Atoms PDF

ATOMS What is an atom?  The word "atom" is derived from the Greek word, “atomos” or indivisible.  Atom is the smallest unit of matter that retains the identity of the substance.  Atoms are the smallest particle of an element. Each element is made of only one kind of atom. The overall shap...

ATOMS What is an atom?  The word "atom" is derived from the Greek word, “atomos” or indivisible.  Atom is the smallest unit of matter that retains the identity of the substance.  Atoms are the smallest particle of an element. Each element is made of only one kind of atom. The overall shape of all atoms is spherical but they vary in size.  Each atom has a nucleus located at its center, surrounded by an electron cloud, which is mostly empty space outside of the nucleus. The nucleus is very small, compared with the overall size of the atom.  Atoms are made of three different kinds of sub-atomic particles: 1. protons 2. neutrons 3. electrons  An atom is neutral (has no charge). The negative charge of the electrons is enough to neutralize the positive charge of the protons. An atom contains equal number of protons and electrons. NEUTRAL ATOM A neutral atom is an atom that has not lost or gained an electron; for an atom to become stable, it loses or gains one or more electrons in its valence shell. When an atom loses one or more electron(s), it becomes positively charged and negatively charged when it gains one or more electron(s). Parts of an Atom NUCLEUS  Every atom has a core called a nucleus.  Much of the mass of an atom is concentrated at the nucleus (about 99.9% of it’s mass).  Within the nucleus are very small particles called protons and neutrons.  Electrons are in an orbit around the nucleus. PROTONS  Particles found inside the nucleus of an atom.  Protons are the positively charged.  Eugen Goldstein was the first to observe the proton (then unnamed)  Ernest Rutherford coined the word “proton” NEUTRONS  Neutronsare the other particle found inside the nucleus of an atom.  Neutrons have no charge.  Discovered by James Chadwick. ELECTRONS  Negatively charged particles found orbiting around the nucleus.  Electrons are very small.  Electrons are very much lighter than the protons and neutrons, to the point that its mass does not significantly contribute to the mass of the entire atom.  Discovered by Joseph John Thomson (J. J. Thomson)  Some properties of the three main subatomic particles Subatomic Charge Mass (grams) Location in the particle Atom (symbol) Electrons (e-) -1 9.109 x 10-28 Outside nucleus Protons (p+) +1 1.672 x 10-24 Nucleus Neutrons (n0 ) 0 1.675 x 10-24 Nucleus The History of the Atom Timeline: 400 BC Scientist: Democritus (Greek Philosopher)  Democritus was a Greek philosopher who was the first person to use the term atom (atomos: meaning indivisible). He thought that if you take a piece of matter and divide it and continue to divide it you will eventually come to a point where you could not divide it anymore. This fundamental or basic unit was what Democritus called an atom. He called this the theory of the universe: All matter consists of atoms, which are bits of matter too small to be seen. There is an empty space between atoms. Atoms are completely solid. Atoms have no internal structure. Each atom (of a different substance) is different in size, weight and shape. Timeline: 1800’s Scientist: John Dalton John Dalton was an English chemist. His ideas form the atomic theory of matter. Here are his ideas. 1. All matter is composed of indivisible particles called atoms. 2. All atoms of a given element are identical in mass and properties. Atoms of different elements have different masses and different properties. 3. Compounds are formed by a combination of 2 or more atoms. 4. Atoms cannot be created, destroyed, or converted into other kinds of atoms during chemical reactions. Timeline: 1890’s Scientist: J.J Thomson  J.J Thomson was a physicist who is credited for discovering the electron. He used his research on cathode ray tube technology in this discovery.  J.JThomson was an excellent physicist and thus did not stop when he had found this negative charge. Through a series of clever experiments, he was able to predict the mass of this charge. He then found out that this charge was 1000 times lighter that a hydrogen atom. He made a bold statement saying that this negative charge must be inside an atom. This negative charge (he called corpuscles) later became known as the electron. THOMSON’S ATOMIC MODEL (PLUM PUDDING MODEL) Using what he had discovered, Thomson predicted what an atom should look like. These are the key points to Thomson’s Atomic Model: 1. Because of its design this model is known as the plum pudding model. 2. Each atom is a sphere filled with positively charged ‘fluid’. This resembles the sticky jam part of a pudding. 3. Corpuscles (later called electrons), are the negatively charged particles suspended in this ‘fluid’. This resembles the plums in the pudding. 4. He did not predict the movement of these electrons. Plum Pudding Models of the Atom Thomson proposed that the negatively-charged electrons were embedded in a cloud of positive charge. Since plums and puddings are not commonly known in the Philippines, it may work better for you to use the other name for the model, the raisin bread model or a watermelon fruit model. Plum Pudding Models of the Atom Timeline: 1910’s Scientist: Ernest Rutherford Ernest Rutherford was not convinced about the model of the atom proposed by Thomson. He thus set up his now famous Gold Foil Experiment. 1. He fired alpha particles (positively charged) at a gold foil. 2. He measured the deflection as the particles came out the other side. 3. Most of the particles did not deflect at all. Every now and then a particle would deflect all the way back. 4. He said that there must be a positive center of the foil. He called this center the nucleus. Nuclear Models of the Atom Nuclear Models of the Atom Using a setup, Rutherford and his coworkers, after doing a series of experiments, observed the following:  Most alpha particles were undeflected.  Some are deflected at smaller angles.  Few alpha particles deflected almost back towards the source. RUTHERFORD’S ATOMIC MODEL (NUCLEAR MODEL) 1. The nucleus of the atom is a dense mass of positively charged particles. 2. The electrons orbit the nucleus. 3. A problem raised was: Why are the negatively charged particles not attracted by the positively charged nucleus. 4. Rutherford stated that the atom was like a mini solar system and that the electrons orbited the nucleus in a wide orbit. That is why it is known as the planetary model. Timeline: 1910’s Scientist: Niels Bohr Niels Bohr agreed with the planetary model of the atom, but also knew that it had a few flaws. Using his knowledge of energy and quantum physics he was able to perfect Rutherford’s model. He was able to answer why the electrons did not collapse into the nucleus BOHR’S ATOMIC MODEL (THE PLANETARY MODEL) 1. Electrons orbit the nucleus in orbits that have a set size and energy. 2. The lower the energy of the electron, the lower the orbit. 3. This means that as electrons fill up the orbitals, they will fill the lower energy level first. 4. If that energy level is at fill (or at capacity), a new energy level will begin. 5. Radiation is when an electron moves from one level to another. However, here is the problem with this theory: Electrons do not travel on a specific orbit or path. Timeline: 1920’s Scientist: Erwin Schrödinger  Erwin Schrödinger was a revolutionary physicist who used Heisenberg’s uncertainty principle to come up with the atomic model that we still use today.  SCHRÖDINGER’S ATOMIC MODEL (THE QUANTUM MECHANICAL MODEL) 1. An electron does not travel in an exact orbit. 2. We can predict where it will probably be. 3. We cannot say for certain where it is, but only where it ought to be. 4. The type of probability orbit is dependent on the energy level described by Bohr. Atomic number  Atoms of a particular element have a set number of protons. For example, every atom of hydrogen has one proton, and every atom of gold has 79 protons.  The number of protons is called the element’s atomic number.  The total number of protons and neutrons in an atom’s nucleus is called the mass number of that atom.  Remember: Atomic Number = Number of Protons Mass Number = Number of Protons + Number of Neutrons Number of Neutrons = Mass number ─ Number of Protons Atomic mass  Atomic mass is the mass of an atom of a particular element. It is the average of the mass numbers of the naturally occurring isotopes of the element multiplied with their respective abundance. You will not compute for atomic mass. However, you have to know, at least, where to find it in the periodic table. Atomic Notation A shorthand way to represent information of elements from the periodic table of elements. A standard atomic notation shows the element symbol, atomic number, mass number and charge (in case of an ion) of the element simultaneously. Atomic Notation  The figure on the right shows another shorthand notation. Information on the subatomic particles may be derived from this shorthand.  The base is the element’s symbol.  The left subscript denotes the atomic number.  The superscript at the left denotes the mass number.  The superscript at the right denotes the charge wherein the number of electrons may be determined. When there is no superscript at the right, it means that the charge is zero (0). Ions  Atoms may gain charges. This happens when electrons are lost or gained by the atom. When this happens, the atom become an ion.  Atoms with a net positive charge (cation) or negative charge (anion).  Atoms that are electrically neutral will have the same number of protons and electrons. ATOMS (neutral) VS. IONS (charged) ISOTOPES  An isotope is a variation of an element that possesses the same atomic number but a different mass number. A group of isotopes of any element will always have the same number of protons and electrons. They will differ in the number of neutrons held by their respective nuclei. ISOTOPES  While the number of protons is the same with atoms of a particular element, the number of neutrons may vary. Atoms having the same number of protons but different number of neutrons are referred as isotopes. The isotopes are identified through their mass number which is the sum of the number of protons and the number of neutrons in an atom. ISOTOPES  Isotopes are primarily represented by writing the name of the element followed by a hyphen and the mass number of the isotope. For example, uranium-235 and uranium-239 are two different isotopes of the element uranium. A shorthand notation for isotope includes the element’s symbol and mass number, for instance, U–235. Determining the Number of Neutrons in an Isotope  The total number of neutrons in the nucleus of an isotope can be determined by subtracting the atomic number of the element from the mass number of the isotope. Three isotopes of hydrogen Protium, or hydrogen-1. This isotope of hydrogen contains 1 proton, 1 electron, and no neutrons. Deuterium, or hydrogen-2. This isotope of hydrogen contains 1 proton, 1 electron, and 1 neutron. Tritium, or hydrogen-3. This isotope of hydrogen contains 1 proton, 1 electron, and 2 neutrons. It can also be noted that this isotope of hydrogen is radioactive. Isotopes of hydrogen Isotopes Atomic Mass Number Number Number number number of of of protons neutrons electrons Hydrogen-1 1 1 1 0 1 Hydrogen-2 1 2 1 1 1 Hydrogen-3 1 3 1 2 1 Carbon occurs naturally in three isotopes  Carbon-12  Carbon-13  Carbon-14 Carbon-14 is unstable and undergoes radioactive decay with a half-life of about 5,730 years. This decay means the amount of carbon-14 in an object serves as a clock, showing the object’s age in a process called “carbon dating.” Isotopes of carbon Isotopes Atomic Mass Number Number Number number number of of of protons neutrons electrons Carbon-12 6 12 6 6 6 Carbon-13 6 13 6 7 6 Carbon-14 6 14 6 8 6 There are three naturally occurring isotopes of uranium  Uranium-238, the heaviest and most abundant  Uranium-235 Uranium-235 is the only isotope that undergoes fission  Uranium-234