Atomic Structure and the Periodic Table

Early Models of the Atom Democritus (4 th century BC) –atomos – “unable to be divided.” Democritus (4 th century BC) –atomos – “unable to be divided.” John Dalton (19 th century) – developed an atomic theory John Dalton (19 th century) – developed an atomic theory

Dalton’s Atomic Theory (pg ) All elements are composed of tiny indivisible particles called atoms. All elements are composed of tiny indivisible particles called atoms. Atoms of the same element are identical. The atoms of any one element are different from those of any other element. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

Dalton’s Atomic Theory (cont) Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds. Atoms of different elements can physically mix together or can chemically combine with one another in simple whole-number ratios to form compounds. Chemical Reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another elements as the result of a chemical reaction. Chemical Reactions occur when atoms are separated, joined, or rearranged. Atoms of one element, however, are never changed into atoms of another elements as the result of a chemical reaction.

Structure of the Atom Atoms are now known to be divisible – they can break down into subatomic particles Atoms are now known to be divisible – they can break down into subatomic particles Protons Protons Neutrons Neutrons Electrons Electrons

Structure of the Atom (cont) Electrons are negatively charged particles. Electrons are negatively charged particles. Discovered by JJ Thomson using a cathode ray tube Discovered by JJ Thomson using a cathode ray tube Anode (+) Cathode (-)

Structure of the Atom (cont) Thomson also determined the charge to mass ratio of electrons. Thomson also determined the charge to mass ratio of electrons. Robert Millikan used this knowledge and through experimentation determined the mass of electrons. Robert Millikan used this knowledge and through experimentation determined the mass of electrons.

Structure of the Atom (cont) ParticleSymbol Relative electrical charge Relative mass (mass of proton =1 Actual mass (g) Electron e-e-e-e-1-1/ x Proton p+p+p+p x Neutron nononono01

Structure of the Atom (cont) JJ Thomson hypothesized that atoms were like a “plum pudding” – protons and electrons distributed evenly throughout. JJ Thomson hypothesized that atoms were like a “plum pudding” – protons and electrons distributed evenly throughout.

Structure of the Atom (cont) Ernest Rutherford proved him wrong. Ernest Rutherford proved him wrong. He aimed positive alpha particles at gold foil. He aimed positive alpha particles at gold foil. If plum pudding model was true, the particles should have passed straight through. If plum pudding model was true, the particles should have passed straight through. However, some were deflected. However, some were deflected.

Structure of the Atom (cont) From his experiment, Rutherford hypothesized that there was a dense, positively charged center to the atom, called a nucleus. From his experiment, Rutherford hypothesized that there was a dense, positively charged center to the atom, called a nucleus. Protons and neutrons are found in the nucleus Protons and neutrons are found in the nucleus

Distinguishing between atoms Atomic Number – number of protons in an element. All elements are defined by their number of protons. Atomic Number – number of protons in an element. All elements are defined by their number of protons. Mass Number – The total number of protons and neutrons. Mass Number – The total number of protons and neutrons. # of neutrons = mass # – atomic #

Distinguishing between atoms (cont) Isotopes – Atoms that have the same number or protons but different numbers of neutrons. Isotopes – Atoms that have the same number or protons but different numbers of neutrons. Chemically alike Chemically alike Differ in some of the physical properties Differ in some of the physical properties Written as the element name followed by mass number (carbon-12, carbon-14, etc) Written as the element name followed by mass number (carbon-12, carbon-14, etc)

Distinguishing between atoms (cont) Atomic Mass Unit (amu) – 1/12 the mass of the carbon-12 atom Atomic Mass Unit (amu) – 1/12 the mass of the carbon-12 atom The mass of protons and neutrons are each 1 amu. The mass of protons and neutrons are each 1 amu. Are the atomic masses on the periodic table whole numbers? Are the atomic masses on the periodic table whole numbers?

Distinguishing between atoms (cont) Average atomic masses (found on the periodic table) are a weighted average of the isotopes. Average atomic masses (found on the periodic table) are a weighted average of the isotopes. Ex: chlorine-35 (75.77% abundance, amu) and chlorine-37 (24.23% abundance, amu) are the two isotopes of chlorine. Ex: chlorine-35 (75.77% abundance, amu) and chlorine-37 (24.23% abundance, amu) are the two isotopes of chlorine.

Distinguishing between atoms (cont) By multiplying the average mass of each isotope by its relative abundance and adding these numbers together, you can determine the average atomic mass of the isotope. By multiplying the average mass of each isotope by its relative abundance and adding these numbers together, you can determine the average atomic mass of the isotope. ( * ) + ( * ) = amu ** Notice that the avg atomic mass is closer in mass to the more abundant isotope **

Development of the periodic table Dmitri Mendeleev (mid-1800s) – arranged elements by similarities in properties and mass. Dmitri Mendeleev (mid-1800s) – arranged elements by similarities in properties and mass. Left blank spaces in the table for undiscovered elements. Left blank spaces in the table for undiscovered elements. Henry Moseley arranged the elements by atomic number (number of protons), which is the current form of the table. Henry Moseley arranged the elements by atomic number (number of protons), which is the current form of the table.

Modern Periodic Table Horizontal rows are called periods. Horizontal rows are called periods. Vertical Columns are called groups or families. Vertical Columns are called groups or families.

Modern Periodic Table (cont) Periodic Law – When the elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties. Periodic Law – When the elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties. As you move across a period, the properties of the elements change. As you move across a period, the properties of the elements change. As you move down a group, the properties are similar. As you move down a group, the properties are similar.

Modern Periodic Table (cont) Group 1(1A) – Alkali Metals Group 1(1A) – Alkali Metals Group 2 (2A) – Alkaline Earth Metals Group 2 (2A) – Alkaline Earth Metals Groups 3-12 (3B-8B, 1B, 2B) –Transition and Inner Transition (Lanthanides & Actinides) Metals Groups 3-12 (3B-8B, 1B, 2B) –Transition and Inner Transition (Lanthanides & Actinides) Metals Group 17 (7A) – Halogens Group 17 (7A) – Halogens Group 18 (0) – Noble (Inert) Gases Group 18 (0) – Noble (Inert) Gases

Modern Periodic Table (cont) B, Si, Ge, As, Sb, Te, Po are metalloids. B, Si, Ge, As, Sb, Te, Po are metalloids. C, N, O, P, S, Se are non-metals. C, N, O, P, S, Se are non-metals.

Modern Periodic Table (cont) Most elements are solids at 25 C. Most elements are solids at 25 C. Exceptions: Exceptions: Liquids – Mercury (Hg) and Bromine (Br) Liquids – Mercury (Hg) and Bromine (Br) Gases – Hydrogen (H), Nitrogen (N), Oxygen (O), Fluorine (F), Chlorine (Cl), all of Group 18 (He, Ne, Ar, Kr, Xe, Rn) Gases – Hydrogen (H), Nitrogen (N), Oxygen (O), Fluorine (F), Chlorine (Cl), all of Group 18 (He, Ne, Ar, Kr, Xe, Rn) Man-made elements – Technetium (Tc) and all elements 93 and above. Man-made elements – Technetium (Tc) and all elements 93 and above.