Atoms and The Periodic Table

Chapter 3 – p. 70Blue Book

Objectives for this chapter:1) Understand the history of how early scientists discovered the atom.

2) To understand the structure of an atom and be able to classify the three main subatomic particles according to their location and charge.

3) Understand and know how to figure half- life of substances.4) Be able to describe the design of the modern periodic table.

5) Understand and know certain elements from the periodic table.6) Know and be able to explain certain vocabulary words.

• History of Matter• Greeks: 2500 yrs. Ago

– Had the idea that all matter was composed of 4 elements: fire, air, water and earth (but this didn’t make sense).

– Decades later had the idea: take matter – cut in half, again & again.– Eventually you couldn’t cut it anymore…– One particle left was named atoms: a small particle that makes up most

types of matter.– term means “cannot be divided”– IE … string of beads…end with one bead.

• Eighteenth century: atom discussion restarted.– Studied matter in laboratories…determined that different types of atoms

existed for every type of matter and that the atom’s identity explained the characteristic of each type of matter.

– Found out that matter is anything that has mass and takes up space. Made of:

• elements: cannot be broken down into simpler substances

• John Dalton…English teacher.• He combined element idea

with atom idea from Greeks.• He proposed…

• matter is made of elements

• atoms cannot be divided into smaller pieces

• all atoms of an element are alike

• atoms are hard spheres & the same throughout – like a marble

• William Crookes…1870…better equipment to experiment with.

• Cathode Ray Tube – CRT – (p. 76)• Found negatively charged particles.

• JJ Thomson– Repeated CRT with different metals…same

result– http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::100%25::100%25::/sites/dl/

free/0072512644/117354/01_Cathode_Ray_Tube.swf::Cathode%20Ray%20Tube

– Negatively charged particles…now called = electrons

• Why isn’t all matter negatively charged then?• Is there something that neutralizes the negative

particles?• Evidence of a positive charge was found in 1920.

– Scientists identified the proton : positive charged particle

– 2000 times heavier than the electron…

• - Ernest Rutherford…Rutherford’s Atom (p. 77)• - Wanted to test Thomson’s model of the atom:• He bombarded a thin sheet of gold with alpha particles: 2 protons & 2

neutrons– These come from unstable atoms & have a positive (+) charge

• He expected most alpha particles would pass right through the gold foil.– Why? … he hypothesized there was not enough matter to change path

BUT…more particles bounced and angled off than expected.• Some bounced back – p. 77• Conflicted with cookie dough model.

• Therefore…he proposed almost all the mass of the atom was in a very small region in the center of the atom.

• This area is now called the nucleus:– Center of the atom that

contains the protons and neutrons

• The rest of the atom is empty space occupied by electrons

• Summary – HOW did Rutherford find the nucleus?– The alpha particles hit

center & bounced/ repelled off

• Other scientists reviewed data and reasoned that since electrons have no mass, most mass of the atom should then be equal to protons.

• BUT, it isn’t…• Mass of most atoms is twice that of

its protons.• Where does this extra mass come

from?• Rutherford reasoned there must be

another particle in nucleus• Today we know this as the neutron:

• neutral charged particle• same mass as proton• took 20 yrs to find and prove

the existence of the neutron

• Atom size? – There are roughly 1 trillion atoms

in a period at the end of a sentence

• Where are the electrons?• Electrons go around nucleus

somewhere• They don’t know precise location at

a certain moment…electrons are constantly moving.

• Niels Bohr model: electrons are arranged according to energy levels.

– Lowest energy levels are close to nucleus.

– Highest energy levels are farthest from nucleus

• Therefore…electrons are located in a region around the nucleus called:– the electron cloud

Matter Timeline

• Identify 12 main/important events in the discovery of matter.– What year did they happen?– Who was involved?

• Using a unique shape (not just a line…), map out the 12 events from first to most recent.

• Add a picture to illustrate each event.

• What makes atoms different from each element?• Atomic number = number of protons

• Listed on Periodic Table – main number at top• Increasing from #1 - #116

• Atoms of the same element ALWAYS have the same number of protons• Therefore…protons make different elements• Proton # = Atomic #• What else equals the proton number? ELECTRONS!• Size of Nucleus? (p. 77)

• If the nucleus was the size of a poppy seed…atom would be the size of a football stadium!

• Atomic Summary…Three Main Subatomic Particles…• 1) proton … positive …(+)… nucleus ………….… +• 2) neutron … neutral …(=)… nucleus………………N• 3) electron … negative …(-)… electron cloud…e-

• Which particle is the most important in determining the type of element?• = proton

– If you change proton #, you change the element

• Atomic Number = number at top of square.

• Atomic Symbol• Element Name• Atomic Mass• P = _____ (same as

atomic number)• N = _____ (Atomic

mass – atomic number)• E = _____ (same as

protons)

• How to draw Bohr’s Diagram:– Find the element on the periodic table.– Determine the number of electrons (same as

atomic number)– The number of electrons is how many you will

draw in Bohr’s model.– With your element in the center, draw a circle

around your nucleus.• The first circle can only have 2 electrons• The second circle can only have 8 electrons.• The third circle can only have 18 electrons.

• Carbon has 6 electrons…– The first shell has 2– Since you already have 2, you only need to

draw 4 more (2+4=6)

- What about the neutron #? Can it change?

• YES = Isotope: atoms of the same element that have a different # of neutrons

• IE - Carbon 12– Carbon13– Carbon 14

• Number of protons? same• Number of neutrons? different

• - Mass number = # of protons & neutrons• - Atomic number = # of protons• Holding protons in nucleus?• = strong nuclear force = “nuclear glue”

• not fully understood…Discuss…

• - If isotope has too many or too few neutrons, it can be unstable and it may go through…

• - Radioactive Decay: release of nuclear particles and decay

• when particles are ejected from nucleus…

• atomic # can change therefore the element can change…

• = Transmutation : changing of one element into another element through radioactive decay

• Some radioactive isotopes release…• 1) alpha particle : 2 protons and 2 neutrons• 2) beta particle : high energy electron• - Half – Life: (radioactive isotope) amount of time it

takes for half of a sample of the element to decay– convenient way to measure the rate of decay of

a nucleus– unaffected by conditions…weather, pressure,

electric or magnetic currents, chemical reactions– rate of time is different for each isotope…from

seconds…to billions of years!• IE – If half life is 1 yr. – 1000 g sample…left after 3

yrs.?• Carbon dating…C14 = 5730 yrs. (p. 51)

• Living things have certain amount of carbon• Find fossil…relate amount of carbon back to when it was

living…Half = 5730yrs.!!• http://www.youtube.com/watch?v=31-P9pcPStg

• Periodic Table• Early civilizations…familiar with a few of the elements.

– Coins and jewelry from gold– Battle of Troy…armor from bronze (copper & tin)– Assyrians built empire with weapons of steel (iron &

carbon)• Nineteenth century…began search for new elements

– 1830 – had isolated and named 55 different elements– 1860 – Russian chemist – Dmitri Mendeleev

• used knowledge to build a table of elements• arranged in order of increasing atomic mass

• He began to see a pattern.– Elements with similar properties fell into groups.– Published first periodic table in 1869

• Even though all elements weren’t known…Mendeleev predicted missing elements and left gaps for undiscovered elements…

• Early 20th century, English physicist Henry Moseley– Improved Mendeleev’s table by arranging the elements in order of

increasing atomic number.

• Modern Periodic Table:• Arranged in increasing atomic number

– Rows or periods:• = row of elements on the periodic table whose properties change

gradually and predictably– Columns = group or family:

• = contains elements that have similar physical or chemical properties

• Color coded? Different colors…• (88)= metals: element that has luster and is a good conductor of heat and electricity

and is malleable• (luster = ability to reflect light)• (16)= nonmetals: gases or brittle solids at room temperature, poor conductor of heat

and electricity• (8)= metalloids: shares properties of both metals and nonmetals• IE – boron has a luster, but is a poor conductor

• Element Keys• Each element box on the periodic

table contains:– Element name (Some do not)– Atomic number– Symbol– Average atomic mass– Bull’s eye symbol – not

naturally on Earth – Synthetic = man made

• Others are labeled according to key - solid, liquid or gas at room temperature

• Most stable elements are right column of table – all gases

• Symbols – don’t make sense?… gold = Au

– due to Latin or Greek origin– some new names – geographic

areas, names, countries…• Unnamed – IE - Uun, Uuu, Uub

– temporary name