Grade 7

Intermediate

10 min

Lesson 12 of 12

Putting it All Together: Atomic Review

Not Started

A comprehensive review of subatomic particles, isotopes, and electron configuration.

Did you know that you are 99.99% empty space? If an atom's nucleus were the size of a marble in the center of a football stadium, the electrons would be like tiny gnats buzzing in the very top row of the stands.

Learning Objectives

1.By the end of this lesson, you will be able to calculate the mass and charge of any atom using its subatomic particles.
2.You'll understand how to identify an element even when its neutron or electron counts change.
3.You will be able to predict the electron arrangement for the first 20 elements using the 2-8-8 rule.

The Nucleus: The Atom's Heavy Heart

At the center of every atom lies the nucleus, containing two types of particles: protons (

p^+

) and neutrons (

n^0

). The number of protons is the atom's 'social security number'—it defines the Atomic Number (

Z

). If you change the number of protons, you change the element itself! Neutrons act as the 'nuclear glue,' adding mass without changing the charge. Together, these two particles determine the Mass Number (

A

), calculated as:

A = \text{protons} + \text{neutrons}

Because protons and neutrons are roughly 1,800 times heavier than electrons, we ignore electrons when calculating mass.

Let's look at a Carbon atom. 1. Identify the protons: Carbon always has $6$ protons. 2. Identify the neutrons: A standard Carbon atom has $6$ neutrons. 3. Add them together: $6 + 6 = 12$ . 4. The Mass Number is $12$ .

Quick Check

If an atom has 11 protons and 12 neutrons, what is its mass number and which element is it?

Check a periodic table for atomic number 11.

Answer

The mass number is 23, and the element is Sodium (Na).

The Cloud: Electrons and Electrical Balance

Surrounding the nucleus are electrons (

e^-

), which carry a negative charge. In a neutral atom, the number of negative electrons perfectly balances the number of positive protons. However, atoms often gain or lose electrons to become ions. If an atom loses an electron, it becomes a positive cation (think 'plussy cat'). If it gains an electron, it becomes a negative anion. The Net Charge is calculated by:

\text{Charge} = (\text{number of protons}) - (\text{number of electrons})

An Oxygen atom ( $8$ protons) gains $2$ electrons to fill its outer shell. 1. Protons = $+8$ 2. Electrons = $10$ (the original $8$ plus $2$ extra) 3. Calculation: $8 - 10 = -2$ 4. The result is an Oxygen ion with a $-2$ charge, written as $O^{2-}$ .

Quick Check

A Magnesium atom has 12 protons and 10 electrons. What is its charge?

Subtract the number of electrons from the number of protons.

Answer

The charge is +2.

Isotopes and Electron Shells

Not all atoms of the same element are identical. Isotopes are versions of an element with the same number of protons but different numbers of neutrons. This makes them heavier or lighter, but they behave the same chemically. Speaking of behavior, electrons live in energy levels or shells. For the first 20 elements, we follow the 2-8-8 rule: the first shell holds $2$ electrons, the second holds $8$ , and the third holds $8$ . The electrons in the outermost shell are called valence electrons, and they are the 'hands' the atom uses to bond with others.

Identify this atom: It has a mass of $35$ , a charge of $-1$ , and $18$ neutrons. 1. Find Protons: $\text{Mass} (35) - \text{Neutrons} (18) = 17$ protons. 2. Identify Element: Atomic number $17$ is Chlorine ( $Cl$ ). 3. Find Electrons: Since the charge is $-1$ , it has one more electron than protons. $17 + 1 = 18$ electrons. 4. Shell Arrangement: $2$ in the first, $8$ in the second, and $8$ in the third.

Key Takeaways

1Protons define the element's identity (Atomic Number).
2Mass Number is the sum of protons and neutrons ( $A = p + n$ ).
3Isotopes have the same number of protons but different neutron counts.
4Ions are formed when atoms gain or lose electrons, changing the net charge.
5The 2-8-8 rule governs how electrons fill energy levels for the first 20 elements.

Quick Check

Multiple Choice

Which subatomic particle has a positive charge and is located in the nucleus?

Multiple Choice

An isotope of Carbon has 6 protons and 8 neutrons. What is its mass number?

True/False

A neutral atom always has the same number of protons and electrons.

What's Next?

Review Tomorrow

In 24 hours, try to explain to a friend the difference between an isotope and an ion without looking at your notes.

Practice Activity

Pick any element from the first 20 on the Periodic Table and draw its Bohr model using the 2-8-8 rule.

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Putting it All Together: Atomic Review

Learning Objectives

The Nucleus: The Atom's Heavy Heart

The Cloud: Electrons and Electrical Balance

Isotopes and Electron Shells

Key Takeaways

Quick Check

What's Next?

Putting it All Together: Atomic Review

Learning Objectives

The Nucleus: The Atom's Heavy Heart

The Cloud: Electrons and Electrical Balance

Isotopes and Electron Shells

Key Takeaways

Quick Check

What's Next?