group 3 or d-block elements. these dense metals with high boiling points can have different oxidation states and all are solid at room temperature with the exception of mercury. transition metals this is the highly reactive family of elements with 7 valence electrons. this is an element with full valence shell, very unreactive. this is a group of elements with few valence electrons that conducts heat and electricity. one of a class of elements having properties intermediate to metals and nonmetals. this is a type of element that has many valence electrons, not a conductor.