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Chemistry: Materials and their structure

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Chemistry: Materials and their structure
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In this chapter

  1. 1The Basic Atomic Model
  2. 2Subatomic Particles
  3. 3Atomic and Mass Numbers
  4. 4Understanding Isotopes

1. The Basic Atomic Model

Everything in the universe is made of tiny particles called atoms. They are the fundamental building blocks of all elements. The modern model of an atom describes it as having two main regions: a tiny, dense, positively charged nucleus at the centre, and a much larger region of mostly empty space around it where negatively charged electrons are found. The nucleus contains protons and neutrons, while the electrons orbit the nucleus in specific energy levels or 'shells'. Because the nucleus is so small compared to the overall size of the atom, it's often said that atoms are over 99.9% empty space.

Key term

Atom: The smallest particle of a chemical element that can exist and still retain the properties of that element.

Fun fact

If you scaled an atom up so that its nucleus was the size of a pea, the nearest electron would be orbiting about a kilometre away.

Worked example 14 marks

The diagram shows a simple model of an atom. Identify the particles labelled A, B, and C, and the structure labelled D. [4 marks]

  1. 1

    Step 1: Identify the central part of the atom. This is the nucleus, so D is the nucleus.

  2. 2

    Step 2: Identify the particles inside the nucleus. These are protons and neutrons. Protons have a positive charge (often shown as '+') and neutrons are neutral. A is a proton.

  3. 3

    Step 3: The other particle in the nucleus, B, has no charge shown, so it is a neutron.

  4. 4

    Step 4: Identify the particle orbiting the nucleus. This is the electron, which has a negative charge. C is an electron.

Recap

  • Atoms are the fundamental building blocks of all matter.
  • An atom consists of a central nucleus and electrons orbiting it.
  • The nucleus contains protons and neutrons.
  • Electrons are found in energy levels or shells around the nucleus.
  • Most of an atom is empty space.

Quick check

  1. Name the two main regions of an atom.2 marks
  2. Where is most of the mass of an atom concentrated?1 mark

2. Subatomic Particles

Atoms are composed of three fundamental types of particles, known as subatomic particles: protons, neutrons, and electrons. Their properties determine the overall characteristics of an atom. Protons and neutrons are located together in the nucleus and are sometimes called 'nucleons'. Electrons are much smaller and orbit the nucleus. In any neutral atom, the number of positive protons is exactly balanced by the number of negative electrons, resulting in an overall charge of zero. The table below summarises their key properties.

ParticleRelative MassRelative ChargeLocation
Proton1+1Nucleus
Neutron10Nucleus
Electron1/1840 (negligible)-1Shells/Orbitals

Key term

Subatomic Particle: A particle smaller than an atom, such as a proton, neutron, or electron, that is a constituent of atoms.

Examiner insight

Examiners require precise answers for the properties of subatomic particles. Stating an electron's mass is 'very small' may lose a mark, whereas 'negligible' or '1/1840' is credited.

Common pitfall

Confusing the mass and charge of the particles. A common mistake is to think neutrons are massless because they are neutral, but they have the same mass as a proton.

Worked example 13 marks

Complete the following table about the particles found in an atom. [3 marks]

  1. 1

    Step 1: Fill in the properties for a proton. It is in the nucleus, has a relative mass of 1, and a relative charge of +1.

  2. 2

    Step 2: Fill in the properties for a neutron. It is also in the nucleus, has a relative mass of 1, and a relative charge of 0.

  3. 3

    Step 3: Fill in the properties for an electron. It is in shells, has a negligible relative mass (or 1/1840), and a relative charge of -1.

Recap

  • Protons have a relative mass of 1 and a relative charge of +1.
  • Neutrons have a relative mass of 1 and a relative charge of 0.
  • Electrons have a negligible mass and a relative charge of -1.
  • Protons and neutrons are found in the nucleus.
  • Electrons orbit the nucleus in shells.
  • An atom is neutral because the number of protons equals the number of electrons.

Quick check

  1. What is the relative charge of a neutron?1 mark
  2. Which two particles contribute to the mass of an atom but not its charge?1 mark

3. Atomic and Mass Numbers

Two key numbers are used to describe the composition of an atom's nucleus: the Atomic Number and the Mass Number. The Atomic Number (symbol Z) is the number of protons in the nucleus. This number is unique to each element and defines it; for example, any atom with 6 protons is a carbon atom. The Mass Number (symbol A) is the total count of protons and neutrons in the nucleus. We can use these numbers to work out the composition of any atom. For a neutral atom, the number of electrons is equal to the number of protons (the atomic number).

Mass Number (A) = Number of Protons + Number of Neutrons

Number of Neutrons = Mass Number (A) - Atomic Number (Z)

For a neutral atom: Number of Electrons = Number of Protons = Atomic Number (Z)

Key term

Atomic Number (Z): The number of protons in the nucleus of an atom, which uniquely identifies that chemical element.

Common pitfall

Mixing up the atomic number (protons only) and the mass number (protons + neutrons). Remember, the atomic number is usually the smaller of the two numbers shown in standard notation.

Worked example 13 marks

An atom of sodium is represented by the symbol ^23_11Na. Determine the number of protons, neutrons, and electrons in a neutral sodium atom. [3 marks]

  1. 1

    Step 1: Identify the atomic number and mass number. The bottom number is the atomic number (Z = 11). The top number is the mass number (A = 23).

  2. 2

    Step 2: The number of protons is equal to the atomic number. So, Protons = 11.

  3. 3

    Step 3: The number of electrons in a neutral atom is equal to the number of protons. So, Electrons = 11.

  4. 4

    Step 4: The number of neutrons is the mass number minus the atomic number. So, Neutrons = 23 - 11 = 12.

Worked example 23 marks

An atom contains 17 protons, 18 neutrons, and 17 electrons. State its atomic number, mass number, and identify the element. [3 marks]

  1. 1

    Step 1: The atomic number (Z) is the number of protons. Z = 17.

  2. 2

    Step 2: The mass number (A) is the number of protons + neutrons. A = 17 + 18 = 35.

  3. 3

    Step 3: The element is identified by its atomic number (number of protons). The element with 17 protons is Chlorine (Cl).

Recap

  • The atomic number (Z) is the number of protons and defines the element.
  • The mass number (A) is the total number of protons and neutrons.
  • To find the number of neutrons, subtract the atomic number from the mass number.
  • In a neutral atom, the number of electrons is equal to the number of protons.

Quick check

  1. An atom has 19 protons and 20 neutrons. What is its atomic number and mass number?2 marks
  2. How many electrons does a neutral atom of Fluorine (atomic number 9) have?1 mark

4. Understanding Isotopes

While all atoms of a particular element must have the same number of protons, they can have different numbers of neutrons. Atoms of the same element with the same number of protons but different numbers of neutrons are called isotopes. For example, all carbon atoms have 6 protons. However, most carbon atoms have 6 neutrons (Carbon-12), but some have 8 neutrons (Carbon-14). Because they have the same number of protons and therefore the same number of electrons, isotopes have identical chemical properties. Their different number of neutrons means they have different mass numbers and can have different physical properties, such as density or radioactivity.

Key term

Isotopes: Atoms of the same element that have the same number of protons but different numbers of neutrons.

Examiner insight

A high-scoring definition of isotopes must explicitly state that the number of protons is the same AND the number of neutrons is different. Just mentioning one of these is insufficient for full marks.

Common pitfall

Stating that isotopes have different chemical properties. Chemical reactions are determined by the electron arrangement, which is identical for isotopes of the same element.

Fun fact

Heavy water, used in some nuclear reactors, is made with an isotope of hydrogen called deuterium (^2_1H), which has one proton and one neutron instead of just one proton.

Worked example 14 marks

Chlorine has two main isotopes: Chlorine-35 (^35_17Cl) and Chlorine-37 (^37_17Cl). State the number of protons, neutrons, and electrons in an atom of each isotope. [4 marks]

  1. 1

    Step 1: For Chlorine-35 (^35_17Cl): Protons = Atomic Number = 17. Electrons = Protons (in a neutral atom) = 17. Neutrons = Mass Number - Atomic Number = 35 - 17 = 18.

  2. 2

    Step 2: For Chlorine-37 (^37_17Cl): Protons = Atomic Number = 17. Electrons = Protons (in a neutral atom) = 17. Neutrons = Mass Number - Atomic Number = 37 - 17 = 20.

  3. 3

    Step 3: Present the answer clearly for both isotopes. Cl-35 has 17p, 18n, 17e. Cl-37 has 17p, 20n, 17e.

Recap

  • Isotopes are atoms of the same element.
  • Isotopes have the same number of protons and electrons.
  • Isotopes have different numbers of neutrons.
  • Isotopes have the same atomic number but different mass numbers.
  • Isotopes have identical chemical properties but may have different physical properties.

Quick check

  1. Boron has two main isotopes: Boron-10 and Boron-11. State the number of neutrons in each. (The atomic number of Boron is 5).2 marks
  2. Why do isotopes of an element have the same chemical reactivity?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'atom' and describe its basic structure.3 marks
  2. Complete the table below showing the properties of subatomic particles. | Particle | Relative Mass | Relative Charge | |---|---|---| | Proton | | | | Neutron | | | | Electron | negligible | |3 marks
  3. An atom of element X has 13 protons and 14 neutrons. State its atomic number, mass number, and use the periodic table to identify element X.3 marks
  4. Describe the structure of a lithium atom, represented by the symbol ^7_3Li, in terms of the number and location of its subatomic particles.4 marks
  5. Explain, in terms of subatomic particles, why an atom is electrically neutral.2 marks
  6. Define the term 'isotope'. Use the element chlorine, which has an atomic number of 17 and isotopes with mass numbers 35 and 37, to illustrate your answer by stating the composition of each isotope.4 marks
  7. Atom A has 12 protons and 12 neutrons. Atom B has 12 protons and 13 neutrons. Atom C has 13 protons and 12 neutrons. Which two atoms are isotopes of each other? Explain your reasoning.3 marks
  8. An ion of magnesium has a charge of 2+, an atomic number of 12 and a mass number of 24. Determine the number of protons, neutrons, and electrons in this ion.3 marks
  9. Where is almost all the mass of an atom located? Explain your answer with reference to subatomic particles.2 marks
  10. Compare the properties of a proton and an electron. You must refer to their relative mass and relative charge in your answer.3 marks

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