Atomic Number, Mass Number and Isotopes

Lab Brief #2

🎯 Your mission

Master the concept. Use it tomorrow.

⚡ The twist

Mixing two safe things doesn't always make a safe thing.

What you'll learn

The atomic number is the proton count and fixes which element an atom is. The mass number is protons plus neutrons. Isotopes are atoms of the same element with different neutron counts — chlorine-35 and chlorine-37 behave identically in reactions.

Key words

atomic number
The number of protons in an atom, which identifies the element.
isotope
Atoms of the same element with different numbers of neutrons.

Make a hypothesis first

Predict: will the mixture get hotter, colder, or stay the same?

Say your guess out loud — then find out if you were right.

Step 1 of 4 · Read

Think about what you just learned: say what "atomic number" and "isotope" mean in your own words before you start.

Step 2 of 4 · Do

Given six nuclide symbols, work out protons, neutrons and electrons for each. Then calculate the relative atomic mass of chlorine from 75% chlorine-35 and 25% chlorine-37 and compare with the periodic table value.

Step 3 of 4 · Observe

Record what actually happened, including anything that did not match what you expected.

Step 4 of 4 · Record

Write a conclusion that answers the question this lesson opened with, and state your uncertainty.

Question 1 of 2

Isotopes of an element differ in their number of:

How sure are you?😐 maybe

Question 2 of 2

An atom with 17 protons and 20 neutrons has a mass number of:

How sure are you?😐 maybe

Mind = blown

💧 Hot water can freeze faster than cold water under the right conditions. Nobody fully understands why.

Nice work!

You got 0 of 2 right.

Next: Electron Shells and Configuration All lessons

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✨ More to explore

🌍 Where you see this in real life: This is happening in your kitchen right now — every time food cooks.

🏆 Stretch challenge (try it this week): Find one ingredient in a snack and look up what it does.

👨‍👩‍👧 For the dinner table: “What's something we cook that mixes things together this way?”

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Atomic Number, Mass Number and Isotopes

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