Aim: How are electrons arranged around a nucleus? Do Now: Take out a reference table and a calculator. Calculate the number of neutron the an atom of the following elements will have. Chlorine-37 Uranium-235 Iodine-127

Answers to the Do Now

***Reminder – Calculating Average Atomic Mass*** 1) Rubidium is a soft, silvery-white metal that has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?

Another Problem 2) Uranium is used in nuclear reactors and is a rare element on earth. Uranium has three common isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is 0.71%, and the abundance of 238U is 99.28%, what is the average atomic mass of uranium?

How can we tell the number of electrons in an atom? In an electrically neutral atom (does not have a net charge), the atom will have the same number of protons and electrons. Example: An atom of Helium has 2 protons. Therefore, if it is electrically neutral, it will have 2 electrons. How many electrons does an atom of chlorine have? How many electrons does an atom of gold have?

What about atoms that are not electrically neutral? It depends on whether the atom has a positive or negative charge. Atoms with charges are called ions. If an atom has a positive charge, it has more protons than electrons. If an atom has a negative charge, it has more electrons than protons.

How do we write a symbol if it is an ion? The actual symbol remains the same. If the atom is charged, write the symbol and the charge on the upper right. Example: Mg2+ has a positive charge. How many protons and electrons does it have? Example: Cl- has a negative charge. How many protons and electrons does it have?

Where do we find electrons in an atom? Remember that electrons are found outside the nucleus in orbitals. Orbitals form a series of energy levels in which electrons may be found. Each electron has its own distinct amount of energy that corresponds to the energy level it occupies. The electron configuration for an element can be found on the periodic table of the elements.

What is the electron configuration for… Magnesium? Copper? Hafnium? Gold?

What if an atom is excited? When the electrons in an atom occupy the lowest available orbitals, they are said to be in the ground state. This is the information you can find on the periodic table of elements. When an atom is subjected to stimuli such as heat, light, or electricity, an electron may absorb the energy and temporarily move to a higher orbital. This unstable condition is called an excited state. An excited electron will quickly return to its ground state emitting the same amount of energy it absorbed.

Recognizing Electron Configurations of Excited Atoms When an electron is excited, its configuration has to illustrate that it is at a different level. Example: Na-2-7-2 is one of many possible excited arrangement for an excited atom of sodium. How will you know when an electron is in an excited state by looking at the configuration? You can use the periodic table of elements. Count the number of electrons in the configuration. Use that information to find the element on the table. Then compare to see if it is in the ground state or excited state.

Are the following ground state or excited? Examples Are the following ground state or excited? 2-8-8-2 2-7-6 2-8-18-8 2-8-17-2

Activity Answer the regents review questions on the sheet you were given in class.

Homework #