chapter 3 · • in 1911, rutherford and his coworkers at the university of manchester, england,...
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3.1 THE ELEMENTS
• 118 elements
•92 occur naturally, the rest are synthesized
•All matter in the universe can be chemically broken down into elements
•Compounds are made by combining atoms of elements like words are formed from letters of the alphabet.
Words Compounds
THE WORD ELEMENT CAN HAVE SEVERAL MEANINGS
Element
Element
Microscopic form
Single atom of that
element
Macroscopic form
Sample of that
element large enough
to weigh on a balance
Element
Generic form
When we say the human body contains the element
sodium or lithium, we do not mean that free
elemental sodium or lithium is present. Rather we
mean that atoms of these elements are present in
some form.
NAME AND SYMBOLS FOR THE ELEMENTS
• Each element has a unique name and symbol
• Symbol usually consists of first one or two letters of elements name • Oxygen = O
• Krypton = Kr
• Sometimes the symbol is taken from the original Latin or Greek name
FORMULAS OF COMPOUNDS
• The types of atoms and the number of each type is shown using chemical symbols and subscripts.
• Write the formula for each of the following, listing the elements in the order given
• One atom of sulfur and three atoms of oxygen
• Two atoms of nitrogen and five atoms of oxygen
3.2 EARLY IDEAS ABOUT MATTER
• Greek philosophers – formed explanations based on life experiences
•Proposed four ‘elements’
DEMOCRITUS – 400 BC
•Believed matter was made of tiny particles called atomos • Cannot be created, destroyed, or divided
•Aristotle disagreed
ALCHEMY
• Origin of the word Chemistry
• Tried to make gold out of abundant elements
• Invented many of the techniques still used today like filtration and distillation
ANTOINE LAVOISIER
• 1743-1794
• French scientist known as the “Father of Chemistry”
• Made Chemistry quantitative by taking accurate measurements
• Wrote 1st chemistry textbook
• Beheaded in 1794 for being funded by taxes
PROUST (1754-1826)
• Proposed the Law of Constant Composition (1794): • A given compound always contains the same proportion by mass of
the elements of which it is composed.
JOHN DALTON (1803)
• Revived and Revised Democritus’ idea
• Came up with his own Atomic Theory • All elements are composed of tiny particles called atoms
which cannot be broken into smaller parts.
• All atoms of the same element are identical, atoms of different elements are different
• Atoms of one element can combine with atoms of other elements to form compounds. A given compound always has the same relative numbers and types of atoms
• Chemical reactions take place when atoms rearrange. Atoms are not created or destroyed during chemical reactions.
3.3 DEFINING THE ATOM
• The smallest particle of an element that retains the properties of the element
•Atoms are very small One Cu atom = 1.28 x 10-10 m
Solid copper penny = 2.9 x 1022 atoms
If 1 atom = size of an orange, an orange would be as big as the earth!
• Scanning tunneling microscope (STM) allows individual atoms to be seen Scans across the surface of atoms
silicon
By measuring the effects of both magnetic and electric fields on a cathode ray, Thomson determined the mass of the charged particle is less than the hydrogen atom (the lightest atom known)
Why is this significant?
•Thomson’s model Matter is neutral so some positive charge needs to balance
out the negative electrons
Plum pudding model
• In 1911, Rutherford and his coworkers at the University of Manchester, England, directed a narrow beam of alpha particles at a very thin sheet of gold foil.
• Based on Thomson’s model of the atom he expected the alpha particles to pass through the foil but not all did!
Rutherford concluded: Atom consists of mostly empty space
If an atom had a diameter of 2 football fields the nucleus would be the size of a nickel
Almost all + charge & mass are found in nucleus
ATOMIC STRUCTURE
Ernest Rutherford showed that atoms have internal structure. The nucleus, which is at the center of the
atom, contains protons (positively charged) and neutrons (uncharged).
– Electrons move around the nucleus.
JAMES CHADWICK - 1932
What we knew
1. Electrons & Protons have opposite charges
2. Neutral atom has same number of electrons as protons
3. Mass of atom was too much to just be made of protons & electrons
The number of protons in an atom identifies it as a particular element
Number of protons = atomic number
Periodic table is organized in order of increasing atomic number
MASS NUMBER
Mass number = protons + neutrons
Neutrons =
Mass numbers are given in atomic mass units (amu) 1 amu = 1/12 mass of Carbon-12 atom
ATOMIC SHORTHAND
• Beryllium- - -atomic number 4
mass number 9
The atomic number is written as a subscript.
The mass number is written as a superscript.
Be
• Most naturally occurring elements have two or more stable isotopes One is usually more common than the others
Oxygen – 16, oxygen – 17, oxygen - 18
• mass number vs. atomic mass • Mass number = protons + neutrons
• Atomic mass (on periodic table) = weighted average of all the isotopes of an element
Unknown element X has two isotopes. 6X has an atomic mass of 6.015 amu and a 7.59 % abundance. 7X has an atomic mass of 7.016 amu and a 92.41% abundance. What is the atomic mass of the unknown element? What is the identity of the unknown element?
IONS •Atoms can form ions by gaining or losing electrons. Metals tend to lose one or more electrons to form positive ions called
cations.
– Cations are generally named by using the name of the parent atom.
IONS • Nonmetals tend to gain one or more electrons to form negative ions called anions.
• Anions are named by using the root of the atom name followed by the suffix
–ide.
WHAT CAN CHANGE IN AN ATOM?
Protons – NEVER!!!
Neutrons – if changed an isotope is formed
Electrons – if changed an ion is formed
PROTON CHANGED
Whole new atom!!!
Oxygen loses a proton it becomes Nitrogen
Oxygen gains a proton it becomes Fluorine
NEUTRON CHANGED
Different version of the same atom is formed (isotope)
Oxygen – 16 has 8 Neutrons
Oxygen – 17 has 9 Neutrons
Oxygen – 18 has 10 Neutrons
ELECTRONS CHANGED
Charged version of the same atom is formed (ion)
Oxygen gains 2 electrons – 10 electrons & 8 protons Anion
Oxygen loses 2 electrons – 6 electrons & 8 protons Cation
COMPOUNDS THAT CONTAIN IONS
•Ions combine to form ionic compounds.
•Properties of ionic compounds High melting points
Conduct electricity
If melted
If dissolved in water
Ionic compounds are electrically neutral Total positive charge must cancel out total negative charge
FORMULAS FOR IONIC COMPOUNDS
Write cation symbol followed by anion symbol
The number of cations and ions must be correct so their charge sums to zero
• Write the formula for compounds containing the following ions: Potassium and oxygen
Calcium and sulfur
Magnesium and nitrogen
3.4 INTRODUCTION TO THE PERIODIC TABLE
• In 1913 our current periodic table was developed by Henry Moseley who arranged the atoms by atomic number
• Periodic law – when elements are arranged according to increasing atomic number, there is a periodic repetition of chemical and physical properties
THE MODERN PERIODIC TABLE
•Columns = groups Numbered 1 – 18
Correspond to the number of outermost electrons
• Have similar properties
Some have special names
•Rows = periods Numbered 1 – 7
Correspond to outermost energy level
PHYSICAL STATES AND CLASSES OF ELEMENTS
•Most elements are solid at room temperature
•Br & Hg are liquid
•N, O, F, Cl, and Noble gases are gas
METALS
•Shiny
•Solid at room temperature
•Good conductors of heat & electricity
•Malleable
•Ductile
•Found to the left of the staircase • 1 exception =
NONMETALS
•Gasses or dull looking solids • 1 exception =
•Poor conductors of heat and electricity
•Found to the right of the staircase
METALLOIDS
Border the staircase
Have properties of both metals and nonmetals
Semi-conductors = conduct electricity only under certain conditions
NATURAL STATES OF THE ELEMENTS
Most elements are very reactive.
Elements are not generally found in uncombined form. Exceptions are:
Noble metals – gold, platinum and silver
Noble gases – Group 8
NATURAL STATES OF THE ELEMENTS
Diatomic Molecules
Nitrogen gas contains
N2 molecules.
Oxygen gas contains
O2 molecules.