Post on 19-Nov-2021
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Unit 2 continued-Chemical Foundations
Atoms, Ions, &Elements
The Elements• Most abundant elements in/on
Earth:– Oxygen- 49.2%
– Silicon-25.7%
• Most abundant in the human body:– Oxygen-65.0%
– Carbon-18.0 %
– Hydrogen-10.0%
Microscopic vs. Macroscopic– Chemists use the word element for
macroscopic (things we can see with the eye)- meaning an amount that we would mass on a balance
– Chemists also use the word element to describe microscopic form of an element when we are talking about a single atom of that element.
Element Defined• Element- a substance that
cannot be decomposed into a simpler substances by chemical or physical means.
• It consists of atoms all having the same atomic number.
Element Symbols• Symbols are used to represent the
element.
• They usually consist of the first letter of the first two letters of the element names.
• The first letter is always capitalized, and the second is not.
Most Common Elements
You will need to memorize the elements name and symbol of the elements (on your sheet)—Polyatomic ions included.
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The Language of ChemistryCHEMICAL ELEMENTS
SodiumBromine
Aluminum
The Language of Chemistry
• The elements, their names, and symbols are given on the
PERIODIC TABLE• How many elements
are there?
The Periodic Table
Dmitri Mendeleev (1834 - 1907)- Known as the Father of the Periodic Table.
An atom consists of a
• nucleus
– (of protons and neutrons)
• electrons in space about the nucleus.
The Atom
Nucleus
Electron cloud
Copper atoms on silica surface.
• An atom is the smallest particle of an
element that has the chemical properties of the element.
Distance across = 1.8 nanometer (1.8 x 10-9 m)
ATOM COMPOSITION
•protons and neutrons in the nucleus.
•the number of electrons is equal to the number of protons.
•electrons in space around the nucleus.
•extremely small. One teaspoon of water has 3 times as many atoms as the Atlantic Ocean has teaspoons of water.
The atom is mostlyempty space
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ATOMIC COMPOSITION• Protons (p+)
– + electrical charge– Located in the nucleus– mass = 1.672623 x 10-24 g– relative mass = 1.007 atomic mass units (amu)
we round to 1
• Electrons (e-)– negative electrical charge– Located outside of nucleus– relative mass = 0.0005 amu
we round to 0
• Neutrons (no)– no electrical charge– Located in the nucleus– mass = 1.009 amu
We round to 1
Subatomic Particles
• Quarks
– component of protons & neutrons
– 6 types
– 3 quarks = 1 proton or 1 neutron
He
You do not have to know the types, just that quarks exist! Scientists are learning more everyday!
The red compound is composed of• nickel (Ni) (silver)• carbon (C) (black)• hydrogen (H) (white)• oxygen (O) (red)• nitrogen (N) (blue)
CHEMICAL COMPOUNDS are two or more elements chemically combined.
They can be broken back down into the elements by a chemical process. (Usually requires much energy!)
Compounds
– composed of 2 or more elements in a fixed ratio
–properties differ from those of individual elements
– EX: table salt (NaCl)
A MOLECULE is the smallest unit of a
compound that retains the chemical characteristics of the compound. (Covalent compounds are called molecules)
Composition of molecules is given
by a MOLECULAR FORMULA
H2O C8H10N4O2 - caffeine
ELEMENTS THAT EXIST AS DIATOMIC MOLECULES
Remember:
BrINClHOF
These elements
only exist as
PAIRS. Note that
when they combine
to make
compounds, they
are no longer
elements so they
are no longer in
pairs!
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How do we know?
How do we know all of these things
about the atom?
As part of the chemistry curriculum, you will need to know about the development of the model of the atom. In the next few days, we will explore and see what each scientist learned as the current model of the atom developed.
Atomic Models• Models are used to help us visualize something that
cannot be seen
Democritus• This Greek philosopher
is credited with coining the term “atomos”
• This was the term for the indivisible units.
The idea ONLY- He had no Proof!
Another Greek -Aristotle• All substances are made of 4 elements
• Fire - Hot
• Air - light
• Earth - cool, heavy
• Water - wet
• Blend these in different proportions to get all substances
• He was better at debating, this was believed until 1700’s--Alchemy
Late 1700’s - John Dalton
An English Scientist who studied the nature of materials.
He developed what is known as the
Atomic Theory.
The brown box on page 52 of text.. You will also need to know the main ideas of his theory.
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Dalton’s Atomic Theory
John Dalton (1766-1844) proposed an atomic theory
While this theory was not completely correct, it revolutionized how chemists looked at matter and brought about chemistry as we know it today instead of alchemy
Thus, it’s an important landmark in the history of science.
Dalton’s Atomic Theory
1. matter is composed, indivisible particles (atoms)
2. all atoms of a particular element are identical3. different elements have different atoms4. atoms combine in certain whole-number
ratios 5. In a chemical reaction, atoms are merely
rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.
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Problems with Dalton’s Atomic Theory?
1. matter is composed, indivisible particlesTrue, but Atoms Can Be Divided, but only in a nuclear reaction
2. all atoms of a particular element are identical**WRONG**Does Not Account for Isotopes (atoms of the same element but a different mass due to a different number of neutrons)!
3. different elements have different atomsTrue!
4. atoms combine in certain whole-number ratiosTrue! Called the Law of Definite Proportions
5. In a chemical reaction, atoms are merely rearranged to form new compounds; they are not created, destroyed, or changed into atoms of any other elements.True, except for nuclear reactions that can change atoms of one element to a different element
J.J. Thomson
• Deflected a beam of particles with magnets
• Determined that particles must have a negative charge
Thomson’s Experiment
Voltage source
+-
Metal Disks
Passing an electric current makes a beam
appear to move from the negative to the
positive end
Thomson’s Experiment
Voltage source
+-
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Voltage source
Thomson’s Experiment
By adding an electric field
+
-
Voltage source
Thomson’s Experiment
By adding an electric field he found that the
moving pieces were negative
+
-
By adding an electric field
Thomson’s Model
• Found the electron.
• Couldn’t find positive (for a while).
• Said the atom was like plum pudding.
• A bunch of positive stuff, with the electrons able to be removed.
About Millikan
• Millikan determined the charge of the electron.
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Millikan’s Experiment
Atomizer
Microscope
-
+
Oil
Metal Plates
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Millikan’s Experiment
Oil
Atomizer
Microscope
-
+
Oil droplets
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Millikan’s Experiment
X-rays
X-rays give some drops a charge by knocking off
electrons38
-
Millikan’s Experiment
+
39
Millikan’s Experiment
They put an electric charge on the plates
++
--
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Millikan’s Experiment
Some drops would hover
++
--
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Millikan’s Experiment
+
+ + + + + + +
- - - - - - -
Some drops would hover
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Millikan’s Experiment
From the mass of the drop and the charge on
the plates, he calculated the charge on an electron
++
--
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The modern view of the atom was developed by Ernest Rutherford (1871-1937).
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Rutherford’s Experiment
• Ernest Rutherford English physicist. (1910)
• Believed the plum pudding model of the atom was correct.
• Wanted to see how big electrons were.
• Used radioactivity.
• Alpha particles - positively charged pieces given off by uranium.
• Shot them at gold foil which can be made a few atoms thick.
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Rutherford
• A stream of positive alpha particles were directed at foil
• Thomson’s model predicted that particles should pass through neutral material
Rutherford’s experiment.
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Rutherford’s experiment
• When the alpha particles hit a florescent screen, it glows.
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Lead
blockUranium
Gold Foil
Flourescent
Screen
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He Expected
• The alpha particles to pass through without changing direction very much.
• Because…
• The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles.
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What he expected
51
Because
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Because, he thought the mass was
evenly distributed in the atom
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Because, he thought
the mass was evenly
distributed in the atomResults of foil experiment if Plum Pudding
model had been correct.
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What he got
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How he explained it
+
• Atom is mostly empty.
• Small dense, positive piece
at center.
• Alpha particles are deflected by
it if they get close
enough.
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+
What Actually Happened
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Rutherford
• Rare deflections indicated two things:
A positive mass existed in the atom
The positive center is very small
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Density and the Atom
• Since most of the particles went through, it was mostly empty.
• Because the pieces turned so much, the positive pieces were heavy.
• Small volume, big mass, big density.
• This small dense positive area is the nucleus.
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Modern View
• The atom is mostly empty space.
• Two regions.
• Nucleus- protons and neutrons.
• Electron cloud- region where you might find an electron.
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Which Model is this?Is it correct?
Atomic Number, Mass Number, and Element Symbol
• There are 3 different parts to identify an element.
– Atomic Number• The number of protons in the nucleus of an atom.
• Symbolized by Z
– Mass Number• The sum of the protons and neutrons.
• Symbolized by A
– Element Symbol• The one or two letter symbol given to every element.
XAZ
Na2311
{Plus}
• If you see an element written this way:
Carbon-14
mass number is 14
Atomic Number, Z
All atoms of the same element have the same number of protons in the
nucleus, Z
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Al
26.981
Atomic number
Atom symbol
AVERAGE Atomic Mass
Mass Number, A• C atom with 6 protons and 6 neutrons is the
mass standard • = 12 atomic mass units
• Mass Number (A)= # protons + # neutrons
• NOT on the periodic table…(it is the AVERAGE atomic mass on the table)
• A boron atom can have A = 5 p + 5 n = 10 amu
A
Z
10
5B
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Isotopes
• Atoms of the same element (same Z) but different mass number (A).
• Boron-10 (10B) has 5 p and 5 n
• Boron-11 (11B) has 5 p and 6 n
10B
11B
Figure 3.10: Two isotopes of sodium.
Isotopes & Their Uses
Bone scans with radioactive technetium-99.
Isotopes & Their Uses
The tritium content of ground water is used to discover the source of the water, for example, in municipal water or the source of the steam from a volcano.
Atomic Symbols
Show the name of the element, a hyphen, and the
mass number in hyphen notation
sodium-23
Show the mass number and atomic number in
nuclear symbol form
mass number23 Na
atomic number 11
Isotopes?
Which of the following represent isotopes of the same element? Which element?
234 X
234 X
235 X
238 X
92 93 92 92
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Answer:
234 U
234 Np
235 U
238 U
92 93 92 92
234 Np is not an isotope of Uranium.
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Counting Protons, Neutrons, and Electrons
• Protons: Atomic Number (from periodic table)• Neutrons: Mass Number minus the number of
protons (mass number is protons and neutrons because the mass of electrons is negligible)
• Electrons:– If it’s an atom, the protons and electrons must be the SAME
so that it is has a net charge of zero (equal numbers of + and -)
– If it does NOT have an equal number of electrons, it is not an atom, it is an ION. For each negative charge, add an extra electron. For each positive charge, subtract an electron (Don’t add a proton!!! That changes the element!)
Proton Electron Neutron
+ charge - charge 0 charge
In nucleus Outside nucleus in
specific levelsIn nucleus
1 1/1836 1
Learning Check – Counting
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C 13C 14C6 6 6
#p+ _______ _______ _______
#no _______ _______ _______
#e- _______ _______ _______
Answers
12C 13C 14C6 6 6
#p+ 6 6 6
#no 6 7 8
#e- 6 6 6
Learning Check
An atom has 14 protons and 20 neutrons.A. Its atomic number is
1) 14 2) 16 3) 34
B. Its mass number is1) 14 2) 16 3) 34
C. The element is1) Si 2) Ca 3) Se
D. Another isotope of this element is1) 34X 2) 34X 3) 36X
16 14 14
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Solution
An atom has 14 protons and 20 neutrons.A. It has atomic number
1) 14
B. It has a mass number of3) 34
C. The element is1) Si
D. Another isotope of this element would be
3) 36X14
IONS • IONS are atoms or groups of atoms with a
positive or negative charge.
• Taking away an electron from an atom gives a
CATION with a positive charge
• Adding an electron to an atom gives an ANIONwith a negative charge.
• To tell the difference between an atom and an ion,
look to see if there is a charge in the superscript!
Examples: Na+ Ca+2 I- O-2
Na Ca I O
Forming Cations & Anions
A CATION forms when an atom loses one or more electrons.
An ANION forms when an atom gains one or more electrons
Mg --> Mg2+ + 2 e- F + e- --> F-
PREDICTING ION CHARGES
In general
• metals (Mg) lose electrons ---> cations
• nonmetals (F) gain electrons ---> anions
Learning Check – Counting
State the number of protons, neutrons, and electrons in each of these ions.
39 K+ 16O -2 41Ca +2
19 8 20
#p+ ______ ______ _______
#no ______ ______ _______
#e- ______ ______ _______
Learning Check – Counting
State the number of protons, neutrons, and electrons in each of these ions.
39 K+ 16O -2 41Ca +2
19 8 20
#P 19 8 20
#N 20 8 21
#E 18 10 18
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One Last Learning Check
Write the nuclear symbol form for the following atoms or ions:
A. 8 p+, 8 n, 8 e- ___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 46 e- ___________
Answers
Write the nuclear symbol form for the following atoms or ions:
A. 8 p+, 8 n, 8 e- 16O8
B. 17p+, 20n, 17e- 37Cl17
C. 47p+, 60 n, 46 e- 107Ag+
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Charges on Common Ions-1-2-3
+1
+2
By losing or gaining e-, atom has same number of e-’s as nearest Group 8A atom.
AVERAGE
ATOMIC MASS
• Because of the existence of isotopes, the mass of a collection of atoms has an average value.
• Boron is 20% 10B and 80% 11B. That is, 11B is 80 percent abundant on earth.
• For boron atomic weight
= 0.20 (10 amu) + 0.80 (11 amu) = 10.8 amu
10B
11B
Isotopes & Average Atomic Mass
• Because of the existence of isotopes, the mass of a collection of atoms has an average value.
• 6Li = 7.5% abundant and 7Li = 92.5%
– Avg. Atomic mass of Li = ______________
• 28Si = 92.23%, 29Si = 4.67%, 30Si = 3.10%
– Avg. Atomic mass of Si = ______________
Honors only
The Periodic Table
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Periods in the Periodic Table Groups in the Periodic Table
Elements in groups react in similar ways!
Regions of the Periodic Table Group 1A: Alkali Metals
Cutting sodium metal
Reaction of potassium + H2O
Magnesium
Magnesium oxide
Group 2A: Alkaline Earth MetalsGroup 7A: The Halogens (salt
makers) F, Cl, Br, I, At
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Group 8A: The Noble (Inert) Gases
He, Ne, Ar, Kr, Xe, Rn
• Lighter than air balloons
• “Neon” signs
• Very Unreactive because they have full electron levels
XeOF4
Transition Elements
Lanthanides and actinides
Iron in air gives iron(III) oxide