Daniel R. BarnesInit:10/2/09

. . . draw an atom when given a chemical symbol with a mass number and an electric charge.

“SWBAT = “Students will be able to”

There are many ways to draw atoms, but in this class, you’re going to learn the Mr. Barnes method.

It’s not the only good way to draw atoms. In fact, as you probably already know, it’s unrealistic in many ways, but it’s useful for learning purposes.

So here we go. If you haven’t already got paper and something to write with, get’em out, because you’re going to need to TAKE NOTES on the process.

You’re also going to need use the periodic table side of your CST Reference Sheet. If you’re at home right now and don’t have one, click the red button and the pdf will pop up. CST Reference Sheet

If you don’t have time to watch a few examples done, and just want to skip to a numbered list of steps, click the stopwatch below.

BWe’ll start by drawing a normal boron atom.

Boron is element number 5 on the periodic table, and its chemical symbol is a capital “B”.

Boron atoms can help turn regular glass into heat-resistant glass.

You’ll be using borosilicate glassware during labs this year whenever we need to heat up some chemicals.

BStep #1 in drawing an atom in Mr. Barnes’ class is to copy the element’s box from the periodic table.5

B10.81

As we’ve noted already, boron is element #5 on the periodic table.

The “atomic number” of boron is 5.

The atomic number of an element is how many protons are in an atom of that element.

Protons are found in the nucleus of an atom, which is in the center of the atom.

So, the next step is to draw the protons. You should make your protons about as big as a pencil eraser.

According to the “liquid drop” model of the nucleus, the protons and neutrons move around a lot, like people wandering around in a crowded party.

B 5

B10.81

REALITY CHECK!In a real nucleus, the protons would be mixed in with neutrons, but here we’re drawing them clumped together with their own kind, as though they liked each other.

When free to move, protons will try to get away from each other as much as they can, since they find each other’s positive electric charge to be quite repulsive.

In fact, if it weren’t for the “strong force” holding the nucleons together, the protons would just fly out of the nucleus altogether, disgusted by each other’s positivity.

According to the “liquid drop” model of the nucleus, the protons and neutrons move around a lot, like people wandering around in a crowded party.

B 5

B10.81

REALITY CHECK!In a real nucleus, the protons would be mixed in with neutrons, but here we’re drawing them clumped together with their own kind, as though they liked each other.

When free to move, protons will try to get away from each other as much as they can, since they find each other’s positive electric charge to be quite repulsive.

In fact, if it weren’t for the “strong force” holding the nucleons together, the protons would just fly out of the nucleus altogether, disgusted by each other’s positivity.

B 5

B10.81

Anyway, the next step in drawing an atom is to figure out how many neutrons to draw.

11

Although all boron atoms have five protons, a boron atom can have any number of neutrons, in theory.

Realistically, though, if an atom has too many neutrons or too few neutrons, it will be unstable and fall apart.

Such unstable atoms are called “radioactive” and they’re very dangerous.

Let’s assume, though, that the atom we’re drawing is a typical boron atom, an example of the “isotope” of boron that has the usual number of neutrons

To get the number of neutrons for a typical atom of an element, round the average atomic mass of the element to the closest whole number.

B 5

B10.81

The funny looking symbol next to the eleven, “”, is called “sigma”, and it’s the letter “S” from the Greek alphabet.

11

Sigma is a symbol used in math and science to represent “sum” or “total”.

I use it here because when you round the average atomic mass to the closest whole number, you get the “mass number” of the most common isotope of the element.

The mass number of an atom is the total number of things in the nucleus of the atom.

In other words, the mass number is the sum of the number of protons and the number of neutrons added together.

When you’re drawing an atom, once you’ve got your mass number, you subtract the atomic number, which is the number of protons.

B 5

B10.81

Now, it stands to reason that if there are eleven total things in the nucleus and five of them are protons, the other six must be neutrons. Let’s do the math to show this.

11-56

Mass number – atomic number = number of neutrons.

I like to use black circles to represent neutrons because when you fill in a circle with blackness, there’s no room to write anything in the circle -- no room for a plus, no room for a minus. Draw neutrons just as big as you draw protons, and shade them in.

Neutrons are neither positive nor negative. They’re neutral.

Now that we’ve drawn the protons and neutrons, we’re done drawing the nucleus of the atom.B 5

B10.8111

-56

However, an atom just isn’t an atom unless it’s got some electrons orbiting the nucleus.

The letter “B” representing this boron atom doesn’t have any plus or minus signs attached to it, so this boron atom is assumed to be neutral.

If an atom is neutral, the number of minuses equals the number of plusses. Therefore, the number of electrons equals the number of protons. This boron atom has five electrons.

B 5

B10.8111

-56

Where and how do we draw those five electrons?

This is one of the places where the drawing becomes VERY unrealistic, but we’re going to ignore that and draw it nice and pretty anyway.

As you will learn soon enough, the properties of the elements follow a repeating rhythm on the periodic table.

Let’s look at the PT for a second and see what I mean.

Most of the elements in the first column of the periodic table, the “alkali metals”, react violently with water.

The elements at the far right hand side of the table are all “noble” gases that don’t react with anything.

The elements in column 7A, the “halogens”, are all reactive nonmetals that form salts when combined with metals.

B 5

B10.8111

-56

Because of the rhythmic patterns in the periodic table, scientists regard electrons as existing in “shells”

The shells are nested around the nucleus of the atom like the layers in an onion.

The number of layers in an atom depends on which row of the periodic table it’s from

Since boron is from the second row of the periodic table, it only has two shells.

B 5

B10.8111

-56

A normal boron atom has five electrons existing in two shells, but how many electrons are in each shell?

Once again, for our answer, we go to the periodic table.

Because there are only two elements in the first row of the periodic table, the first shell can hold only two electrons.

Because there are eight elements in the second row of the periodic table, the second shell can hold only eight electrons.

. . . and so on . . .

B 5

B10.8111

-56

Electrons are negative and protons are positive, so electrons try to get as close to the nucleus as possible.

Therefore, you should always fill the lowest shell with electrons before you start putting electrons in the farther-out shells.

Electrons feel an “electrostatic” attraction toward the protons in the nucleus because of their electric charges.

The first shell is full. We can put electrons in the 2nd shell now.

The second shell isn’t full, but we’ve got five electrons now, so that’s it.

B 5

B10.8111

-56 Awesome atom!

Great job!

5

# = #neutral, so 5

If I ever ask you, on a test, to draw an atom, this is what your paper should look like.

You need to draw the atom, but you also need to show, with numbers, arrows, and symbols, how you got the amount of each particle.

Q1: What are the three main subatomic particles that atoms are made of?A: protons neutrons, and electrons

Q2: Where are protons, neutrons, and electrons found?A: Protons and neutrons are found in the nucleus. Electrons orbit the nucleus, grouping into shells.

Q3: What determines the number of protons in an atom?

A: The atomic number of the element is the number of protons in any atom of that element.

Q4: What is the equation for number of neutrons?

A: # neutrons = mass number – atomic number

Q6: In what kind of atoms is the number of electrons equal to the number of protons?A: neutral atoms only

Q7: How many electrons can fit in each shell?A: two in the first, eight each in the second and third, 18@ in the 4th & 5th, and 32@ in the 6th & 7th shells.

Q5: If you are not given the mass number of an atom, how should you determine the mass number?A: Round the average atomic mass of the element to the closest whole number.

Now, let’s try a slightly more complicated atom.

Sometimes, an atom’s symbol will have a number in the upper left superscript position.

O15This is the mass number of the atom.

The mass number tells you which isotope of the element the atom belongs to.

If the mass number is very close to the average atomic mass of the element, the atom probably isn’t radioactive.

If the mass number of the atom is substantially different from the average atomic mass of the element, the nucleus of the atom will probably be unstable and dangerous. When it decays, it could cause the dreaded disease cancer by causing genetic mutations in your DNA.

The real reason an isotope with a weird mass number is radioactive is because of the following equation:O15

Mass number = number of protons + number of neutrons

As we said before, if an atom has too many neutrons or too few neutrons, that makes the nucleus radioactively unstable.

What does the “15” on this oxygen atom’s symbol mean?

The mass number of this particular oxygen atom is fifteen.

This particular atom belongs to the isotope “oxygen-15”.

The average atomic mass of oxygen is 16.00, so the isotope oxygen-15 probably is radioactive.

Is this isotope of oxygen radioactive?

O15

It turns out that oxygen-15 is radioactive. It’s very unstable, in fact, so unstable that it has a “half-life” of only 122.24 seconds. That’s a short amount of time, which means oxygen-15 atoms decay very quickly.

This doesn’t affect our drawing, though, so let’s continue.

As always, the first step in drawing the atom is to copy the correct box from the periodic table.O15

8

O16.00

8

The atomic number of oxygen is eight, so this atom is going to have eight protons.

Because this atom has a mass number specified, we ignore the average atomic mass of oxygen when calculating the number of neutrons.

Instead of using 16, we use 15 as our mass number.

Otherwise, the neutron math is the same as the last atom.

15- 87

Now we have enough information to draw the nucleus of the atom.

O158

O16.00

8

15- 87

As always, the nucleus of an atom is in the center of the atom.

That’s a great nucleus, but we still need electrons to complete the atom.

Like the boron atom we did before, this atom of oxygen has no electrical charge indicated in the upper right superscript position.

Like the boron atom before, this oxygen atom is neutral.

In this atom, # of electrons = # of protons, so there are eight electrons.

Where do we put our eight electrons? In shells, just like before.

Here’s how I want you to show your line of reasoning on your paper on tests . . .

O158

O16.00

8

15- 87

# = #neutral, so 8

How do we draw a shell?O158

O16.00

8

15- 87

# = #neutral, so 8

Yes, as a nice, big circle.

How many electrons can fit into the first shell?

Two.

What if you forget that only two electrons go in the first shell?

Look at the periodic table and count the number of elements in the first row.

O158

O16.00

8

15- 87

# = #neutral, so 8

How do we draw the 2nd shell?

As an even bigger circle.

How many electrons go in the 2nd shell?

8 – 2 = 6

O158

O16.00

8

15- 87

# = #neutral, so 8

Q7: What do you call a number that appears to the upper left or the upper right of an atom’s chemical symbol?A: a “superscript”

Q8: What piece of information appears in the left superscript position in an atom’s symbol?A: mass number

Q9: What does an atom’s mass number tell you about that atom?A: which isotope the atom belongs to; total # of protons & neutrons & # of neutrons when atomic # subtracted from it.

Now, we need to draw some ions.

Be 4

Be9.01

42+

This is a beryllium atom, so it is guaranteed to have four protons.

Be 4

Be9.01

4

9-45

2+

No mass number is specified, so we round the average atomic mass to the closest whole number to get the mass number of this very typical beryllium atom.

, so 4 – 2 =

Be 4

Be9.01

4

9-45

2 missing 2

2+

The positive electric charge indicates that there are missing electrons. The number two tells us how many.

If this atom were neutral, it would have just as many electrons as protons (4), but two electrons are missing, so the atom has two electrons in its possession.

With only two electrons, we need only one shell to stick them in.

, so 4 – 2 =

Be 4

Be9.01

4

9-45

2 missing 2

2+

9, so 6 + 3 =

C 6

C12.01

6

13- 67

3 extra

3-13

9, so 6 + 3 =

C 6

C12.01

6

13- 67

3 extra

3-13

Q10: What is an ion?

A: an atom with a non-zero electric charge (must be + or – but can’t be neutral)

Q11: What makes an ion have a positive electric charge? A negative electric charge?

A: losing electrons positive charge; gaining electrons negative charge

Q12: Given an equation for number of electrons.

A: # of electrons = atomic number – electric charge

ORA: more protons than electrons positive charge; more electrons than protons negative charge.

Let’s do some more to make sure everybody remembers how to do this.

# = #

H 1

H1.01

1

2-11

neutral, so 1

2

10, so 8 + 2 =

O 8

O16.00

8

16- 88

2 extra

2-

1. # of protons = atomic number of element (see PTotE)2. If mass number is not given, round average atomic mass to

closest whole number to get mass number.3. # of neutrons = mass # - atomic # 4. # of electrons = atomic number – electric charge5. Draw protons and neutrons together in center to form nucleus6. Do the following until you run out of electrons: Put 1st 2 e- in 1st shell. Put next 8 e- in 2nd shell. Put next 8 e- in 3rd shell. Put next 18 e- in 4th shell. Put next 18 e- in 5th shell. Put next 32 e- in 6th shell. Put next 32 e- in 7th shell.

Atom-Drawing Steps

C3-13

massnumber

electriccharge

chemicalsymbolPTotE = periodic table of the elementse- = electron

Now, try drawing the atoms listed on the back of the “Atom Drawing Worksheet”

Press the green button below if and when you want to open up the Atom Drawing Worksheet Answer Key Power Point. It shows you, step by step, how each atom on the worksheet is supposed to be drawn.

Atom Drawing Worksheet ANSWER KEY

Power Point green button

Atom Drawing Worksheet

What follows is just a junkyard of construction scraps.

POWER POINT OVER. GO AWAY!