Grade 9 Chemistry

Teacher: M. ColeSt. Mary HighOffline course pack

002-CHEM-G9-ATOM

Atomic Structure

Structure of atoms and calculation of subatomic particles.

CHEM-G9-ATOM-01 · Atomic Structure20 content · 1 resources

Explore the atom, its subatomic particles, neutral charge, chemical notation, atomic number and mass number, then apply A = Z + N to calculate particle numbers.

Lesson objectives

State the parts of an atom.
Differentiate between protons, neutrons and electrons by location, relative mass and charge.
Calculate protons, neutrons and electrons for given elements.

Starter: Match Elements to Atomic Numbers

Match each element from the lesson starter to its correct atomic number.

Starter Question

What do you call the smallest particle into which an element can be divided while retaining the properties of that element?

Use your answer as a prediction before reading the next section.

What Is an Atom?

An atom is the smallest particle of an element that can exist and still have the properties of that element. It is the basic building block of chemistry and the smallest particle of an element that can take part in a chemical reaction.

Subatomic Particles and Regions of the Atom

Atoms are made of three fundamental subatomic particles: protons, neutrons and electrons.

The atom has two main regions:

Nucleus: the centre of the atom; contains protons and neutrons.
Shells (or orbits in the source lesson): the area surrounding the nucleus where electrons are found.

Match & Sort: Subatomic Particle Properties

Sort each property into the correct subatomic-particle category.

Proton

Neutron

Electron

Label the Atom

Drag each label onto the correct particle or region of the atom.

Properties of the Three Subatomic Particles

Particle	Relative mass	Relative charge	Location
Proton	1	+1	Inside the nucleus
Neutron	1	0	Inside the nucleus
Electron	1/1836	−1	Outside the nucleus in shells or orbits

An atomic mass unit (amu) is approximately the mass of a proton or neutron.

Charge of Atoms

Protons are positively charged and electrons are negatively charged. In a neutral atom, the number of protons equals the number of electrons, so the overall charge is zero.

Example: helium has atomic number 2. A neutral helium atom therefore has 2 protons and 2 electrons.

Chemical Symbols

Each element has a chemical symbol. Symbols are usually one or two letters long. The first letter is capitalized; if there is a second letter, it is lower case.

Examples: H for hydrogen, O for oxygen and Mg for magnesium.

Try it: choose the correctly written symbols for potassium, fluorine and sulphur.

Match & Sort: Correct Chemical Symbols

Match each element to its correctly written chemical symbol.

Atomic Number and Mass Number

Atomic number (Z) is the number of protons in the nucleus. The number of protons determines the identity of the element.

Mass number (A) is the total number of protons and neutrons.

For a neutral atom: electrons = protons = atomic number.

To find neutrons: neutrons = mass number − number of protons.

Atomic Structure Relationship

A = Z + N

Remember

A = mass number Z = atomic number (proton number) N = neutron number.

Therefore, N = A − Z.

Worked Examples

Silver

For silver with mass number 108 and 61 neutrons: Z = 108 − 61 = 47. Therefore it has 47 protons.

Aluminium

If Al has 13 protons and 14 neutrons, then a neutral atom has 13 electrons, atomic number 13, and mass number 27.

Sodium

For Na with atomic number 11 and mass number 23: protons = 11, electrons = 11, neutrons = 12.

Magnesium

For neutral Mg with 12 electrons and mass number 24: protons = 12, atomic number = 12, neutrons = 12.

PhET Investigation — Build an Atom

Use PhET's Build an Atom simulation as an exploration before or after the calculation section.

Open PhET: Build an Atom

Spend a few minutes freely building atoms. Record two observations.
Build a neutral helium atom. Record its protons, neutrons, electrons, charge and mass number.
Add one proton. What changes: element identity, charge, or mass number?
Return to a neutral atom and add one electron. What changes?
Add or remove a neutron. What changes and what stays the same?
Build sodium with atomic number 11 and mass number 23. Check that your particle counts agree with your calculations.
Challenge: create two atoms with the same number of protons but different numbers of neutrons. Compare their mass numbers.

This activity needs internet access unless the simulation has been made available locally beforehand. The rest of this course pack remains usable offline.

Match & Sort: What Does Each Quantity Tell You?

Sort each statement under Atomic Number, Mass Number, or Neutral Atom.

Atomic Number

Mass Number

Neutral Atom

Practice Table

Complete the missing values using the relationships in this lesson.

Element	Protons	Neutrons	Electrons	Mass #	Atomic #
Calcium	?	?	?	40	?
Phosphorus	?	?	?	31	?
Silicon	?	?	?	28	?
Argon	?	?	?	40	?
Neon	?	?	?	20	?
Nitrogen	?	?	?	14	?
Hydrogen	?	?	?	1	?

Practice Table — Answer Check

Element	p	n	e	Mass #	Atomic #
Calcium	20	20	20	40	20
Phosphorus	15	16	15	31	15
Silicon	14	14	14	28	14
Argon	18	22	18	40	18
Neon	10	10	10	20	10
Nitrogen	7	7	7	14	7
Hydrogen	1	0	1	1	1

Atomic Structure — Multiple Choice Check

Choose the best answer. Check your answer and read the explanation.

1. Which subatomic particle has a relative charge of +1?

2. Which two particles are found inside the nucleus?

3. What is the relative mass of an electron in the lesson?

4. A neutral atom has 12 protons. How many electrons does it have?

5. Which statement defines atomic number?

6. An atom has mass number 23 and atomic number 11. How many neutrons does it have?

7. Which is the correctly written chemical symbol for magnesium?

8. Which quantity determines the identity of an element?

9. If an atom has 13 protons and 14 neutrons, what is its mass number?

10. Why is a neutral atom's overall charge zero?

Final Recap

Define an atom.
Name the three subatomic particles.
State the charge, relative mass and location of each subatomic particle.
Explain why a neutral atom has an overall charge of zero.
Explain the difference between atomic number and mass number.
Use A = Z + N to calculate a missing particle number.

Resources

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ATOMIC STRUCTURENotes · PDF

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