Atomic Structure Atoms are the building blocks of molecules. John Dalton proposed the Atomic Theory in 1808.
Atomic Theory 1. Every element is made up of tiny particles that can be subdivided. 2. Atoms of the same element are exactly alike. Ex. O and O2 and O3 3. Atoms of different elements can join to form different molecules and compounds. Ex. Na+ + Cl- -> NaCl
Atomic Theory Atoms are responsible for properties (physical & chemical) but the atoms themselves DO NOT possess properties.
What are the 3 sub-atomic particles? • Proton • Neutron • Electron
Positive, +1 • In the nucleus • In the electron cloud • Negative, -1 • Neutral, 0 • In the nucleus
Relative Masses of the Sub-atomic Particles • Electrons are the smallest of the three • Protons and neutrons are approximately the same size and are each approximately 2000 times the size of an electron
Nucleus • Extremely dense central portion of an atom that contains the protons and neutrons • Has an overall positive charge • Extremely small compared with the atom as a whole, though it accounts for nearly all of the atom’s mass; baseball stadium analogy • Atoms are mostly empty space
Electron Cloud • Electron Cloud • The region of space surrounding the nucleus where the electrons are found • Has an overall negative charge • Is extremely HUGEcompared to the nucleus • Each dot represents a place where an electron could possibly be… the dots DO NOT tell us where the electrons actually are!
Scanning Tunneling Microscope - 1981 • Picture of the surface of gold atoms
Distinguishing Between Atoms Recall: all atoms contain p, n, and e- How are the atoms of one element different from the atoms of another atom? • They contain different numbers of protons
Atomic Number • Gives the number of protons in the nucleus of an atom of a particular element • All atoms of a particular element have the same atomic number • No two elements have the same atomic number.
Atomic Mass • Atomic Mass refers to the “weight” of the atom. • It is derived at by adding the number of protons with the number of neutrons. This is a helium atom. Its atomic mass is 4 (protons plus neutrons). What is its atomic number? H
Atomic Mass and Isotopes • While most atoms have the same number of protons and neutrons, some don’t. • Some atoms have more or less neutrons than protons. These are called isotopes. • An atomic mass number with a decimal is the total of the number of protons plus the average number of neutrons.
Atomic Mass Unit (AMU) • The unit of measurement for an atom is an AMU. It stands for atomic mass unit. • One AMU is equal to the mass of one proton.
Atomic Mass Unit (AMU) • There are 6 X 1023 or 600,000,000,000,000,000,000,000 amus in one gram. • (Remember that electrons are 2000 times smaller than one amu).
Symbols • All elements have their own unique symbol. • It can consist of a single capital letter, or a capital letter and one or two lower case letters. C Carbon Cu Copper
Mass Number • The total number of protons and neutrons in the nucleus of an atom. • Notice… the mass number does not appear on the periodic table. • The number of electrons does not count toward the mass number because the vast majority of the mass of an atom is concentrated in its nucleus
Protons:______________ • Electrons:____________ • Neutrons:____________
Calcium-41 • Tells us it has a mass number of 41 • Protons:______________ • Electrons:____________ • Neutrons:____________ • Tells us it has an atomic number of 20 which gives us the number of protons
Isotope • Atoms of the same element that have different masses because they have different numbers ofneutrons • Like Carbon-12 & Carbon-14 • Have the same number of protons and electrons. • Because they have different numbers of neutrons, they also have different mass numbers
Isotope • General Rule – if the # of p+ is different than the # of n, then the element has isotopes! • The more neutrons you add to an element, the more unstable it becomes. • Radioactive Isotopes: an unstable isotope • Unstable nuclei breakdown in a process called radioactive decay. • Where radioactive isotopes are transformed (broken down) into other stable elements
Practice Problems • What is the atomic number of iron (Fe)? • How many electrons does a neutral nitrogen atom have? • How many neutrons does Lithium-8 have? • Using the two notation methods, indicate a gold atom with 118 neutrons? • What is the mass number of an element with 9 protons and 10 neutrons? What is the element? • How many p, n, and e- does oxygen-16 have? • How many p, n, and e- does oxygen-17 have?
Practice Problems • What is the atomic number of iron (Fe)? • Tells us it has an atomic number of 26
2 . How many electrons does a neutral nitrogen atom have? • Tells us it has 7 protons… • If an atom has 7 protons (which are positive), then it will need 7 electrons(which are negative) in order to be neutral.
Tells us it has a mass number of 8 • Tells us it has 3 protons 3. How many neutrons does Lithium-8 have? • We know that… • Mass # = protons + neutrons • 8 = 3 + ?
Since we know that… • Mass # = protons + neutrons • ? = 79 + 118 • Then the mass number has to be 197 • Tells us it has 79 protons 4. Using the two notation methods, indicate a gold atom with 118 neutrons?
5. What is the mass number of an element with 9 protons and 10 neutrons? What is the element? • We know that… • Mass # = protons + neutrons • ? = 9 + 10 • Therefore the mass # is 19 • What’s the important number that identifies an atom? • Protons • Neutrons • Mass number
Tells us it has a mass number of 16 • Tells us it has8 protonswhich means that it must also have8 electrons 6. How many p, n, and e- does oxygen-16 have? • Since we know that… • Mass # = protons + neutrons • 16 = 8 + ? • Then it must have8 neutrons
Tells us it has a mass number of 17 • Tells us it has8 protonswhich means that it must also have8 electrons 7. How many p, n, and e- does oxygen-17 have? • Since we know that… • Mass # = protons + neutrons • 17 = 8 + ? • Then it must have9 neutrons
Complete and turn in the following… • Atomic Structure Worksheet • On the back do questions #1-9 found on page 112 of your book.
First International Congress of Chemists • 1860 • Agreed upon a method to accurately determine atomic mass
Mendeleev • In 1869,Dmitri Mendeléev created the first accepted version of the periodic table. • He grouped elements (about 60 known elements) according to their atomic mass, and as he did, he found that the families had similar chemical properties. • Blank spaces were left open to add the new elements he predicted would occur. • "...if all the elements be arranged in order of their atomic weights a periodic repetition of properties is obtained." - Mendeleev
Mendeleev tried different arrangements looking for trends • When arranged according to increasing atomic mass he found that certain similarities in their chemical properties appeared at regular intervals…such a trend is said to be periodic
Mendeleev’s Periodic Table • Elements with similar properties grouped together… thus the name “Periodic Table” • His table contradicted norm of listing the elements in order of increasing atomic mass
2 Questions Still Unanswered… • Why could most of the elements be arranged in order of increasing atomic mass but a few could not? • What was the reason for chemical periodicity?
Henry Moseley • Discovered that elements fit into patterns better when arranged according to increasing nuclear charge • His work led to… • Modern definition of atomic number • PT organization based on atomic # instead of atomic mass
Periodic Law • Combo of Mendeleev and Moseley’s work • Definition: the physical and chemical properties of the elements are periodic functions of their atomic numbers
Periods and Groups Horizontal rows in the periodic table are called periods Vertical columns are called groups We will learn later why the elements can be arranged in this fashion based on the arrangements of the electrons outside the nucleus
Columns of elements are called groups or families. Elements in each family have similar but not identical properties. For example, lithium (Li), sodium (Na), potassium (K), and other members of family IA are all soft, white, shiny metals. All elements in a family have the same number of valence electrons. Each horizontal row of elements is called a period. The elements in a period are not alike in properties. In fact, the properties change greatly across even given row. The first element in a period is always an extremely active solid. The last element in a period, is always an inactive gas. Families Periods