Chapter 2 I CHEMICAL ELEMENTS AND COMPOUNDS A Elements most basic form of matter substance that can t be broken down to other substances by chemical reactions cannot be broken down further by chemical reaction 92 elements essential elements elements that are essential to live a healthy life 20 25 of elements Carbon Hydrogen Oxygen Nitrogen B Compounds substance consisting of 2 or more elements in a fixed ratio 2H2 2O2 2H2O Think of a cake recipe 2 eggs 2 cups flour etc Ratios don t change if you want to bake multiple cakes A compound has characteristics different from those of its elements II ATOMS AND MOLECULES A Subatomic particles An atom is the smallest unit having the properties of an element these tiny bits of matter are composed of even smaller parts called subatomic particles 1 proton positive charge 1 2 neutron no charge 0 3 electron e negative charge 1 Protons and neutrons are packed together tightly in a dense core or atomic nucleus at the center of an atom protons give the nucleus a positive charge The electrons form a sort of cloud of negative charge around the nucleus and it is attraction between opposite charges that keeps the electrons in the vicinity of the nucleus The neutron and proton are almost identical in mass B Atomic number and mass number for my shorthand All atoms of a particular element have the same number of protons in their nuclei This number of protons which is unique to that element is called the atomic number Protons must be balanced by an equal number of electrons The atomic number tells us the number of protons and also the number of electrons in an electrically neutral atom The mass number is the sum of protons plus neutrons in the nucleus of an atom Because the atomic number indicates how many protons there are we can determine the number of neutrons by subtracting the atomic number from the mass number The mass number is an approximation of the total mass of an atom called its atomic number 1 Atomic number protons e2 Mass number protons neutrons C Isotopes 1 Isotope elements with the same of protons but different of neutrons All atoms of a given element have the same number of protons but some atoms have more neutrons than other atoms of the same element and therefore have a greater mass These different atomic forms of the same element are called isotopes 2 Characteristics a occur naturally as mixtures of elements In nature an element occurs as a mixture of isotopes b isotopes of the same element react EXACTLY the same way chemically WHY Radioactive isotopes the nucleus decays spontaneously giving off particles and energy When the decay leads to a change in the number of protons it transforms the atom to an atom of another element c half life fixed rate of decay breakdown i geological dating date relics of past life ii tracers useful tracers to follow atoms through metabolism iii clinical treatment diagnostic tools to help diagnose D Electron e shells store energy in distinct quanta Only electrons are directly involved in the chemical reactions between atoms Energy is defined as the capacity to cause change for instance by doing work Potential energy is the energy that matter possesses because of its location or structure Matter has a natural tendency to move to the lowest possible state of potential energy water runs downhill and to restore the potential energy of a reservoir work must be done to elevate the water against gravity The electrons of an atom have potential energy because of how they are arranged in relation to the nucleus The negatively charged electrons are attracted to the positively charged nucleus It takes work to move a given electron further away from the nucleus so the more distant an electron is from the nucleus the greater its potential energy 1 e have potential energy that exists in distinct quanta 2 only e participate in chemical reactions 3 e want to be close to nucleus but far from each other An electron s energy level is correlated with its average distance from the nucleus Electrons are found in different electron shells each with a characteristic average distance and energy level The first shell is closest to the nucleus and electrons in this shell have the lowest potential energy Electrons in the second shell have more energy and electrons in the third shell even more energy An electron can change the shell it occupies but only by absorbing or losing an amount of energy equal to the difference in potential energy between its position in the old shell and that in the new shell When an electron absorbs energy it moves to a shell farther out from the nucleus When an electron loses energy it falls back to a shell closer to the nucleus and the lost energy is released to the environment as heat E Electron orbital capacity dictates the number of possible bonds formed The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell We call those outer electrons valence electrons and the outermost electron shell the valence shell Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior An atom with a completed valence shell is unreactive that is it will not interact readily with other atoms Helium neon and argon all have full electron shells these elements are said to be inert meaning chemically unreactive 1 electron orbit 3D space where e found 90 of time no more than 2 electrons can occupy a single orbital 2 energy levels and electron capacity a 1st energy level has 2 electron capacity b 3 2nd energy level has 8 electron capacity Unpaired valence electrons correlate with the reactivity of an atom Each concentric circle represents only the average distance between an electron in that shell and the nucleus F 1 Atoms combine by chemical bonds to form novel molecules Ionic bonds Ion charged atom e g Na or molecule e g NH4 Anion negatively charged ion via e gain Cation positively charged ion via e loss Ionic bond formed after complete transfer of a valence e from a donor to an acceptor because of the opposite charges of anions and cations they attract each other this attraction is called an ionic bond the transfer of an electron is not the formation of the bond rather it allows a bond to form because it results in two ions of opposite charge Any two ions of opposite charge can form an ionic bond Compounds formed by ionic bonds are called ionic compounds or salts 2 Covalent formed by sharing valence electrons sharing a pair of valence electrons by