Atoms and the Periodic Table

Atomic Structure ATOM: the smallest particle that has the properties of an element. From the early Greek concept of the atom to the modern atomic theory, scientists have built on and modified existing models of the atom.

ATOM BASICS Atoms are composed of a positively charged nucleus surrounded by an electron cloud. Nucleus (99% of atom’s mass): uncharged neutrons and positively charged protons. Electron cloud: negatively charged electrons in constant motion creating a “cloud” like a fan.

DEMOCRITUS In 400 B.C., this Greek philosopher suggested that the universe was made of indivisible units. “Atom” (atomos)– Greek word meaning “unable to be divided”

Do these ideas still hold true today? JOHN DALTON In 1808, this English schoolteacher proposed his own atomic theory that became widely accepted. Dalton’s Atomic Theory: All elements are made of tiny atoms. Atoms cannot be subdivided. Atoms of the same element are exactly alike. Atoms of different elements can join to form molecules. Do these ideas still hold true today?

As it turns out, the atom can be divided into subatomic particles. Thompson and Millikan are given credit for the first discoveries relating to electrons. Millikan Thompson http://www.shs.d211.org/science/faculty/hlg/history%20of%20atom%20sanjay%20karan/millikanpage.htm

Ernest Rutherford Rutherford discovered the positively charged nucleus (GOLD FOIL EXPERIMENT)

NIELS BOHR In 1913, this Danish scientist suggested that electrons “orbit” the nucleus. In Bohr’s model, electrons are placed in different energy levels based on their distance from the nucleus.

Dmitri Mendeleev: 1834-1907 1869: created first periodic table of elements. Arranged elements in order of increasing atomic mass.

Henry Moseley 1887-1915 One of Rutherford’s students. 1914: Arranged the elements in order of increasing atomic number (responsible for TODAY’S periodic table).

Electrons are like books in a book case Electrons are like books in a book case. They can only exist on certain levels.

Only certain orbits are allowed

The Bohr Atom was a “Solar System” model.

MODERN ATOMIC MODEL By 1925, Bohr’s model of the atom no longer explained all observations. Bohr was correct about energy levels, but wrong about electron movement. Electrons occupy the lowest energy levels available. Energy increases as distance from the nucleus increases. Electrons move in patterns of “wave functions” around the nucleus. It is impossible to know an electrons velocity and location at any moment in time (Schrödinger).

This is very difficult stuff! Schrödinger In 1926 Schrodinger proposes an equation that gives the probability of finding an electron at any place in the atom. Don’t worry, this won’t be on the test.

ORBITALS s orbital p orbitals ORBITAL: the regions in an atom where there is a high probability of finding electrons. s is the lowest energy orbital, and p is slightly higher s orbital p orbitals “Dumbbell” shaped “Spherical”

d and f are the next two orbitals d and f are the next two orbitals. They occupy even higher energy levels and take on more complex shapes than s & p

VALENCE ELECTRONS Carbon 4 valence electrons Electrons in the outermost energy level are called VALENCE ELECTRONS. Valence electrons determine how an atom will act in a chemical reaction. Atoms with equal numbers of valence electrons have similar properties. We will learn how to determine the # of valence electrons in an atom later in this unit.

Organization of the Periodic Table PERIODICITY: regular variations (or patterns) of properties with increasing atomic number. Both chemical and physical properties vary in a periodic (repeating) pattern.

PERIOD: horizontal row of elements on P.T.

GROUP (FAMILY): vertical column of elements on P.T.

C Periodic Key 6 Atomic number (Z) Element’s symbol Carbon # of electrons = # of protons (in a neutral atom) # of neutrons = A-Z # of protons = Z 6 C Carbon 12.011 Atomic number (Z) Element’s symbol Element’s name Atomic mass (A)

Determining # P+, N, and E- from chemical symbols: Mass # Example 1: atom # protons = 6 #electrons= 6 #neutrons= 14-6 = 8   Example 2: ion # protons = 7 #electrons= 10 #neutrons= 15-7 = 8 No net charge 14 C 6 Atomic # 15 N3- 7 Net charge of -3

ISOTOPES Isotopes are atoms that have the same # of protons, but a different # of neutrons. Example: Carbon-12 vs. Carbon-14 12C Mass # = 12; Atomic # = 6 (6P, 6E, 6N) 14C Mass # = 14; Atomic # = 6 (6P, 6E, 8N)

IONS Ionization: the process of adding or removing electrons from an atom or group of atoms. An ion has a net electric charge. Cation: ion with a positive charge. Ex: Na+ Anion: ion with a negative charge. Ex: O2-

Valence electrons & electron dot diagrams Review: The valence electrons are the outermost electrons in an atom. These are the electrons that are involved when there is a chemical reaction. Looking at a periodic table, you can quickly determine how many valence electrons an atom has by what column it is in.

# of Valence electrons 2 1 2 3 4 5 6 7 8

ELECTRON DOT DIAGRAMS: visual representations of elements and their valence electrons Standard form: 3 6 R 4 1 7 2 5 8 Order of electron/dot placement element symbol Example: O 6 valence electrons Oxygen

How many dots are in an electron dot diagram of chlorine? 1 7 17 35 Cl Chlorine and all of the other halogens (column 17) have seven valence electrons.

Review Questions…

How many protons, electrons and neutrons are in one atom of oxygen-17? 17 p, 17 e, 17 n 17 p, 17 e, 1 n 8 p, 8 e, 8 n 8 p, 8 e, 9 n While most oxygen atoms have a mass of 16 g/mol, oxygen-17 is an isotope with a mass of 17 g/mol. The number of protons in an element is the same for every atom of that element. # electrons = # protons if element has no charge.

The periodic law states that when elements are arranged in order of increasing __________ _________, similarities in their properties occur in a regular pattern. Atomic mass Atomic number Atomic radius Moseley created the modern periodic table when he determined that elements should be placed in order of increasing atomic number (# of protons). It’s a shame that WWI took the life of this brilliant scientist when he was so young.

Carbon-12 and carbon-14 are isomers isotopes radioactive elements different elements Isotopes are atoms of the same element with different numbers of neutrons, and therefore different atomic masses. While carbon-14 is used in radioactive dating, carbon-12 has a more stable nucleus and therefore is not used in this capacity.

How many protons, electrons and neutrons are in O2-? 10 p, 8 e, 8 n 8 p, 8 e, 8 n 8 p, 10 e, 8 n 8 p, 6 e, 8 n Every atom or ion of oxygen has 8 protons. The net charge of -2 indicates that the ion has two more negative charges (electrons) than positive charges (protons) All atoms and ions of standard oxygen-16 have 8 neutrons.