Electrons, Nucleus, and Atomic Structure Help (page 2)
In 1897, J.J. Thomson, a physicist from England, discovered negatively charged particles by removing all the air from a glass tube that was connected to two electrodes. One electrode was attached to one end of the tube and negatively (–) charged. It was called the cathode . The other end of the tube was attached to a positive (+) electrode and called the anode . A cathode ray tube uses a current to excite atoms of different gases contained in the tube. The electricity is beamed directionally through the length of the tube to the other electrode. By using this piece of equipment, scientists of a century ago began to separate the individual particles of atoms.
Through his experiments with several different colored gases, Thomson found that electrons had a negative charge and seemed to be common to all elements.
Electrons are small negatively charged sub-atomic particles that orbit around an atom’s positively charged nucleus.
However, since Thomson’s results showed that the overall charge of atoms was neutral in nature, something within the atom must be positive to counteract the negative charge. This something made the atom neutral.
Thomson came up with the “plum pudding” model of sub-particle arrangement made up of a blob of positively (+) charged particles, the pudding, and specks of negatively (–) charged particles floating around in it like raisins. He probably ate dessert right before or after working in his lab, so the idea came to him fairly easily. The plum pudding model of electrons and protons is shown in Figure 5.1 .
In 1906, Thomson was awarded the Nobel Prize for physics for his research and electrical work with gases. Later research found that an electron has a mass of 9.1 × 10 –31 kg and that it has a charge of 1.6 × 10 –19 Coulombs.
It wasn’t until a student of Thomson’s, Ernest Rutherford, started working to support his teacher’s ideas that the data for a plum pudding model just didn’t hold up. The floating negatively charged “raisins” acted differently in electrical current, for different elements, than what Thomson expected. This seemed to suggest they had different energy levels. (Maybe that is where the expression “the proof is in the pudding” came from.)
It wasn’t until scientists discovered that the atom was not just a solid core, but made up of smaller building block sub-particles located in the nucleus, that some of their data made sense.
In 1907, Rutherford, teaching at Cambridge, developed the modern atomic concept. He received the Nobel Prize for Chemistry in 1908 and was knighted in 1914 for his work. (Whoever said chemistry was not a glory science?)
Through his experiments with radioactive uranium in 1911, Rutherford described a nuclear model. By bombarding particles through thin gold foil, he predicted that atoms had positive cores that were much smaller than the rest of the atom.
Instead of thinking that atoms were the same all the way through (“plum pudding” model) as Thomson suggested, Rutherford’s experiments pointed more toward something like a fruit with a small, dense pit. His experiments along with those of his student, Hans Geiger, showed that over 99% of the bombarded particles passed easily through the gold, but a few (one out of eight thousand) ricocheted at wild angles, even backwards. Figure 5.2 shows how Rutherford’s dense pit model of the nucleus might look.
Rutherford thought this scattering happened when positive nuclei of the test particles collided and were then repelled by heavy positively charged gold nuclei. It was later proven that Rutherford’s dense pit model was correct. When an accelerated alpha particle collided with an electron of a gold atom in a gas, a proton was knocked out of the nucleus.
Later research done along the same lines as Rutherford’s early work found that protons in a nucleus have a mass over 1800 times that of an electron. In fact, the positively charged nucleus of the atom that contained most of its mass was very dense and took up only a tiny part of an atom’s total space.
To get an idea of size, if an atomic nucleus were the size of a ping-pong ball, then the rest of the atom with its encircling negatively charged electrons would measure nearly 3 miles across. More precisely, nuclei are roughly 10 –12 meters in diameter. The total diameter of an atom is around 10 −8 meters or roughly 10,000 times larger.
A proton has a positive charge and roughly 1800 times greater mass than an electron.
A proton is a smaller bit of positively charged matter or sub-atomic particle within the nucleus.
The atomic number (Z) of an element is taken from the number of protons in the nucleus of an atom. A pure element is one that is made up of particles that all have the same atomic number.
To obtain the atomic number of an element, you must identify the number of protons in the nucleus.
What is the atomic number (Z) of (a) boron, (b) gold, (c) zinc, (d) iridium, and (e) bismuth? Did you get (a) 5, (b) 79, (c) 30, (d) 77, and (e) 83?
The nucleus of an atom contains sub-atomic particles called nucleons. Nucleons are divided into two kinds of particles, neutrons and protons . Protons make up the dense nucleus core, but when chemists made calculations based on atomic weights of atoms, the numbers didn’t add up. They knew there must be something they were missing. This is when neutrons were discovered. Neutrons are nuclear particles that have no charge and are located inside the crowded nucleus with positively charged protons.
Neutrons are sub-atomic particles with a similar mass to their partner proton in the nucleus but with no electrical (+ or –) charge.
Table 5.1 Common characteristics of electrons, protons, and neutrons indicate their special nature.
Though Thomson, Rutherford, Meyer and Mendeleyev didn’t quite understand what caused many of the reactions they observed, they recorded the patterns they saw among the elements. They noted that elemental properties seemed to reflect atomic weight and atomic number, but weren’t really sure why. Modern chemistry has discovered the answers to these puzzles.
Figure 5.3 illustrates a beryllium atom with its energy levels. The atom is composed of 4 protons and 5 neutrons in the nucleus, and 4 electrons arranged in 2 shells (or orbital layers) outside the nucleus. The first shell contains 2 electrons and the second shell contains 2 electrons.
Through detailed experiments, scientists discovered that the way an element behaves is largely due to the number of electrons in its outer orbital shells. For example, elements with one electron in the outer shell behave alike; those with two electrons behave alike, and so on. This knowledge allowed early chemists to place similar elements in the same Periodic Table groups according to their outermost or valence electrons.
Practice problems for these concepts can be found at - Atoms, Elements, and Compounds Practice Test
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