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Download 4.1Atoms and Isotopes
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Atoms, Ions, and Isotopes “Up and at them” Atoms ■ ■ An atom is composed of a central nucleus which consists of protons and neutrons, along with orbiting electrons that exist within ‘clouds’ or orbitals. These protons, neutrons, and electrons are commonly known as SUB-ATOMIC PARTICLES. Sub-Atomic Particles ■ Protons ■ ■ ■ ■ ■ Found in nucleus Positive charge (+1) Mass 1 atomic mass unit (a.m.u.) If an atom gains or loses one or more protons, it becomes an atom of a different element E.g. If N lost a proton, it would become C (losing or gaining a proton takes a massive amount of energy – can really only occur in the laboratory if the proper equipment is available). ■ Electrons ■ ■ ■ ■ ■ Exist outside the nucleus in orbitals Negative charge (-1) Very small mass compared to protons (Mass approx 1/1800 a.m.u.) If an atom gains or loses one or more electrons, it becomes an ion (this occurs often in nature since the energy required is somewhat available). Neutrons ■ ■ ■ ■ ■ Found in nucleus of atom Neutral charge (0 charge) Made up of a proton fused with an electron Mass 1 a.m.u. (in fact, slightly more than 1 amu), but assumed to be 1 amu. Each element has atoms with different amounts of neutrons – the different atoms are called isotopes Atomic Number ■ ■ ■ ■ ■ Equal to the number of protons in the atom. Therefore, each atomic number is unique and defines each atom Eg: Oxygen has an atomic number of 8 because it has 8 protons Easily recognized on the periodic table for each element (see next slide) In chemical symbol notation, the atomic number is written on the bottom left 12 e.g. C 6 ■ The atomic number is found at the top of each square on the periodic table. Atomic number If you have a periodic table you can instantly find the number of protons that each atom possesses. It should also be noted that any element existing in its neutral state will also have the same number of electrons as its Atomic #. Atomic Mass ■ ■ ■ ■ Total number of all protons and neutrons in an element (always a whole number) [units are in a.m.u.] Note - electrons are too small to affect total mass In chemical symbol (isotope) notation, the atomic mass is a whole number and is written on the top left 35Cl E.g. 17 Mass Number ■ ■ To find mass number from a periodic table, look under the symbol (or name) It will be explained later why decimal values are given on the periodic table Mass number Number of Electrons ■ ■ ■ In any neutral (uncharged) atom the number of protons is always equal to the number of electrons. The number of electrons, therefore, is also equal to the atomic number. If there are 8 protons, there will be 8 electrons. Number of Neutrons ■ ■ Can be determined with the following formula: Atomic Mass = Protons + Neutrons Calculating Protons, Neutrons and Electrons ■ Find number of protons, neutrons, and electrons and write chemical symbol notation for each of the following Atomic Mass ■ ■ ■ Carbon-13 Sodium-23 Uranium-235 Ions ■ ■ ■ Atoms which have either gained or lost electrons by a chemical reaction of some kind. Ions which have lost electrons carry a positive charge and are called CATIONS Ions which have gained electrons carry a negative charge and are called ANIONS In chemical symbol notation, the ionic charge is written in the top right if there is a charge 32S2e.g. 16 How many protons, electrons, and neutrons? 16 protons, 16 neutrons, 18 electrons ■ 1. 2. 3. Write chemical symbol notation for an element with 19 protons, 21 neutrons, and 18 electrons Write chemical symbol notation for an element with 15 protons, 17 neutrons, and 18 electrons How many protons, electrons, and neutrons? 206Pb4+ 82 Answers: 1. 2. 3. 40K1+ 16 32P315 82 protons, 78 electrons, and 124 neutrons. Isotopes ■ ■ ■ ■ ■ Isotopes are atoms of the same element that have different atomic masses due to different amounts of neutrons Hydrogen has three isotopes Ordinary hydrogen (protium) 1H Heavy hydrogen (deuterium) 21H Radioactive hydrogen (tritium)1 3H 1 ■ ■ ■ ■ Carbon has three isotopes: C-12 (most abundant), C-13 (used in medical imagingMRI), and C-14 (used for dating fossils) Tin (Sn) has the most isotopes of any element at 10 Many isotopes are radioactive (unstable nucleus that will eventually break apart and release energy in sometimes harmful forms – eg. Gamma rays) Any isotope with an atomic number greater than 82 is radioactive Why Decimal Mass Numbers? ■ ■ ■ The mass reported on the periodic table is actually a weighted average of the masses of all the isotopes of an element. Most hydrogen is Hydrogen – 1, therefore the average mass is very close to 1 (1.0079). For most atoms, one isotope is much more common than the other(s). Examples ■ ■ ■ ■ ■ How many protons, neutrons, and electrons in C-12, C-13, C-14, Th-196? C-12 = 6 P, 6 N, 6 E C-13 = 6 P, 7 N, 6 E C-14 = 6 P, 8 N, 6 E Th-196 = 90 P, 106 N, 90 E