Atomic Number, Ions, and Isotopes Making sense of some basic chemistry numbers: Atomic Number Mass Number Atomic Mass Charge Values.

Atomic Number, Ions, and IsotopesAtomic Number, Ions, and Isotopes

Making sense of some basic chemistry numbers:Making sense of some basic chemistry numbers:

Atomic NumberAtomic Number

Mass NumberMass Number

Atomic MassAtomic Mass

Charge ValuesCharge Values

To review…To review…

At this point, it is assumed that you know about the three At this point, it is assumed that you know about the three main parts of an atom. Below is a synopsis...main parts of an atom. Below is a synopsis...

ElectronElectron– The first subatomic particle discovered (by JJ Thomson).The first subatomic particle discovered (by JJ Thomson).– Exists outside the nucleus in orbitals.Exists outside the nucleus in orbitals.– About 1/2000About 1/2000thth the mass of a proton or neutron. the mass of a proton or neutron.– Holds a negative (-) charge.Holds a negative (-) charge.

ProtonProton– The second subatomic particle discovered (by Ernest The second subatomic particle discovered (by Ernest

Rutherford).Rutherford).– Exists in the small, dense nucleus.Exists in the small, dense nucleus.– Has a mass of about 1 amu (atomic mass unit) .Has a mass of about 1 amu (atomic mass unit) .– Holds a positive (+) charge.Holds a positive (+) charge.

NeutronNeutron– The last subatomic particle discovered (by James Chadwick).The last subatomic particle discovered (by James Chadwick).– Exists with the proton in the small, dense nucleus.Exists with the proton in the small, dense nucleus.– Has a mass similar to that of a proton.Has a mass similar to that of a proton.– Holds NO charge.Holds NO charge.

Other points of note…Other points of note…

The VAST majority of the mass of an atom is The VAST majority of the mass of an atom is held in the nucleus.held in the nucleus.

The atom is mostly empty space. The atom is mostly empty space. All positive charge is held in the nucleus.All positive charge is held in the nucleus. All negative charge is held by electrons, All negative charge is held by electrons,

outside the nucleus. outside the nucleus. Protons and neutrons are both termed Protons and neutrons are both termed

“nucleons,” in that they are found in the “nucleons,” in that they are found in the nucleus.nucleus.

Electrons are much easier to remove from an Electrons are much easier to remove from an atom than nucleons.atom than nucleons.

Atomic NumberAtomic Number

The The atomic numberatomic number is simply the number of protons held in is simply the number of protons held in an atom. It an atom. It definesdefines the element in question. the element in question.

We could just as easily call carbon “element number six” or We could just as easily call carbon “element number six” or helium “element number two,” but the names of many of helium “element number two,” but the names of many of the elements were established long before we knew the elements were established long before we knew anything about protons, so we don’t do that.anything about protons, so we don’t do that.

You CAN NOT separate atomic number from the name of an You CAN NOT separate atomic number from the name of an element. For example, nitrogen will NEVER have an atomic element. For example, nitrogen will NEVER have an atomic number of 12. It will always be 7. That is, the element with number of 12. It will always be 7. That is, the element with 7 protons is always called 7 protons is always called nitrogennitrogen. .

If an element has an atomic number of 12, on the other If an element has an atomic number of 12, on the other hand, it MUST be called hand, it MUST be called magnesiummagnesium..

If we were somehow able to add one proton to magnesium, If we were somehow able to add one proton to magnesium, we would call our new element we would call our new element aluminumaluminum. If you could take . If you could take one proton away from magnesium, we would call our new one proton away from magnesium, we would call our new element element sodiumsodium..

Mass NumberMass Number

Now let’s consider both the protons and the Now let’s consider both the protons and the neutrons (the two nucleons) at the same time.neutrons (the two nucleons) at the same time.

The The mass numbermass number is the sum of the # of protons is the sum of the # of protons and # of neutrons in a and # of neutrons in a particular atom’s particular atom’s nucleus. nucleus.

Notice that we are talking about one specific Notice that we are talking about one specific ATOM of an element in this case, not the ATOM of an element in this case, not the element as a whole.element as a whole.

A specific atom and its mass are generally A specific atom and its mass are generally expressed in (element name)-(mass number) expressed in (element name)-(mass number) format (that’s a dash, not a subtraction sign).format (that’s a dash, not a subtraction sign).

For example: For example: nitrogen-15nitrogen-15 vs. vs. nitrogen-14…nitrogen-14… or or Carbon-12Carbon-12 vs. vs. Carbon-13Carbon-13..

Check Your Check Your Understanding…Understanding… Q: How many protons are in calcium-41? How many Q: How many protons are in calcium-41? How many

neutrons?neutrons? A: By definition, calcium must have 20 protons. And A: By definition, calcium must have 20 protons. And

because its mass number is 41, there are 21 neutrons.because its mass number is 41, there are 21 neutrons. Do NOT look at your periodic table to determine mass Do NOT look at your periodic table to determine mass

number! It’s not there! The mass values that are on number! It’s not there! The mass values that are on periodic tables will be addressed later in this lesson.periodic tables will be addressed later in this lesson.

Q: How many protons and how many neutrons are in Q: How many protons and how many neutrons are in beryllium-10?beryllium-10?

A: 4 protons and 6 neutrons.A: 4 protons and 6 neutrons. Q: What is the mass number of titanium- 48?Q: What is the mass number of titanium- 48? A: 48… yeah, it’s that easy.A: 48… yeah, it’s that easy. Q: How many protons and neutrons are in titanium- 48?Q: How many protons and neutrons are in titanium- 48? A: 22 protons and 26 neutronsA: 22 protons and 26 neutrons

Fill in the table…Fill in the table…

Element No. of protons

No. of neutrons

Mass number

Sodium 11 11 22

26 58

Lithium 4

Oxygen 15

15 16

Notice that in every question you need to be provided either the mass number or the number of neutrons in the atom. We can never infer the number of neutrons by looking at the periodic table.

-Answers are on the next slide-

Table Answers…Table Answers…

Element No. of protons

No. of neutrons

Mass number

Sodium 11 11 22

Iron 26 32 58

Lithium 3 4 7

Oxygen 8 7 15

Phosphorous 15 16 31

Again, notice that in every question you need to be provided either the mass number or the number of neutrons in the atom. Mass numbers are never decimal values. If you were tempted to report the phosphorous mass number as 30.974, then you were dealing with atomic mass, which is quite different from mass number.

IsotopesIsotopes

Isotopes are like different ‘versions’ of a given element. Isotopes are like different ‘versions’ of a given element. They have the same number of protons, but a different They have the same number of protons, but a different number of neutrons. number of neutrons.

In other words, they are atoms of the same element In other words, they are atoms of the same element that have different mass numbers.that have different mass numbers.

Below are two examples. The first shows 3 isotopes of Below are two examples. The first shows 3 isotopes of hydrogen and the second shows 2 isotopes of iron.hydrogen and the second shows 2 isotopes of iron.

Isotope Symbols Isotope Symbols

One way to express all the One way to express all the information about a given information about a given isotope is with the generic isotope is with the generic notation to the right, where…notation to the right, where…

““X” = Element symbolX” = Element symbol– Found via the periodic tableFound via the periodic table

““Z” = Atomic numberZ” = Atomic number– The # of protons The # of protons – Also the # of eAlso the # of e-- in the neutral atom in the neutral atom

““A” = Mass numberA” = Mass number– (# protons) + (# neutrons)(# protons) + (# neutrons)– # nucleons# nucleons

XAZ

Atomic MassAtomic Mass

This is very different than the mass This is very different than the mass number!number!

Mass numberMass number: the mass (in amu) of one : the mass (in amu) of one particularparticular atom of an element. atom of an element.

Atomic massAtomic mass: the : the weighted averageweighted average of of allall the isotope masses of that element that the isotope masses of that element that have been observed in nature.have been observed in nature.

Before we jump headlong into these mass Before we jump headlong into these mass values, let’s consider a couple of values, let’s consider a couple of analogies…analogies…

An example…An example…

Imagine a sample of 10 cans of soda. 3 of them are Imagine a sample of 10 cans of soda. 3 of them are Diet Cola and 7 of them are regular Cola. Assume the Diet Cola and 7 of them are regular Cola. Assume the Diet Colas each weigh 1 pound, while the regulars Diet Colas each weigh 1 pound, while the regulars each weigh 2 pounds.each weigh 2 pounds.

Is the average weight of each soda in our sample of Is the average weight of each soda in our sample of 10 sodas going to be 1½ pounds, the average of the 10 sodas going to be 1½ pounds, the average of the two weights? two weights?

No, that would only be the case if it was a 50/50 No, that would only be the case if it was a 50/50 mixture. However, we have 3 of one and 7 of the mixture. However, we have 3 of one and 7 of the other.other.

The average is going to be closer to the weight of the The average is going to be closer to the weight of the regular Cola.regular Cola.

Average = (0.7*2)+(0.3*1) = 1.7 pounds per soda.Average = (0.7*2)+(0.3*1) = 1.7 pounds per soda. The weight of the 7 regular colas is 1.4 pounds. The 3 The weight of the 7 regular colas is 1.4 pounds. The 3

diet colas contribute 0.3 pounds to the average mass.diet colas contribute 0.3 pounds to the average mass.

Another example…Another example…

Older pennies weigh more than modern pennies (this Older pennies weigh more than modern pennies (this is due to a change in composition that took place in is due to a change in composition that took place in 1982). Newer pennies are mostly zinc, while older, 1982). Newer pennies are mostly zinc, while older, pre-1982 pennies are mostly copper. As such, older pre-1982 pennies are mostly copper. As such, older pennies weigh more than newer ones. pennies weigh more than newer ones.

Imagine you have a sample of 100 pennies, 72 of Imagine you have a sample of 100 pennies, 72 of them are new ones and 28 of them are old. them are new ones and 28 of them are old.

Let’s assume that the new pennies weigh 2.5 grams Let’s assume that the new pennies weigh 2.5 grams and the older ones weigh 3.0 grams.and the older ones weigh 3.0 grams.

What is the average mass of a coin in our sample of What is the average mass of a coin in our sample of 100 coins? 2.5 grams? 3.0 grams? 2.75 grams?100 coins? 2.5 grams? 3.0 grams? 2.75 grams?

To find out, we would take into account each ‘isotope’ To find out, we would take into account each ‘isotope’ of our ‘pennium’ element (there are two ‘isotopes’ –of our ‘pennium’ element (there are two ‘isotopes’ –old and new).old and new).

Cont’d.Cont’d.

In our analogy…In our analogy…– Our ‘element’ is “pennium” and it has two isotopes.Our ‘element’ is “pennium” and it has two isotopes.– Our ‘isotopes’ are “old pennium” and “new pennium.”Our ‘isotopes’ are “old pennium” and “new pennium.”

The average (weighted) mass of our pennies will take into The average (weighted) mass of our pennies will take into account individual isotope masses AND the relative abundance account individual isotope masses AND the relative abundance of each isotope via the formula:of each isotope via the formula:– Average mass = (relative abundanceAverage mass = (relative abundance11*mass*mass11) +(relative abundance) +(relative abundance22*mass*mass22) )

+ …+ …– Relative AbundanceRelative Abundance is the number of atoms of a particular element expressed is the number of atoms of a particular element expressed

as a percentage of the total sample.as a percentage of the total sample.– Note that we can add more terms to our equation if there are more isotopes.Note that we can add more terms to our equation if there are more isotopes.

So, the average mass of a penny in our sample is (0.72*2.5) + So, the average mass of a penny in our sample is (0.72*2.5) + (0.28*3.0), or (0.28*3.0), or 2.64 grams2.64 grams. .

Notice that there is no single penny that actually weighs 2.64 Notice that there is no single penny that actually weighs 2.64 grams. It is just a weighted average of all the pennies’ masses.grams. It is just a weighted average of all the pennies’ masses.

Also notice that the average is closer to the weight of the more Also notice that the average is closer to the weight of the more abundant penny.abundant penny.

A A chemistrychemistry example example

75.53% of all chlorine observed is 75.53% of all chlorine observed is chlorine-35.chlorine-35.– Cl-35 = 75.53% of all chlorine atomsCl-35 = 75.53% of all chlorine atoms

24.47% of all chlorine observed is 24.47% of all chlorine observed is chlorine-37. chlorine-37. – Cl-37 = 24.47% of all chlorine atomsCl-37 = 24.47% of all chlorine atoms

What, then, is the average mass of What, then, is the average mass of chlorine?chlorine?

Ave = (.7753*35)+(.2447*37)Ave = (.7753*35)+(.2447*37)

= 35.5 amu= 35.5 amu

Atomic masses Atomic masses are found on the are found on the periodic table, typically underneath periodic table, typically underneath the symbol.the symbol.

Check Your Check Your Understanding…Understanding… Given the following information, solve for the atomic mass of each Given the following information, solve for the atomic mass of each

element.element. Gallium has two isotopes. gallium-69 is present in 60.10% of all Gallium has two isotopes. gallium-69 is present in 60.10% of all

gallium, while gallium-71 comprises the other 39.9%. Q:What is the gallium, while gallium-71 comprises the other 39.9%. Q:What is the atomic mass of Gallium? Calculate it, don’t simply look at your periodic atomic mass of Gallium? Calculate it, don’t simply look at your periodic table.table.

A: 69.798 amu A: 69.798 amu – (note: we are taking some liberties with the mass of each isotope in assuming (note: we are taking some liberties with the mass of each isotope in assuming

them to be exactly 69 and 71. Actual isotope masses are a bit different, them to be exactly 69 and 71. Actual isotope masses are a bit different, which is why our answer is a bit off from what you’ll find on a periodic table.)which is why our answer is a bit off from what you’ll find on a periodic table.)

There are three isotopes of magnesium. Mg-24 (78.9%), Mg-25 There are three isotopes of magnesium. Mg-24 (78.9%), Mg-25 (10.0%), and Mg-26 (11.1%). What is the atomic mass?(10.0%), and Mg-26 (11.1%). What is the atomic mass?

A: 24.322 amuA: 24.322 amu Q: What is the mass of element X if X’s three isotopes are X-123 (15%), Q: What is the mass of element X if X’s three isotopes are X-123 (15%),

X-125 (30%) and X-126 (55%)?X-125 (30%) and X-126 (55%)? A: 125.25 amuA: 125.25 amu

But how do we know But how do we know the masses?the masses? Dalton and others helped to determine Dalton and others helped to determine

the relative masses of elements on the the relative masses of elements on the periodic table via experiment.periodic table via experiment.

Take HCl, for example…Take HCl, for example…– There is a 1:1 ratio of H atoms to Cl atoms.There is a 1:1 ratio of H atoms to Cl atoms.– Experimentally, we’ve found that there is Experimentally, we’ve found that there is

always about 35.2 times as much Cl mass always about 35.2 times as much Cl mass than H mass in HCl.than H mass in HCl.

Therefore, chlorine must have a mass Therefore, chlorine must have a mass that is about 35.2 times as great as that is about 35.2 times as great as hydrogen.hydrogen.

Carbon-12 is the Carbon-12 is the standardstandard Carbon-12 is defined as having a mass Carbon-12 is defined as having a mass

value of 12.0000 (repeating).value of 12.0000 (repeating). All other elements are compared All other elements are compared

against this isotope of carbon.against this isotope of carbon. Hydrogen, then, has a mass of 1.008. Hydrogen, then, has a mass of 1.008. Chlorine has a mass of 35.45 (Not Chlorine has a mass of 35.45 (Not

35.2, as it would if it was compared 35.2, as it would if it was compared against H).against H).

Masses are expressed as Masses are expressed as amuamu (atomic (atomic mass units).mass units).

So, why do we care so much So, why do we care so much about masses? Another about masses? Another analogy…analogy… In the world of chemistry, we rely on masses to determine In the world of chemistry, we rely on masses to determine

whether or not we have the right amount of a substance.whether or not we have the right amount of a substance. Imagine that a head chef requires 1,000 oranges and 1,000 Imagine that a head chef requires 1,000 oranges and 1,000

cantaloupes. You are in charge of making sure that the cantaloupes. You are in charge of making sure that the correct number of each is delivered. However, you only have correct number of each is delivered. However, you only have 15 minutes to verify this and you are armed with nothing but 15 minutes to verify this and you are armed with nothing but a scale.a scale.

Because Because everyone everyone knows that an average orange is 150 knows that an average orange is 150 gg and and an average cantaloupe is 450 an average cantaloupe is 450 gg (we’ll suppose), you can (we’ll suppose), you can weigh the supplies of each to determine how many of each weigh the supplies of each to determine how many of each fruit you have.fruit you have.

As long as you have at least 150 kg of oranges and 450 kg of As long as you have at least 150 kg of oranges and 450 kg of cantaloupe, your chef should be happy.cantaloupe, your chef should be happy.

Note: just like chemists don’t actually Note: just like chemists don’t actually countcount atoms, you didn’t atoms, you didn’t actually actually countcount fruit. Your work was all based on mass. fruit. Your work was all based on mass.

Therein lies the value of knowing the masses of our elements! Therein lies the value of knowing the masses of our elements! We can simply weigh out an amount of a substance and use We can simply weigh out an amount of a substance and use that mass to infer how many atoms/molecules we have.that mass to infer how many atoms/molecules we have.

Now, on to electrons…Now, on to electrons…

In the previous slides we have In the previous slides we have considered protons and the considered protons and the combination of protons with combination of protons with neutrons. neutrons.

In the upcoming slides we will In the upcoming slides we will bring electrons into the fold.bring electrons into the fold.

The # of electrons The # of electrons helps to determine helps to determine chargecharge Neutral atoms have the same number of Neutral atoms have the same number of

protons as they do electrons.protons as they do electrons. This is because each proton has a +1 This is because each proton has a +1

charge and each electron has a -1 charge.charge and each electron has a -1 charge. You CAN NOT change the number of You CAN NOT change the number of

protons in an atom without significantly protons in an atom without significantly changing the atom itself and its changing the atom itself and its properties.properties.

You CAN change the number of electrons, You CAN change the number of electrons, though, and rather easily.though, and rather easily.

Determining chargeDetermining charge

Suppose element #1 has 9 protons and 10 electrons.Suppose element #1 has 9 protons and 10 electrons. What element is it?What element is it? Answer: fluorineAnswer: fluorine What is its charge?What is its charge? Answer: -1 (so FAnswer: -1 (so F-- would be our symbol) would be our symbol) Suppose element #2 has 12 protons and 10 electrons.Suppose element #2 has 12 protons and 10 electrons. What element is it? What is its charge?What element is it? What is its charge? Answer: MgAnswer: Mg2+2+

Suppose element #3 has 47 protons and 46 electrons.Suppose element #3 has 47 protons and 46 electrons. What element is it? What is its charge?What element is it? What is its charge? Answer: AgAnswer: Ag++

Suppose element #4 has 16 protons and 18 electrons.Suppose element #4 has 16 protons and 18 electrons. What element is it? What is its charge?What element is it? What is its charge? SS2-2-

Naming charged Naming charged particlesparticles

Charged particles are called Charged particles are called ionsions.. Positively charged ions are called Positively charged ions are called cationscations..

– One way to remember this is to think of the One way to remember this is to think of the tt as a (+) as a (+) sign (as in sign (as in ca+ionca+ion).).

Negatively charged ions are called Negatively charged ions are called anionsanions..– I have no clever way of remembering that one…I have no clever way of remembering that one…

With cations we need not change anything about the With cations we need not change anything about the name. For example, we would just term Mgname. For example, we would just term Mg2+2+ as the as the magnesium cationmagnesium cation. .

However, with anions we need to change the ending to ‘-However, with anions we need to change the ending to ‘-ide.’ide.’

Ex: chlorine with one extra electron is called the Ex: chlorine with one extra electron is called the chloride chloride anionanion (not the (not the chlorinechlorine anion). anion).

Other examples: oxide anion, sulfide anion, sodium cation, Other examples: oxide anion, sulfide anion, sodium cation, fluoride anion, copper cation.fluoride anion, copper cation.

To summarize…To summarize…

In this slideshow we attempted to explain…In this slideshow we attempted to explain…– Atomic numberAtomic number– Mass numberMass number– IsotopesIsotopes– Atomic massAtomic mass– Relative massesRelative masses– The utility of using masses of elementsThe utility of using masses of elements– IonsIons

That’s That’s a lot a lot of information. If there’s something that of information. If there’s something that doesn’t make sense, please go back and re-visit the doesn’t make sense, please go back and re-visit the appropriate slides. If something still doesn’t make appropriate slides. If something still doesn’t make sense, please use the “QnA” feature associated with sense, please use the “QnA” feature associated with this presentation.this presentation.