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Discovering that atoms of the same element can have different numbers of neutrons.
Imagine two identical-looking twins: they have the same name, the same face, and the same DNA. But when they step on a scale, one is slightly heavier because they are wearing a heavy backpack. Did you know atoms do the exact same thing?
In chemistry, the number of protons in an atom's nucleus is like its social security number—it defines which element it is. For example, every single Carbon atom in the universe has exactly 6 protons. However, the number of neutrons can change! Atoms that have the same number of protons but different numbers of neutrons are called isotopes. Because neutrons have mass, adding more of them makes the atom heavier without changing what element it is. Think of it like a car: a truck is still a truck whether the trunk is empty or full of bricks.
Quick Check
If two atoms have the same number of protons but different numbers of neutrons, what are they called?
Answer
Isotopes
Scientists use a special shorthand to tell isotopes apart. We write the Mass Number (protons + neutrons) as a superscript before the element symbol. For example, is Carbon-12 and is Carbon-14. Even though Carbon-14 is heavier and used by scientists for carbon dating to find the age of fossils, it still behaves chemically just like Carbon-12 because they both have 6 protons and 6 electrons. The only difference is the 'extra weight' in the nucleus.
Compare Carbon-12 () and Carbon-14 (): 1. Both have 6 protons (this makes them Carbon). 2. Carbon-12 has 6 neutrons (). 3. Carbon-14 has 8 neutrons (). 4. Result: Carbon-14 is heavier by 2 neutrons.
Quick Check
How many neutrons does an isotope of Oxygen-18 have if Oxygen's atomic number is 8?
Answer
10 neutrons
Have you ever wondered why the mass on the Periodic Table is a decimal like instead of a whole number like ? This is because the Periodic Table shows the average atomic mass. This is a weighted average of all the isotopes found in nature. If 99% of Carbon is Carbon-12 and only 1% is Carbon-14, the average mass will be very close to 12, but slightly higher because of that small amount of heavy Carbon-14.
What subatomic particle is different in two isotopes of the same element?
If an isotope is written as , what does the number 13 represent?
Carbon-12 and Carbon-14 have the same chemical properties.
Review Tomorrow
Tomorrow, try to explain to a friend why the atomic mass of Chlorine is 35.45 instead of a whole number.
Practice Activity
Look at a Periodic Table and find 3 elements. Round their atomic mass to the nearest whole number and subtract the atomic number to find the most common number of neutrons.