Skill : Reading Comprehension
No. 13
One definition of an atom is that it is the smallest part of an element that can take part in a chemical reaction. It is also defined as the smallest part of an element which has all the properties of that element. It follows from this second definition that atoms of silver differ from atoms of lead that atoms of lead differ from those of gold, and so on.
Although atoms of different elements are different from each other, they all have a similar structure. Thus, all atoms consist of a comparatively heavy nucleus around which one or more comparatively light electrons orbit. This nucleus is composed of one or more neutrons and one or more protons.
Because these parts of an atom are smaller than the complete atom, they are called subatomic particles. Because atoms are so small, the familiar metric units are not used in measuring them. Instead, the unit that is employed is the angstrom. This is symbolized by A. There are 100,000,000 A in one centimeter. Since most atoms are between 1.0 A and 2.4 A in diameter, it is clear why the angstrom unit is preferred to the comparatively large metric units. Compared to the size of the complete atom, the size of the nucleus is extremely small. If an atom were the size of a football pitch, its nucleus would be the size of a small insect.
In spite of its smallness, however, most of an atom’s weight is concentrated in the nucleus. A proton and a neutron each weigh approximately 201 atomic weight unit (awu). Each electron, on the other hand, weighs only about one over eighteen hundred of the weight of a proton or neutron.
There are other important differences between the subatomic particles that make up an atom. Each proton carries a single unit of positive electricity, while each neutron is electrically neutral, and each orbiting electron carries a single unit of negative electricity. Thus, since the number of protons in the nucleus of a complete atom is equal to the number of electrons, a complete atom is electrically neutral.
We use angstrom unit (A) to measure atoms because _______________.
atoms are very small
it is easy for anybody to use
the metric units are no longer in use
there are 100,000,000 A in a centimeter