Grade 9 Chemistry

Chemistry is the study of matter and the changes that matter undergoes.

As you work through this lesson, think about the materials and changes you observe around you every day.

Matter is anything that has mass and occupies space.

Examples of non-matter given in the lesson are heat, light, sound and energy.

Think: Identify three examples of matter in your surroundings and explain why each qualifies as matter.

Chemistry helps us to:

1. Explain the natural world.
2. Prepare for a career.
3. Become informed citizens.

Chemical reactions are constantly taking place around us. Examples from the lesson include:

• The human body: digestion, muscle movement and other bodily actions involve chemical reactions.
• Photosynthesis: plants convert water, sunlight and carbon dioxide into glucose and oxygen.
• Soaps and detergents: these can be produced by a chemical process called saponification.
• Sunscreen: organic and inorganic compounds can filter or block incoming ultraviolet radiation.
• Cooking: chemical reactions can change the flavour, texture and colour of food.

The lesson identifies three main branches of chemistry:

1. Organic chemistry
2. Inorganic chemistry
3. Physical chemistry

Organic chemistry is the branch of chemistry involving the scientific study of organic compounds. The lesson describes organic compounds as compounds containing carbon atoms bonded to hydrogen.

It deals with the structure and chemical composition of organic compounds, their physical and chemical properties, and the chemical reactions they undergo. Methane is shown as an example.

Inorganic chemistry is the branch of chemistry involving the scientific study of inorganic compounds. The lesson describes these as compounds that do not contain carbon-hydrogen bonds.

Examples given are sodium chloride, magnesium oxide and water. Inorganic chemistry studies the behaviour, properties, and physical and chemical characteristics of these compounds.

Physical chemistry is concerned with interactions and transformations when matter undergoes change.

It uses principles of physics to help explain chemical interactions.

1. Define chemistry.
2. Define matter.
3. Give one example of matter and one example of non-matter.
4. Name the three main branches of chemistry.
5. Differentiate between organic and inorganic chemistry.
6. Define physical chemistry.
7. Give two examples of chemistry in everyday life.
8. Classify familiar compounds as organic or inorganic using the ideas in this lesson.

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.

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.

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

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.

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.

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.

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.

Complete the missing values using the relationships in this lesson.

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