Nursing Entrance Exam

9.2.4 Atomic Structure Atoms Atoms are some of the basic building blocks of matter. Each atom is an element, an identifiable substance that cannot be further broken down into other identifiable substances. There are just over 100 such elements, and each of them can combine with themselves andwith other elements to create all the various molecules that exist in the universe. The poison gas chlorine and the explosive metal sodium, for instance, can combine at the atomic level to form sodium chloride, also known as salt. For thousands of years, atoms were thought to be the smallest thing possible. The word “atom” comes from an ancient Greek word meaning “unbreakable”. However, experiments performed in the mid- to late-19 th century began to show the presence of small particles, known as electrons, in electric current. By the early 20 th century, the electron was known to be a part of the atom that orbited a yet undefined atomic core. A few years later, in 1919, the proton was discovered and found to exist in the nuclei of all atoms. The protons and neutrons inside an atomic nucleus are not fundamental particles. That is, they can be divided into still smaller pieces. Protons and neutrons are known as hadrons, which is a class of particle made up of quarks. Quarks are a fundamental particle. There are two distinct types of hadrons – baryons and mesons – and both protons and neutrons are baryons, meaning they are both made up of a combination of three quarks. Besides, being hadrons, protons and neutrons are also known as nucleons because of their place within the nucleus. Protons have a mass of around 1.6726 × 10-27 kg and neutrons have a nearly identical mass of 1.6929 × 10-27 kg. Both particles have a ½ spin. The number of protons inside an atomic nucleus determines which element the atom comprises. An element with only one proton, for instance, is hydrogen. An element with two is helium. One with three is lithium, and so on. No element (except for hydrogen) can exist with only protons in its nucleus. Atoms need neutrons to bond the protons together using the strong force. In general, atoms have an equal number of protons and neutrons in their nuclei except for hydrogen. Atoms with an uneven number of protons and neutrons are called isotopes. Isotopes have all the same chemical properties as their evenly balanced counterparts, but their nuclei are not usually as stable and are more willing to react with other elements. Two deuterium atoms, which are hydrogen isotopes with one proton and one neutron in their nucleus rather than only one proton, will fuse much more readily than two regular hydrogen atoms. Nearly all of an atoms’ mass is within its nucleus. Outside the nucleus is a lot of empty space occupied only by a few, tiny electrons. Electrons were once viewed as orbiting an atom like planets orbit the sun. We now know that this is wrong in several ways. For one, electrons do not really “orbit” in the sense we know. At the quantum level, no particle is really a particle, but is actually both a particle and

Achieve

Page 158

of 175

© 2018