The structure of Atoms and Subatomic Particles
The study of chemistry begins with atoms, the basic building blocks of matter. Early theories of atom considered atoms to be invisible, but we know now that this idea is wrong. Elements differ from one another because of differences in the internal structure of their atoms. Under the right conditions, smaller particles within atoms – known as subatomic particles – can be removed or rearranged. The term atomic structure refers to the identity and arrangement of these subatomic particles in the atom. An understanding of how atoms combine to form compounds and are rearranged in chemical reactions. Atomic structures also accounts for the properties of materials.
Atomic Structure and Subatomic particles
Electrical charges played an important role in many of the experiments from which the theory of atomic structure was derived. Two types of electrical charge exist: positive and negative charge. Electrical charges of the same type repel one another and charges of the opposite type attract one another. A positively charged particle repels another positively charged particle. Likewise, two negatively charged particles repel each other. In contrast, two particles with opposite signs attract each other.
In 1896, Henri Becquerel discovered that a sample of a uranium ore emitted rays that darkened a photographic plate, even though the plate was covered by a protective black paper. In 1898, Marie and Pierre Curie isolated the new elements polonium and radium, which emitted the same kind of rays. Marie suggested that atoms of such elements spontaneously emit these rays and named the phenomenon radioactivity.
Atoms of radioactive elements can emit three types of radiation: alpha (α), beta (β) and gamma (γ) rays. These radiations behave differently when passed between electrically charged plates. Alpha and beta rays are deflected, but gamma rays are not. These events occur because alpha rays and beta rays are composed of charged particles that come from within the radioactive atom. Alpha rays have 2+ charge and beta rays have a 1 – charge. Alpha rays and beta rays are particles because they have mass – they are matter. Experiments revealed that alpha particles were deflected less so must be heavier than beta particles. Gamma rays did not have any detectable charge or mass as they behaved like light rays. If radioactive atoms can break apart to produce subatomic alpha and beta particles, then there must be something smaller inside the atoms.
Further evidence that atoms are composed of subatomic particles came from experiments with specially constructed glass tubes called cathode-ray tubes. Most of the air was removed from these tubes and metal electrode sealed into each end. When a sufficiently high voltage is applied to the electrodes, a beam of rays flows from the negatively charged electrode (the cathode) to the positively charged electrode (the anode). These rays, known as cathode rays, come directly from the metal atoms of the cathode. The cathode rays travel in straight lines, are attracted toward positively charged plates, can be deflected by a magnetic field, can cast sharp shadows, can heat metal objects red hot and can cause gases and fluorescent materials to glow. When cathode rays strike a fluorescent screen, the energy transferred causes light to be given off as tiny flashes. Thus, the properties of cathode ray are those of a beam of negatively charged particles, each of which produces light flash when it hits a fluorescent screen. Sir Joseph John Thomson suggested that cathode rays consist of the same particles that had earlier been named electrons and had been suggested to be the carriers of electricity. He also observed that cathode rays were produced from electrodes made from different metals. This implied that electrons are constituents of the atoms of those elements.
In 1897, Thomson used a specially designed cathode ray tube to simultaneously apply electric and magnetic fields to a beam of cathode rays. By balancing the electric field against the magnetic field and using the basic laws of electricity and magnetism, Thomson calculated that the mass of charge for the electrons in the cathode ray beam: 5.60 × 10-9 grams per Coulomb (g/C).
When atoms loose electrons, the atoms become positively charged. When atoms gain electrons, the atoms become negatively charged. Such charged atoms, or similarly charged group of atoms are known as ions. From experiments with positive ions, formed by knocking electrons out of atoms, the existence of a positively charged fundamental particle was deducted. Positively charged particles with different mass-to-charge ratios were formed by atoms of different elements. The variation in masses showed that atoms of different elements must contain different numbers of positive particles. Those from hydrogen atoms had the smallest mass-to-charge ratio, indicating that they are the fundamental positively charged particles of atomic structure. Such particles are called protons. The mass of a proton is known from experiment to be 1.67262129 × 10-24 g, which is about 1800 times the mass of an electron. The charge of a proton is 1.602176462 × 10-19 C, equal in size, but opposite in sign, to the charge on an electron. The proton’s charge is represented by 1+. Thus, an atom that has lost two electron has a charge of 2+.
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