Isotopes, Ions, and Electrons

A Brief Recap. . . So far, we have seen that atoms consist of. . . A very small, very dense nucleus containing protons (positively charged) and neutrons (neutral charge) -- protons and neutrons each have a mass of 1 amu An electron cloud containing negatively-charged electrons -- electrons have a mass of 0 amu The number of protons determines the type of atom (or element) and is also called the atomic number The atomic mass is equal to the number of protons plus the number of neutrons Atoms are neutral in charge, so the number of electrons equals the number of protons

Isotopes Even though all atoms of the same element have the same number of protons, NOT all atoms of the same element have the same number of neutrons -- therefore, not all atoms of the same element have the same atomic mass Isotopes are two atoms of the same element with different atomic masses The atomic mass that you see on the periodic table is an isotopic average -- in other words, the masses of all of the atoms of that element in the universe were averaged to find the average atomic mass

Some Examples of Isotopes A perfect example of isotopes are the isotopes of carbon, also called Carbon-12 and Carbon-14 -- Carbon-12 has a mass of 12 amu and is stable -- it has 6 protons and 6 neutrons in its nucleus -- Carbon-14 has a mass of 14 amu and is radioactive -- it has 6 protons and 8 neutrons in its nucleus -- most of the carbon atoms in the world are of stable Carbon-12, so the isotopic average (or the mass you see on your periodic table) is very close to 12 amu (actually 12.011) Different isotopes of the same element have different properties -- radioactive isotopes are called radioisotopes

Naming and Writing Isotopes We name isotopes by writing the name (or symbol) of the element followed by the atomic mass of the element -- For example, Carbon-12, or C-12 We can also write atoms and isotopes in the nuclear chemistry format -- For example, Carbon-12 is written as C -- notice that the atomic mass is written as a superscript to the upper left of the symbol and the atomic number is a subscript to the left of the symbol The isotopes of hydrogen, (H-1, H-2, and H-3) have special names -- H-1 is called hydrogen, H-2 is called deuterium, and H-3 is called tritium 12 6

Uses of Isotopes Isotopes (and especially radioisotopes) are very useful in science For example, carbon-14 is radioactive, and the amount of time carbon-14 takes to decay is used to determine the ages of rocks and fossils through a process called carbon dating Scientists also build compounds with radioactive isotopes and learn about how molecules break down by following the path of the radioisotopes -- this is how we discovered that DNA was genetic material!

The Location of Electrons So far we know five important things about electrons: 1) Electrons are negatively charged 2) Electrons have no mass (0 amu) 3) Electrons are located in the electron cloud 4) Atoms are neutral, so the number of protons equals the number of electrons 5) Electrons orbit at specific distances from the nucleus of the atom, and these distances are called energy levels or shells 6) We can never know the exact location of any electron at a given moment in time The question is, even though we don’t know exactly where they are, can we figure out how much energy electrons have?

Lazy Electrons Electrons are lazy creatures and abide by three major rules: 1) Electrons will remain in the lowest energy level possible (also called the ground state) -- if they do gain energy and move to a higher energy level, they will quickly release that energy and fall back down to their original energy level 2) Electrons are negative and are attracted to the positively-charged nucleus, so they want to be as close to the nucleus as possible, and the lowest energy levels are closest to the nucleus 3) Electrons repel other electrons, so not too many of them can be in the same energy level at a time

The Bohr Model for Electrons The Bohr Model (devised by Neils Bohr in 1911) maps out the location of electrons in energy levels -- as we know it today, the Bohr model only works for the element hydrogen (the simplest element) -- technically, we can use the Bohr model for any element with up to 18 electrons (and that’s what we will do in this class) The Bohr Model follows two simple rules: 1) The lowest (1st) energy level can only hold 2 electrons 2) All higher energy levels can hold up to 8 electrons

Drawing a Bohr Model Let’s say we want to draw a Bohr Model for Sodium (Na). Sodium has an atomic number of 11, so it has 11 protons and 11 electrons We start by drawing the nucleus We then add electrons to the lowest energy levels first, until they are full We stop after we have added all 11 electrons

Practice With Bohr Models Draw a Bohr Model for Boron (atomic number = 5)

Valence Electrons Electrons in an atom that have the most energy (those at the highest energy level) are called valence electrons -- valence electrons are important because they are the electrons involved in chemical bonding -- by drawing a Bohr Model, we can figure out how many valence electrons an element has For example, How many valence electrons does sodium have? How many valence electrons does boron have? Atoms are “happy” when their valence shells (the shell with the valence electrons) are full (either having 2 or 8 electrons)

Ions So far, we have seen that atoms are neutral in charge and the number of protons equals the number of electrons Sometimes, an atom will gain or lose an electron (or two or three) and pick up a positive or negative charge -- this is mainly due to collisions with other atoms and bonding -- atoms tend to lose or gain electrons in order to make a full and happy valence shell, even if it involves losing electrons so that the lower energy shell is the valence shell Ions are atoms that have gained or lost an electron and have thus picked up a charge – they are no longer neutral -- in ions, # of protons ≠ # of electrons

Cations When atoms LOSE an electron (or electrons): -- the atom now has MORE PROTONS than electrons -- the atom is POSITIVELY charged We call positively-charged ions cations Cations usually form from atoms with only one or two valence electrons -- it is very easy for these elements to lose an electron or two and drop down to a full valence shell at a lower energy level Cations are very common in elements like sodium, hydrogen, and magnesium

Anions When an atom GAINS one or more electrons: -- the atom now has MORE ELECTRONS than protons -- the atom has NEGATIVE charge We call negatively-charged ions anions Anions usually form from atoms with six or seven valence electrons -- it is much easier for these atoms to gain an electron or two to complete its valence shell Anions are very common in elements like sulfur, chlorine, and bromine

Writing and Naming Ions We name ions based on the name (or symbol) of the element followed by the charge -- For example, a sodium ion with a positive charge is either called “Sodium plus one” or simple, “Na plus” We write ions by writing the symbol of the element and then writing the charge in a superscript to the upper right -- For example, Na+, Cl-, O2- -- notice that we do NOT write out the number “1” on the charge

Using our periodic table and what we know about atomic number, mass, isotopes, and electrons, we can fill in the following chart: Element Symbol Atomic Number Mass # of protons # of neutron # of electron charge 8 Potassium 39 +1 Br 45 -1 30 65

Extending the Bohr Model We can also write the Bohr Model for ions 1 – 18, as long as we use the charge to figure out how many electrons each ion has For example, write the Bohr Model for F- (fluorine -1)