Example: What is the number of electrons in an atom that has 3 protons and 4 neutrons? A. 3. B. 5. C. 7. D. 10.

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1 Structure of atom: PROTONS Protons are located in the nucleus of an atom. They carry a +1 electrical charge and have a mass of 1 atomic mass unit (u). NEUTRONS Neutrons are located in the nucleus of an atom. They carry no electrical charge and have a mass of 1 atomic mass unit (u). ELECTRONS Electrons are located outside the nucleus of an atom. They carry a - 1 electrical charge and have a mass of 1/1836 atomic mass unit (u). They move rapidly around the heavy nucleus ATOMIC NUMBER OF AN ATOM The atomic number of an atom is equal to the number of protons in the nucleus of the atom. MASS NUMBER OF AN ATOM The mass number of an atom is equal to the sum of the number of protons and neutrons in the nucleus of the atom. Example: What is the number of electrons in an atom that has 3 protons and 4 neutrons? A. 3. B. 5. C. 7. D. 10. A sodium metal, Mg, has A. 12 electrons and 12 protons. B. 11 electrons and 11 protons. C. 10 electrons and 12 protons. D. 12 electrons and 10 protons. PERIODICAL TABLE Group: A vertical column of elements that have similar chemical properties. Traditionally designated by a Roman numeral and a letter (either A or B) at the top of the column. Designated only by a number from 1 to 18 in a modern but as yet not universally-used designation. Period: A horizontal row of elements arranged according to increasing atomic numbers. Periods are numbered from top to bottom of the periodic table. Practice question: Which one of the following elements is a member of the halogen family? A. bromine

2 B. xenon C. hydrogen D. lithium Four elements are identified below by their group and period numbers. The nonmetal in the list is in A. Group 4A, Period 5. B. Group 1A, Period 5. C. Group 7A, Period 3. D. Group 5A, Period 4. The diagram below shows a small section of the periodic table. Na Mg Al K Rb a) Identify the group number for the 1st column b) Identify the period number for the 1st row c) State the trend in electronegativity for the elements in the 1 st row. THE QUANTUM MECHANICAL MODEL OF ELECTRON BEHAVIOR IN ATOMS. According to the quantum mechanical model of electron behavior, the precise paths of electrons moving around the nucleus cannot be determined accurately. Instead of circular orbits, the location and energy of electrons moving around the nucleus is specified using the three terms shell, subshell and orbital. Valence electrons Determine the chemical properties of the elements. Are the electrons in the highest energy level. Are related to the Group number of the element. Example: Phosphorus has 5 valence electrons 5 valence electrons P Group 5A(15) 1s 2 2s 2 2p 6 3s 2 3p 3

3 Electron Configuration An electron configuration Lists the sublevels filling with electrons in order of increasing energy. Uses superscripts to show the number of electrons in each sublevel. Example: For neon is as follows: number of electrons sublevel 1s 2 2s 2 2p 6 NOBLE GAS CONFIGURATIONS With the exception of helium, all noble gases (group VIIIA) have electronic configurations that end with completely filled s and p subshells of the highest occupied shell. These configurations are called noble gas configurations. Noble gas configurations can be used to write electronic configurations in an abbreviated form in which the noble gas symbol enclosed in brackets is used to represent all electrons found in the noble gas configuration. Example: Choose the correct electronic configuration for the following element: As A. [Ar] 4s 2 3d 10 4p 3 B. 4s 2 3d 10 4p 3 C. [Ar] 4s 2 4d 10 4p 3 D. 4s 2 4d 10 4p 3

4 Choose the correct electronic configuration for the following element: As A. [Ar] 4s 2 3d 10 4p 3 B. 4s 2 3d 10 4p 3 C. [Ar] 4s 2 4d 10 4p 3 D. 4s 2 4d 10 4p 3 ISOTOPES Isotopes are atoms that have the same number of protons in the nucleus but different numbers of neutrons. That is, they have the same atomic number but different mass numbers. Because they have the same number of protons in the nucleus, all isotopes of the same element have the same number of electrons outside the nucleus. Practice question: Calculate the atomic weight for the element boron that consists of 19.78% boron- 10, and 80.22% boron- 11. Naturally occurring lithium (Li) consists of only two isotopes, Li-6 (6.02 u) and Li- 7 (7.02 u), where the isotopic masses are given in parentheses. Use the periodic table and determine which isotope is present in the larger percentage in the natural element. A. Li-6

5 B. Li-7 C. each is present at 50% D. cannot be determined from the information available Avogadro s Number: The Mole Mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as the amount of any substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon- 12 (12C, the isotope of carbon with relative atomic mass of exactly 12 by definition. THE MOLE CONCEPT APPLIED TO ELEMENTS The number of atoms in one mole of any element is called Avogadro's number and is equal to 6.022x A one- mole sample of any element will contain the same number of atoms as a one- mole sample of any other element. One mole of any element is a sample of the element with a mass in grams that is numerically equal to the atomic weight of the element. THE MOLE CONCEPT APPLIED TO COMPOUNDS The number of molecules in one mole of any compound is called Avogadro's number and is numerically equal to 6.022x A one- mole sample of any compound will contain the same number of molecules as a one- mole sample of any other compound. One mole of any compound is a sample of the compound with a mass in grams equal to the molecular weight of the compound. Example: Calculate the number of moles of Na contained in a 10.0 g sample of Na.