Elements and Atoms LECTURE 2024.pdf

Elements & Atoms DR. RAMELA R.VILLENA Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early nineteenth century, championed by...

Elements & Atoms DR. RAMELA R.VILLENA Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early nineteenth century, championed by John Dalton. Dalton’s Postulates 1) Each element is composed of extremely small particles called atoms. Dalton’s Postulates 1) All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements. Dalton’s Postulates 1) Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions.  Law of Conservation of Mass The total mass of substances present at the end of a chemical process is the same as the mass of substances present before the process took place. This law was one of the laws on which Dalton’s atomic theory was based. Lavoisier Dalton’s Postulates 1) Atoms of more than one element combine to form compounds; a given compound always has the same relative number and kind of atoms. My fourt posulate is an extension of a law formulated by the French chemist, Joseph Proust (1754-1826), in 1799. Proust’s Law of Definite Proportions states: Different samples of the same compound always contain its con- stituent elements in the same proportion by mass. If we analyzed any number of samples of sodium chloride, NaCl, collected from different sources, we would find in each and every sample the same ratio BY MASS of sodium (Na) to chlorine (Cl). Thus, if the ratio of the MASSES of different elements in a compound is fixed, the ratio of the ATOMS of these elements in the compound must also be constant. Discovery of the Nucleus Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.  Structure of the Atom: The Proton & Nucleus Rutherford’s experiment for measuring scattering of a particles by a thin metal foil: a Rutherford used a particles to probe atomic structure. Thin gold and other metal foils served as targets for a a particles. See the illustration below (from S. Zumdahl slides). E. Rutherford Most of the a particles penetrated the metal foils with little or (1871-1937) a no deflection. However, an a particle would be deflected at a large angle or back toward the source!  QUICK TIME VIDEO (1908 Nobel Prize in Chemistry) a particle velocity ~ 1.4 x 107 m/s (~5% speed of light) 1. atoms positive charge is concentrated in the nucleus 2. proton (p) has opposite (+) charge of electron 3. mass of p is 1840 x mass of e- (1.67 x 10-24 g) Structure of the Atom: The Proton & Nucleus Rutherford formulated his model of the atom, based on the results of his a -scattering experiments: Mostly empty space, which would explain why most a a particles passed through the foil undetected. Atom’s positive charges concentrated in the nucleus, a dense E. Rutherford a central core within the atom. This would explain why those a (1871-1937) particles that came near or directly at the nucleus experienced large deflections or reversed directions (repulsion of positive charge by positive charge). We now know these positively charged particles in the nucleus as protons: Mass of proton = 1.67262 x 10-24 g -- about 1840x the mass of a an electron. Typical atomic radius is about 100 pm (pm = picometers). a Radius of atomic nucleus ONLY about 5 x 10-3 pm !!! IT’S LIKE PLACING A MARBLE IN THE MIDDLE OF THE HOUSTON ASTRODOME!!! What is the charge on the particles that form the beam? a. –1, because the beam consists of beta particles. b. +2, because the beam consists of alpha particles. c. +1, because the beam consists of beta particles. d. –2, because the beam consists of alpha particles. What is the charge on the particles that form the beam? a. –1, because the beam consists of beta particles. b. +2, because the beam consists of alpha particles. c. +1, because the beam consists of beta particles. d. –2, because the beam consists of alpha particles.  The Nuclear Atom Since some particles were deflected at large angles, Thomson’s model could not be correct. The Nuclear Atom Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. Most of the volume is empty space. Atoms are very small; 1 – 5 Å or 100 – 500 pm. Other subatomic particles (protons and neutrons) were discovered. What is the approximate diameter of the nucleus in units of pm? a. 100 pm b. 1 pm c. 10–2 pm d. 10–4 pm What is the approximate diameter of the nucleus in units of pm? a. 100 pm b. 1 pm c. 10–2 pm d. 10–4 pm  Subatomic Particles Protons (+1) and electrons (–1) have a charge; neutrons are neutral. Protons and neutrons have essentially the same mass (relative mass 1). The mass of an electron is so small we ignore it (relative mass 0). Protons and neutrons are found in the nucleus; electrons travel around the nucleus. Atomic Mass Atoms have extremely small masses. The heaviest known atoms have a mass of approximately 4 × 10–22 g. A mass scale on the atomic level is used, where an atomic mass unit (amu) is the base unit. 1 amu = 1.66054 × 10–24 g  Atomic Weight Measurement Atomic and molecular weight can be measured with great accuracy using a mass spectrometer. Masses of atoms are compared to the carbon atom with 6 protons and 6 neutrons (C-12).  Symbols of Elements Elements are represented by a one or two letter symbol. This is the symbol for carbon. All atoms of the same element have the same number of protons, which is called the atomic number, Z. It is written as a subscript BEFORE the symbol. The mass number is the total number of protons and neutrons in the nucleus of an atom. It is written as a superscript BEFORE the symbol.  Isotopes Isotopes are atoms of the same element with different masses. Isotopes have different numbers of neutrons, but the same number of protons.  ATOMIC NUMBER (Z) 8 O The number of protons in the nucleus of an atom of each element 16 In a neutral atom: # of protons = # of electrons The element considered above is oxygen. How many protons does a neutral oxygen atom contain? How many electrons? Where are the protons located in the oxygen atom?  ATOMIC MASS 8 Mass Number O (A) The total number of protons + neutrons in the nucleus of an atom of each element 16 In a neutral atom: mass number = Where are neutrons located? # of protons + # of neutrons What is their charge? A = Z + # neutrons  An atom refresher Matter is anything that takes up space and has mass. All matter is made of atoms Atoms are the building blocks of matter, sort of how bricks are the building blocks of houses.  An atom refresher An atom has three parts: Proton = positive Neutron = no charge Electron = negative The proton & neutron are found in the center of the atom, a place called the Picture from nucleus. http://education.jlab.org/qa/atom_model_03.gi f The electrons orbit the nucleus.  What are elements? Elements are the alphabet to the language of molecules. To make molecules, you must have elements. Elements are made of atoms. While the atoms may have different weights and organization, they are all built in the same way. Graphic from http://education.jlab.org/atomtour/fact2.html  More about Elements.. Elements are the building blocks of all matter. The periodic table is a list of all of the elements that can build matter. It’s a little like the alphabet of chemistry. The periodic table tells us several things… Periodic Table Atomic Number: Number of protons 8 and it is also the O number of electrons in an atom of an element. Elements Name Oxygen Atomic Mass/Weight: 16 Number of protons + neutrons. Element’s Symbol: An abbreviation for the element.  Atom Models There are two models of the atoms we will be using in class. Bohr Model Lewis Dot Structure  Bohr Model The Bohr Model shows all of the particles in the - atom. In the center is circles. + Each circle represents a + single neutron or proton. Protons should have a plus or P written on - them. Neutrons should be blank or have an N. In a circle around the nucleus are the electrons. Electrons should have a minus sign or an e.  Electrons have special rules…. You can’t just shove all of the electrons into the first orbit of an electron. Electrons live in something called shells or energy levels. Only so many can be in any certain shell.  2n 2 Nucleus 1st shell 2nd shell 3rd shell Adapted from http://www.sciencespot.net/Media/atomsfam.pdf  Electrons have special rules…. You can’t just shove all of the electrons into the first orbit of an electron. Electrons live in something called shells or energy levels. Only so many can be in any certain shell. The electrons in the outer most shell of any element are called valance electrons.