Atomic structure and chemical bonding

Atomic structure and chemical bonding

Do now

• Take out your binder and a pen– Let’s get in the habit of doing this every day

• Do now: Take out your binder and a pen– Mentally prepare for note taking

Review

• What is an atom?An atom is the smallest unit of an element that still

retains the properties of that element

Structure of an atom• 3 kinds of sub-atomic particles– Protons: positively charged, in nucleus– Neutrons: neutrally charged, in nucleus– Electrons: negatively charged, in cloud around

nucleus

Atomic Number

• The atomic number is the number of protons in an atom. – It is the same for all atoms of a given element. – All hydrogen atoms have one proton– All helium atoms have two protons– And so on…

Atomic Number

• In an electrically neutral atom (no net charge), the number of protons is equal to the number of electrons.

• Therefore the atomic number of an atom is equal to the number of its electrons.

Atomic Mass

• Protons and neutrons have equal mass• Each has a mass of 1 dalton or 1 atomic mass

unit (amu)• Electrons have minimal mass Sooooo…..

Atomic mass (in daltons) = # of protons + # of neutrons

Practice problem

• What is C.C. Carbon’s atomic mass?

I have 6 protons, 6 neutrons, and 6 electrons. Do I look fat??!!?!?!

Atomic mass = 6 protons + 6 neutrons = 12 daltons*Never ask a lady about her atomic mass.

Isotopes• Atoms of the same element with different

numbers of neutrons are called isotopes.– Example: Carbon-12 has 6 protons and 6 neutrons– Carbon-13 has 6 protons and 7 neutrons– Carbon-14 has 6 protons and 8 neutrons– They have different atomic masses, but they’re all

still carbon.

I’m prettier!!!

We’re almost identical!

Carbon-12 Carbon-14

Properties of isotopes• Isotopes of the same element have most of

the same properties– They form the same number of chemical bonds– They can substitute for each other in molecules

• But they are different in some ways– some isotopes are radioactive, like carbon 14

Carbon-14

Radioactive isotopes

• What is radioactivity?– Radioactivity is when an unstable nucleus shoots

out certain particles in a process called radioactive decay

– Alpha particles are made of 2 protons and 2 neutrons

– Beta particles are a single electron

Carbon-14

Real life applications

• How is radioactivity useful in my life?• How is it useful in the lives of others? • Page 21 in text– Imaging regions of Alzheimer’s disease– Radiation poisoning at Chernobyl

Radiation therapy for thyroid cancer

• What is the thyroid?– The thyroid is a gland in your neck– The thyroid gland makes hormones that include

iodine atoms• Therefore, iodine is highly concentrated in the

thyroid.

Normal thyroid

Goiter(Iodine deficient

thyroid)

Radiation therapy for thyroid cancer

• Why does this matter for cancer therapy?• Iodine has several isotopes– Iodine-127 is the only stable isotope– Iodine 131 is highly radioactive and emits beta particles

• Patients with thyroid cancer are injected with small amounts of iodine-131

• Iodine 131 collects in the thyroid and emits beta particles, which tear through and kill the cancerous tissue.

Alzheimer’s treatment• Alzheimer’s is a disease that destroys memory

and cognitive skills• Researchers have found high amounts of a

protein called beta-amyloid in the brains of Alzheimer’s patients

normal brain Alzheimer’s

Alzheimer’s treatment• How do we know where beta-amyloid is?• PIB is a protein that binds beta-amyloid– PIB also binds radioactive isotopes– PIB emits beta particles that can be detected – Therefore PIB shows us where beta-amyloid is.

normal brain Alzheimer’s

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VIRES

Radioactivity at Chernobyl

• Nuclear power plants use radioactive isotopes as fuel.

• Radioactive decay events give off huge amounts of heat that is trapped and used for power

Radioactivity at Chernobyl

• 1986 – nuclear meltdown in Chernobyl, Ukraine• Meltdown sent radioactive isotopes into the air,

water, and land– Death from radiation poisoning– Genetic mutations

Electrons

• “Of the three subatomic particles – protons, neutrons, and electrons – only electrons are directly involved in the chemical activity of an atom.”

Why?Because electrons are responsible for forming

chemical bonds.

Electron Shells• What is an electron shell?

– Electrons move around the nucleus only at certain energy levels, called electron shells

– The first shell holds two electrons

– The second and third shells each hold eight electrons.

– Electrons fill up a lower shell first before filling up a higher shell

Electron shells and the periodic table

• Each row of the periodic table represents one electron shell• The first shell can only hold 2 electrons, so the first row of the

periodic table only has 2 elements (hydrogen and helium)• Shells 2 and 3 have 8 electrons. Rows 2 and 3 have 8 elements• Each time you go across a row of the periodic table, you fill in

a new shell of electrons

Valence shell

• The outermost electron shell is called the valence shell

• Only electrons in the valence shell form bonds with other atoms.

• Atoms form bonds to achieve a full valence shell

Practice problem

• With a partner, determine how many valence electrons each of the following atoms has:– Hydrogen– Helium– Lithum– Carbon– Nitrogen– Oxygen

What if the valence shell is already full?

• The right-most column of the periodic table contains the noble gases.

• The noble gases are at the very end of their row, so their entire valence shell is full.

• Consequently, noble gases do not need to form bonds or take part in reactions

Chemical bonding

• Atoms with incomplete valence shells interact with other atoms to form full valence shells

• These interactions are called chemical bonding

• Atoms can share, donate and receive electrons to get to a full valence shell.

Covalent bonds• Vocabulary: covalent

Break it down!

• co = together (like co-worker, cooperate)• valent = valence

Covalent = valence electrons shared together

Do Now

• With a partner, answer the following:– What is a covalent bond?

Covalent bonds

• Bonds in which two atoms share one or more pairs of valence electrons.

• Two or more atoms held together by covalent bonds form a molecule.

H

H2

H

Double and Triple bonds

• Two atoms can share more than one covalent bond

• In a double bond, four electrons are shared• In a triple bond, six electrons are shared

Triple bond (N2)

Bonding Capacity• In each covalent bond, an atom shares a valence

electron and gets a new one shared with it. • Atoms need to have at least one of their own

valence electrons for every covalent bond they form.

Single bond

Practice problem

• How many valence electrons does boron have?

• What’s the maximum number of covalent bonds boron can form?

• Can boron ever achieve a valence octet by covalent bonding alone?

Bonding capacity

• For each covalent bond an atom forms, it must have a space in its valence shell for a new electron.– Ex: fluorine has 7 electrons in its valence shell. It is

only one electron short of a full octet, so it can only form one bond.

Bonding capacity

• The bottom line: – For each covalent bond an atom forms, it must

have:1) A valence electron to share and 2) a vacancy in its valence shell to accept a new electron.

Practice problem

• Fill in this table in your notes: Element Atomic # Valence e-s Vacancies in

valence shellBonding capacity

Hydrogen 1

Carbon 6

Nitrogen 7

Oxygen 8

Ammonia Water

Methane

Electronegativity

• Not all covalent bonds share electrons equally.• The tendency of an atom to pull electrons

toward itself is called electronegativity.

Electronegativity

• How do we measure electronegativity?• Pauling electronegativity scale– Each element is scored from .7 to 4 (no units)– Higher score = stronger pull on electrons

Pauling electronegativity scale

Electronegativity trend

• Electronegativity increases as:– You move right across a period (row)• Closer to a full octet = less likely to give up electrons

– You move up a group (column)• Less electron shells shielding the nucleus = more pull

from positive nucleus holding on to electrons

Extension question

• Explain two reasons why Francium (group 1, period 7) has the lowest electronegativity of any known element

Polar covalent bonds• Polar covalent bonds are bonds in which

bonding electrons are not evenly shared– PCBs occur when electronegativities differ by

more than .4 and less than 1.7– This results in a partial negative charge (δ-) on the

more electronegative atom and a partial positive charge (δ+) on the less electro- negative atom

Electrostatic potential map

• Map which shows the density of electrons around regions of a molecule

• Electrons are not evenly distributed in polar molecules– More electronegative atoms have higher electron

density (red on map)

Water

Electronegativities Oxygen = 3.4Hydrogen = 2.2

Challenge question

• Why do polar covalent bonds only leave partial charges on participating atoms?– Answer: Because the electrons are still shared,

they are just pulled more strongly toward the electronegative atom.

Non-polar covalent bonds

• When electronegativities differ by less than .4, a covalent bond is considered non-polar.

• Electrons are shared equally in these bonds

Electronegativities Carbon = 2.55Hydrogen = 2.2

Methane (CH4)

Review question

• Does the cartoon below show a polar covalent or a non-polar covalent bond? Explain.

Ionic bonds• In ionic bonds, electrons are transferred from

one atom to another.• Lose e- + charge, gain e- - charge.• Charged atoms are called ions• After e- transfer, the ions attract each other

Ionic bonds

• What atoms form ionic bonds? – Ionic bonds occur between atoms that differ in

electronegativity by more than 1.7

Ion forming atoms:

Ion forming atomsPositive ions Negative ions

Kind of element Metals Non-metals

Location on periodic table

Left side Right side

Valence electrons Few (<4) Many (>4)

Practice question• Which of the following pairs could form ionic

bonds?– Potassium and chlorine– Sodium and fluorine– Aluminum and oxygen– Oxygen and fluorine

Do Now:

• Write a definition of a ionic bond– Are electrons shared, donated or received in an

ionic bond? – What force holds atoms together in an ionic bond?

• Binder quiz!• Homework: Chemical bonding 2 due tomorrow

at 8 AM

Hydrogen bonds

• A hydrogen bond is a weak partial bond caused by the charge attraction between:– An exposed hydrogen nucleus (a proton)– A lone pair (non-bonding pair) of electrons

• Why does this occur?– Nucleus is positive– Electrons are negative – Positive & negative attract

Hydrogen bonds• How do you get an “exposed hydrogen

nucleus?”– An exposed hydrogen nucleus occurs when

hydrogen is bonded to a strongly electronegative atom like oxygen, nitrogen, or fluorine

– The strongly polar bond between H and either O, N, or F pulls electrons away from H and exposes the nucleus

Hydrogen bonds

• Where does the lone pair come from? – Usually a nitrogen or oxygen atom– Nitrogen has 5 valence electrons but it only has a

bonding capacity of 3. It has 2 non-bonding electrons (1 lone pair)

– Oxygen has 6 valence e-s, bonding capacity = 2. It has 4 non bonding electrons (2 lone pairs)

Ammonia Water

Hydrogen bonds in water

• Hydrogen electronegativity = 2.2• Oxygen electronegativity = 3.4• Oxygen pulls strongly on bonding e-s• Hydrogen nucleus (proton) is exposed• Lone pairs are attracted

to this nucleus

• Hydrogen bond

Hydrogen bonds

• The bottom line: When a hydrogen atom is in a strongly polar covalent bond (with N, O, or F), its exposed nucleus can form a weak, partial bond with a lone pair of valence electrons

• Molecule with H is called the hydrogen bond donor

• Molecule with lone pair is called the hydrogen bond acceptor

Challenge question

• Why must a hydrogen be bonded to nitrogen, oxygen, or fluorine for it to form a hydrogen bond with a lone pair? – Answer: Because these are the most

electronegative elements, and they pull the bonding electrons strongly to expose the hydrogen nucleus

Practice question

• How many hydrogen bonds can a single water molecule participate in?

Practice question

• Answer: 4 (2 as H-bond donor, 2 as H-bond acceptor)

Why does this matter?

• Hydrogen bonds make water cohesive– Co = together, -he- = sticking (coherent, adhesive)– Water has strong surface tension because each

molecule is connected to four others by hydrogen bonding

Why does this matter?

Why does this matter?

• Hydrogen bonding drastically increases the boiling point of water– Before liquid water can boil, it must break all its

hydrogen bonds so that water molecules can evaporate

– This keeps us from boiling on a hot day.

Liquid Gas

Why does this matter?

• Hydrogen bonds make ice float!– In liquid water, molecules are bouncing around,

and hydrogen bonds easily break and reform.– When water freezes, individual molecules slooow

down. This allows them to form a full set of hydrogen bonds in an ordered crystal

Why does this matter

• Hydrogen bonds make ice float!– So why does ice float?

• Ordered structure of H bonds more empty space in ice crystals lower density

Tuesday 9/20

• Riddle me this: What would happen to your body if suddenly there was no hydrogen bonding? Why would this happen?

• Do later: Re-read Ch. 2 pgs. 18-27– Answer questions #1, 3, 6, 7, 15, 16, 18, 23, 24– Bring your book tomorrow

Challenge question

• Explain:

I would die over the winter without hydrogen bonds!!!

Yaaaayyy!!!

Solubility effects• Water’s strong polarity allows it to dissolve

other polar substances and ions.– Why? Charge attraction. – The charged regions of water attract to charged

regions of other polar molecules and ions, and they dissolve.

Water dissolving ions Sugar molecule

Solubility effects

• Water does not dissolve non-polar substances because they cant form charge interactions– If a non-polar molecule dissolved in water, it

would break up water’s hydrogen bonds and not replace them with anything.

Solubility effects

• Think another way:– Dissolving in water is sort of like a game of red

rover. – Non polar substances don’t have hands to join

(polar regions to bond) with water, so they can’t penetrate in.

Solubility effects

• So what can non-polar molecules dissolve in? – Solutions of other non-polar molecules.– Why? • There are no hydrogen bonds to disrupt

Solubility effects

• The bottom line: – “Like dissolves like”– Polar molecules and ions dissolve in polar solvents– Non polar molecules dissolve in non-polar

solvents

Practice questions

• Determine which of the following molecules are polar or non-polar– Label δ+ and δ-

vinegar

butter

sugar

cooking oil

ElectronegativitiesHydrogen = 2.2Carbon = 2.55Oxygen = 3.44

Practical experience

• See solubility effects in action!– Try dissolving the following solids:• Salt• Sugar• Butter

– Dissolve them in these liquids: • Water• Oil• Vinegar

– What works? What doesn’t? Do you notice any patterns?

Bond strength

• Recall the different kinds of bonds we’ve discussed (covalent, ionic, hydrogen). Which do you think are strongest and weakest? Why?

• Rank from strongest to weakest.