THE ELEMENTS
When a substance is composed of atoms of the same kind, we call that substance an element. A pure 24-carat gold ring, for example, is composed only of gold atoms. A gold ring with a lower carat value is composed of gold and other elements, such as nickel. The silvery liquid in a thermometer is the element mercury. The entire liquid consists of only mercury atoms. Of course, if a substance contains only a single kind of atom, it can correctly be called an element. An atom of a particular element is the smallest sample of that element. Although atom and element are used interchangeably, element is preferred when referring to macroscopic quantities. For example, we speak of isolating a mercury atom from a flask of the element mercury.
The lightest element of all is hydrogen. In the universe at large, it is the most abundant element. More than 90% of the atoms in the known universe are hydrogen atoms. Helium, the second-lightest elements, provides most of the remaining atoms in the universe. Heavier atoms in our surroundings were manufactured by the fusion of the light elements in the hot, high pressure cauldrons deep within in the interiors of stars. The heaviest elements form when huge stars implode and then explode (supernovas). Nearly all the elements on Earth are remnants of stars that exploded long before the solar system came into existence.
To date, 118 elements are known. Of these, 94 occur in nature. The others are produced in laboratory with high-energy atomic accelerators and nuclear reactors. The laboratory-produced elements are too unstable (radioactive) to occur naturally in appreciable quantities.
From a pantry containing less than 100 elements, we have the atoms that constitute almost every simple, complex, living or non-living substance in the known universe. More than of the material on Earth is formed from only about a dozen of the elements. The other elements are relatively rare. Living things are composed primarily of five elements: oxygen (O), carbon (C), hydrogen (H), nitrogen (N) and calcium (Ca). The letters in parentheses represent the chemical symbols for these elements.
REVIEW QUESTIONS: (can easily be answered from the text)
1- What is the lightest of the elements?
2- What is the most abundant element in the universe?
3- How were elements heavier than hydrogen formed?
4- Where did the heaviest elements originate?
5- What are the five most common elements in things?
AMAZING PHYSICS (Linear Motion)
QUESTION:
A motorist wishes to travel 40 kilometers at an average speed of 40 kilometers per hour (40 km/h). During the first 20 kilometers, an average speed of 40 km/h is maintained. During the next 10 kilometers, however, the motorist goofs off and averages only 20 km/h. To drive the last 10 kilometers and average 40 km/h, the motorist must drive:
a) 60 km/h
b) 80 km/h
c) 90 km/h
d) faster than the speed of light
ANSWER: d (faster than the speed of light!)
EXPLANATION:
He would have to travel at an infinite speed and finish the last 10 kilometers in zero time and attain an average speed of 40 km/h!
Why? Because he has an hour to make the trip and his one hour is up (i.e. it is over) at the 30-km point. He spent ½ hour to the halfway point, 20 kilometers, and another ½ hour when he averaged 20km/h over that 10-kilometer stretch. So, he would have to cover the entire 40 kilometers in 1 hour. This means, the last 10 kilometers in no time at all!
- Enseignant: CHAMS_EDDINE KHIARI (Enseignant)