Introduction to Atomic Structure

1. Introduction to Atomic Structure Courtesy: www.lab-initio.com

2. The Atom • Tiny nucleus: diameter of about 10-13 cm. • Electrons: move about the nucleus at an average distance of about 10-8 from it. If the nucleus were the size of the ball bearing, a typical atom would be the size of the stadium.

3. Nucleus contains protons (positive charge) and neutrons (neutral). • Electrons (negative charge) surround the nucleus.

4. Nucleus • small size with extremely high density • accounts for most of atom’s mass • piece of nuclear material about the size of a pea would have a mass of 250 million tons

5. Electrons • account for most of atom’s volume • their number and arrangement account for an atom’s chemical behavior • involved when different atoms bond

6. Atomic Number • number of protons in the nucleus of an atom • the atomic number identifies the element • has the symbol, Z Mass Number • total number of protons and neutrons in the nucleus of an atom • has the symbol, A • mass number = number of protons + number of neutrons

7. Isotopes • atoms with the same number of protons but different number of neutrons

8. Isotopes are defined by two numbers, its atomic number and its mass number. • Isotopes are represented by nuclear symbols or by using hyphen notation. Nuclear Symbol Mass number (protons + neutrons) Element symbol Atomic number (number of protons) Hyphen notation Element name-mass number

9. Example: symbol for one particular isotope of sodium Mass number Element symbol Atomic number or Sodium-23 Two isotopes of sodium. Both have 11 protons and 11 electrons, but differ in the number of neutrons.

10. Molecules and Ions • Chemical bonds: forces that hold atoms together in compounds. • Covalent bond: results from sharing of electrons between different atoms; the resulting collection of atoms is called a molecule. • Simplest method to represent a molecule is a chemicalformula. • Chemical formula: the symbols for the elements are used to indicate the types of atoms present and subscripts are used to indicate the relative numbers of atoms. Chemical formula for methane: CH4

11. Different representations of molecules. Structural formula for methane. Individual bonds are shown. May or may not indicate the actual structure of the compound. Ball-and-stick model of methane. Space-filling model of methane. This type of model shows the relative sizes of atoms as well as their relative orientation in the molecule.

12. A second type of bond results from attractions between ions. • An ion is an atom or groups of atoms that has a net positive or negative charge resulting from a loss or gain of electrons. • The best known ionic compound is common table salt, or sodium chloride, which forms when neutral sodium and chlorine react.

13. Consider the formation of the sodium ion below. • When an ion forms, electrons are transferred from one atom to another (the number lost must equal the number gained). • With one electron transferred, sodium has 11 protons and only 10 electrons so has a net 1+ charge – it has become a positive ion. • A positive ion is called a cation. • Represented in shorthand form: Na → Na+ + e-

14. Consider the formation of the chlorine ion below. • If an electron is added to chlorine, it has 17 protons and 18 electrons which produce a net 1- charge – it has become a negative ion. • A negative ion is called an anion. • Represented in shorthand form: Cl + e- → Cl-

15. Anions and cations = opposite charges = attraction • Ionic bonding: force of attraction between oppositely charged ions. • Solid composed of oppositely charged ions is called an ionic solid or a salt.

16. Polyatomic ion: an ion composed of two or more atoms. • Can form ionic bonds with other simple ions or with other polyatomic ions. Ammonium nitrate

17. Introduction to the Periodic Table

18. Periodic Table: a chart showing all the elements arranged in columns with similar chemical properties. • Letters in the boxes are the symbols for the elements. • The number above the symbol is the atomic number (number of protons).

19. Metals • Most of the elements are metals (found to the left of the staircase in the periodic table). • Characteristic physical properties: • efficient conduction of heat and electricity • malleability (they can be hammered into thin sheets) • ductility (they can be pulled into wires) • (often) a lustrous appearance • all are solids at room temperature except mercury • Tend to lose electrons to form positive ions.

20. Nonmetals • Found to the right of the staircase on the periodic table. • Encountered mostly in the form of compounds or mixtures of compounds. • Poor conductors of heat and electricity. • Neither malleable nor ductile. • Many are gases at room temperature and many are solids (Bromine is the only liquid). • Usually exist as molecules in their elemental form. • Tend to gain electrons to form negative ions.

21. Metalloids • Grouped along the staircase. • Properties lie between those of metals and nonmetals. • Tend to be semiconductors; they conduct electricity, but not nearly as well as metals. • Generally, metalloids behave as nonmetals, both chemically and physically.

22. Periodic Table • Horizontal rows are called periods. • Vertical columns are called groups or families. • Elements in same column have similar chemical properties. • Group 1 elements are called the alkali metals. • very active elements • readily form ions with a 1+ charge

23. Group 2 elements are called the alkaline earth metals. • reactive and form ions with a 2+ charge • Groups 3 through 12 are called the transition metals. • form colored ions • variable oxidation states

24. Group 17 are called the halogens. • very reactive nonmetals • form ions with 1- charge • exist as diatomic molecules • Group 18 are called the noble gases. • exist as monatomic (single-atom) gases • little chemical reactivity

25. The elements in the two long rows below the main body of the periodic table are called the inner transition elements. • Elements 58-71 are called the lanthanide elements because they follow lanthanum (Z = 57). • Elements 90-103 are called the actinide elements because they follow actinium (Z = 89).