1. Know the atom is composed of 3 subatomic particles the electron, the proton, and the neutron

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1 Chemistry 106 Lecture 1: Chemical Foundations I: The Structure of the Atom 4/03/18 Learning Objectives By the end of this session, you should be able to 1. Know the atom is composed of 3 subatomic particles the electron, the proton, and the neutron 2. Know the relative mass and charge of the 3 subatomic particles 3. Know opposite charges are attracted to one another and like charges repel one another 4. Recognize the importance of charge neutrality the number of electrons orbiting the nucleus must match the number of protons in the nucleus 5. Know electrons will occupy a given region of space around the nucleus. This region of space is known as an orbital and only 2 electrons can ever be in a given orbital 6. Know electrons will occupy orbitals of lowest energy first in trying to match the number of protons in the nucleus 7. Know there is only one orbital (2 electrons) in the 1 st principal energy level, four orbitals in the 2 nd principal energy level (8 electrons), and 9 orbitals in the 3 rd principal energy level (18 electrons). Combined with the fact the 4s orbital is lower in energy than the 3d orbitals, this increasing number of orbitals gives the periodic table its look 8. Know the importance of the noble gas electronic configuration essentially why the noble gases are the only elemental forms that exist as individual atoms and why other atoms will gain, lose, or share electrons to be like them 9. Know what a chemical compound is and that the 2 main types of bonding giving rise to chemical compounds are ionic and covalent bonding 10. Recognize the importance of charge neutrality in any chemical compound the overall charge is zero; that is, the number of electrons perfectly matches the number of protons 11. Define the outermost electrons as the valence electrons and appreciate their role in bonding 1

2 Atom derives from the Greek atomos, meaning not to cut. Initially put forward by Democritus, ca BC, though it took well into the 20 th century to unravel the fundamental structure of this basic unit of chemistry Atoms are not transformed in [non-nuclear] chemical reactions o Chemical reactions occur when atoms change the way they are connected together in a sample of stuff Drugs are chemical compounds chemical compounds consist of 2 or more types of atom bonded to one another o Atoms bond or connect to one another through interactions of their outer electrons While the atom is the fundamental building block of chemistry, it is composed of subatomic particles protons and neutrons in the nucleus, with electrons orbiting around the nucleus o Notice that the atom is mostly empty space electrons are much smaller than protons and neutrons, yet occupy a much larger volume 2

3 One of the fundamental principles of science is that like charges repel and opposite charges attract, and it is very important to understanding chemistry the electron is negatively charged and is attracted to the positively charged protons in the nucleus Question: If like charges repel one another, why doesn t the nucleus fly apart? Answer: The neutron nature s nuclear glue Notice there is an optimum ratio of neutrons to protons to have a stable [nonradioactive] species: 3

4 When considering atomic structure bear in mind overall electrical neutrality must be maintained this is a critical chemistry concept The periodic table reads like an English newspaper you read it left to right, top to bottom. It is organized from lightest to heaviest; that is, hydrogen (H) has 1 proton, helium (He) has 2 protons, lithium (Li) 3 protons, etc. To maintain electrical neutrality, hydrogen must have 1 electron orbiting the nucleus, helium 2 electrons, etc. As we shall see, it is how electrons are placed around the nucleus that determines the stability (or reactivity) of the atom Again for emphasis in a pure elemental form where only one type of atom is present, the number of electrons orbiting the nucleus must match the number of protons in the nucleus 4

5 The question then becomes Just where are these electrons located that are orbiting the nucleus? The distribution of electrons is quantized, which is to say there are discrete steps of increasing energy between electron locations. Simply put, there are some places electrons can be, and some places where they cannot be Electrons are found in orbitals, distinct regions of space around the nucleus The numbers and types of orbitals are given by solving the Schrödinger wave equation, which solves for the number of a given type of orbital and their relative energies o While the Schrödinger wave equation and quantum mechanics in general are one of the great triumphs of science, they have caused tremendous pain and suffering to the undergraduate chemistry student I would not be so cruel as to have you consider them further. The consequences are important however The distribution of orbitals 5

6 Below are the relative energies for the subshell energy levels. Note there may be more than 1 orbital in a given energy subshell (there are 3 for the p designation) all that have the same energy associated with them 6

7 Critical Chemistry Concept the orbital solutions to where electrons can be placed around the nucleus have energies associated with them. The same solutions exist for all atoms. As you might expect, electrons go into the lowest energy orbitals first. For hydrogen, once an electron is placed in the 1s orbital, the number of electrons equals the number protons and no more electrons are required Critical Chemistry Concept orbitals can contain 2 and only 2 electrons. For helium, once a second electron is placed in the 1s orbital, the number of electrons equals the number protons and no more electrons are required. Critical Chemistry Concept There is tremendous stability in completely filling a principal energy level (or the p sublevel) this gives rise to the noble gases and determines the chemical bonding behavior of the elements This is referred to as a noble gas electronic configuration and all atoms want to achieve this distribution of electrons. Consider for example the formation of table salt (NaCl) from elemental sodium (Na) and elemental chlorine: Notice that after the electron is transferred, sodium has the electron look of the [stable] noble gas neon, and chlorine has the [stable] look of the noble gas argon When an electron is transferred between atoms to solve the problem of obtaining a noble gas look, an ionic compound is formed, since the resulting atoms are now charged. We will have much more to say about charge and chemical compounds as it is central understanding their properties 7

8 Taking one more look at the periodic table, notice how those elements on either side of the noble gases are especially reactive Since some of the electrons distributed around a nucleus are buried in low energy orbitals close to the nucleus, they are unavailable for bonding. We are interested in those that are involved in bonding so we introduce the concept of valence electrons Critical Chemistry Concept Valence Electrons: Those electrons in the outermost principal energy level that are involved in bonding. There is a specific way of representing atoms called the Lewis dot symbol that focuses on the valence electrons of the main group elements 8

9 Lewis Dot Symbols Simply include the valence electrons around the symbol for the atom, recognizing electrons will stay away from one another until they have to be paired; there are 4 orbitals to place electrons (except for hydrogen), and each orbital can only hold 2 electrons So what happens when non-metal atoms (those shaded light blue in the periodic table above) get together to solve the noble gas electronic configuration? Since both can best solve the problem of obtaining a noble gas electron distribution by obtaining electrons, neither is willing to give them up as a result they share their electrons, which we call a covalent bond. The result is a molecule. Some Examples Water: H 2 O Methane (natural gas): CH 4 Ammonia: NH 3 Much more on this next time! 9