TEAS Atomic Mass and Mass Number/ chemistry/ pt.2

🧪 1. First, understand the atom

Everything is made of atoms. Inside an atom, we have three important particles:

Particle	Charge	Where is it?
Proton	Positive (+)	Center
Neutron	Neutral (0)	Center
Electron	Negative (-)	Outside

IMPORTANT: Protons and neutrons are in the center (nucleus). They are what we use to calculate the mass number.

2. What is MASS NUMBER?

Mass number = Protons + Neutrons

Think of it like adding two numbers together.

Imagine an atom has:

• 18 protons

• 22 neutrons

Add them:

18 + 22 = 40

Mass number = 40

18

Protons

22

Neutrons

18 + 22 = 40

Memory trick:

🧠 MASS NUMBER = ADD THEM!

Protons + Neutrons = Mass number.

3. What if they ask you to find the number of NEUTRONS?

Instead of adding, you subtract.

Neutrons = Mass number − Protons

Let's use Argon-40.

We know:

• Mass number = 40

• Protons = 18

• Neutrons = ?

40 − 18 = 22 neutrons

Answer: 22 neutrons

Memory trick:

• Need MASS NUMBER? ADD.

• Need NEUTRONS? SUBTRACT.

That's it!

4. What are ISOTOPES?

Isotopes are simply the same element with different numbers of neutrons.

Let's use Argon.

All argon atoms ALWAYS have 18 protons.

But they can have different numbers of neutrons.

Look:

Isotope	Protons	Neutrons	Mass number
Argon-36	18	18	36
Argon-38	18	20	38
Argon-40	18	22	40

Notice something?

ALL THREE have 18 protons.

But their neutrons are different!

Memory trick:

🧠 ISOTOPES = SAME PROTONS, DIFFERENT NEUTRONS.

Think of them as siblings:

• Same family (same element).

• Different amount of neutrons.

5. What is AVERAGE ATOMIC MASS?

Look at your periodic table.

You might see something like this:

The number around 39.95 is the average atomic mass.

Why isn't it exactly 36, 38, or 40?

Because nature has different isotopes of argon!

Think about it:

• Some argon atoms weigh about 36.

• Some weigh about 38.

• MOST weigh about 40.

Since most argon atoms are Argon-40, the average ends up very close to 40.

Look at the abundance:

Isotope	How common?
Argon-36	0.337%
Argon-38	0.063%
Argon-40	99.6%

Argon-40 is the most common, so it has the biggest influence on the average.

Think of it like a classroom:

If 99 students weigh around 40 kg and 1 student weighs around 36 kg, the class average will be much closer to 40.

That's the basic idea!

Remember

Average atomic mass = the weighted average of the masses of an element's naturally occurring isotopes.

6. MASS NUMBER vs. AVERAGE ATOMIC MASS

Mass number	Average atomic mass
Protons + neutrons	Average of isotopes
Always a whole number	Usually a decimal
Example: 40	Example: 39.95

Catchy memory trick:

• MASS NUMBER = ADD the particles.

• AVERAGE ATOMIC MASS = AVERAGE the isotopes.

⭐ Your ultimate cheat sheet

Memorize these 5 things:

1. Proton = Positive (+).

2. Neutron = Neutral (0).

3. Mass number = Protons + Neutrons.

4. Isotopes = Same protons, different neutrons.

5. Average atomic mass = Average of all naturally occurring isotopes, accounting for how common each one is.

1. An atom has 10 protons and 10 neutrons. What is its mass number?

20

10 + 10 = 20.

4. Argon-40 has 18 protons. How many neutrons?

22

40 − 18 = 22.

2. An atom has a mass number of 23 and 11 protons. How many neutrons?

12

23 − 11 = 12 neutrons.

3. What makes isotopes different from one another?

Different neutrons

Isotopes have the same number of protons but different numbers of neutrons.

5. Why is argon's average atomic mass close to 40?

Argon-40 is the most abundant isotope

makes up about 99.6% of naturally occurring argon.

The PERIODIC TABLE gives you

the atomic number, and the atomic number tells you the number of PROTONS.

1. First, what are ELECTRONS?

Remember the three particles inside an atom?

Particle	Charge	Easy way to remember
Proton	+ Positive	Plus
Neutron	0 Neutral	No charge
Electron	− Negative	Minus

What does neutral mean?

Neutral means the atom has NO overall charge.

Why? Because the positive and negative charges balance each other!

Imagine:

• 18 positive protons (+)

• 18 negative electrons (−)

They cancel each other out!

A neutral Argon atom

+18

Protons

−18

Electrons

Net charge = 0

Positive and negative charges cancel!

Memory trick:

🧠 NEUTRAL = EQUAL!

Protons = Electrons.

2. How do I find the number of PROTONS?

Look at the element on the periodic table.

Find its atomic number.

Atomic number = Number of protons.

For example, Argon:

Argon's atomic number is 18.

Therefore:

18 atomic number = 18 protons.

Memory trick:

🧠 ATOMIC NUMBER = PROTON NUMBER.

They are ALWAYS equal.

3. How do I find the number of ELECTRONS?

This is where you use the word NEUTRAL.

If the atom has NO charge written next to it, assume it is neutral.

For a neutral atom:

Electrons = Protons

Example: Argon

• Protons = 18

• Electrons = 18

Why? Because the positive and negative charges must balance.

Easy rule

Neutral atom? Just copy the proton number!

4. How do I find NEUTRONS?

Neutrons = Mass number − Protons.

Example: Argon-40

• Mass number = 40

• Protons = 18

40 − 18 = 22 neutrons.

5. THE ULTIMATE TEAS CHEAT SHEET

protons vs N vs E

What are you finding?	What do you do?
Protons	Atomic number
Neutrons	Mass number − Protons
Electrons (neutral atom)	Same as protons

🧠 Remember: P-N-E

P = Protons

Atomic number tells you!

N = Neutrons

Subtract!

E = Electrons

Equal to protons if neutral!

One more IMPORTANT thing: What if the atom has a charge?

You will eventually see atoms with a little + or − sign.

These are called ions.

An ion is an atom that has gained or lost electrons.

For now, remember:

• Positive (+) charge = Lost electrons.

• Negative (−) charge = Gained electrons.

Example:

Argon example	Electrons
Neutral Ar	18
Ar²⁺	16
Ar²⁻	20

Don't worry about mastering ions just yet. Focus on neutral atoms first!

• Protons = Atomic number.

• Neutrons = Mass number minus atomic number.

• Electrons = Protons (if neutral).

1. An atom has an atomic number of 12. How many protons does it have?

12

Atomic number ALWAYS equals the number of protons.

2. A neutral atom has 15 protons. How many electrons?

15

Neutral means protons and electrons are equal.

3. An atom has a mass number of 35 and 17 protons. How many neutrons?

18

35 − 17 = 18 neutrons.

4. What does NEUTRAL mean?

Equal positive and negative charges

Equal protons and electrons balance the charges.

5. An atom has 20 protons and 20 electrons. What is its overall charge?

0

20 positive and 20 negative charges cancel out.