Atomic structure and the Periodic Table

1. Overview

Atomic structure is the foundation of chemistry, describing how subatomic particles are arranged within an atom. Understanding the distribution of protons, neutrons, and electrons allows us to predict how elements behave, how they bond, and their specific placement within the Periodic Table.

Key Definitions

• Proton Number (Atomic Number): The number of protons in the nucleus of an atom. This defines the identity of the element.
• Mass Number (Nucleon Number): The total number of protons and neutrons in the nucleus of an atom.
• Electronic Configuration: The arrangement of electrons in the shells (energy levels) around the nucleus of an atom.
• Nucleus: The dense, central part of an atom containing protons and neutrons.
• Shells (Energy Levels): Regions surrounding the nucleus where electrons are found.

Core Content

The Structure of the Atom

An atom consists of a central nucleus surrounded by electrons orbiting in shells.

• Nucleus: Contains protons and neutrons. It holds almost all the mass of the atom but occupies a tiny fraction of its volume.
• Shells: Electrons orbit the nucleus at specific distances.

Relative Masses and Charges

You must memorize the properties of the three subatomic particles:

Particle	Relative Mass	Relative Charge	Location
Proton	1	+1	In the nucleus
Neutron	1	0 (Neutral)	In the nucleus
Electron	1/1840 (Negligible)	-1	In the shells

Determining Subatomic Particles

• Number of Protons = Proton Number.
• Number of Electrons = Proton Number (in a neutral atom).
• Number of Neutrons = Mass Number – Proton Number.

Worked Example: Determine the number of protons, neutrons, and electrons in an atom of Sodium ($\text{Na}$).

• Given: Proton number = 11, Mass number = 23.
• Protons: 11
• Electrons: 11 (atoms are neutral)
• Neutrons: $23 - 11 = 12$

Electronic Configuration (Proton numbers 1–20)

Electrons fill the shells closest to the nucleus first.

• 1st Shell: Holds a maximum of 2 electrons.
• 2nd Shell: Holds a maximum of 8 electrons.
• 3rd Shell: Holds a maximum of 8 electrons (for the first 20 elements).

Examples:

• Helium (2): 2
• Carbon (6): 2,4
• Sodium (11): 2,8,1
• Calcium (20): 2,8,8,2

Relationship with the Periodic Table

The electronic configuration of an atom is directly linked to its position in the Periodic Table:

1. Group Number: The number of electrons in the outer shell is equal to the Group number (for Groups I to VII).
   • Example: Magnesium (2,8,2) is in Group II.
2. Period Number: The number of occupied electron shells is equal to the Period number.
   • Example: Chlorine (2,8,7) has 3 shells, so it is in Period 3.
3. Group VIII (Noble Gases): These elements (He, Ne, Ar) have a full outer shell, making them very stable and unreactive (unreactive).

Extended Content (Extended Only)

Note: Per the provided instructions, there is no additional supplement content required for this specific sub-topic (2.2).

Key Equations

While this topic is conceptual, the following "formulas" are essential for calculations:

• Mass Number ($A$) = Protons ($Z$) + Neutrons ($n$)
• Number of Neutrons = $A - Z$

Example Calculation: For an isotope of Carbon with a mass number of 14 and proton number 6: $\text{Neutrons} = 14 - 6 = 8$

Common Mistakes to Avoid

• ❌ Wrong: Confusing the Mass Number with the Proton Number.
• ✅ Right: Remember the Mass number is always the larger number (it includes both protons and neutrons).
• ❌ Wrong: Thinking electrons have no mass at all.
• ✅ Right: Electrons have a "negligible" mass (1/1840th of a proton), but it is not zero.
• ❌ Wrong: Forgetting that ions have a different number of electrons than protons.
• ✅ Right: In a neutral atom, Protons = Electrons. In an ion, you must add or subtract electrons based on the charge.

Exam Tips

• Command Words: If a question says "State the electronic configuration," simply write the numbers (e.g., 2,8,1). If it says "Give the number of neutrons," show the subtraction to ensure you get the mark even if you make a calculation error.
• Typical Values: Expect to work with elements up to Calcium (Proton number 20). Be very comfortable with the numbers 2, 8, 10, and 18, as these represent full shells or common atomic numbers.
• Real-World Contexts: Questions often use "nuclear" contexts, such as the use of isotopes in medical imaging or nuclear fuel. Even in these contexts, the rules for protons and electrons remain the same.
• Drawing: When drawing "dot and cross" diagrams for atoms, always check your total electron count against the proton number. For example, a neutral Oxygen atom must have exactly 8 electrons total ($O_{(g)}$).

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

The element potassium, K, is in Group I of the Periodic Table.

(a) State the number of outer shell electrons in a potassium atom. [1]

(b) Potassium reacts with water to form potassium hydroxide and hydrogen gas. Write a balanced chemical equation for this reaction. [2]

(c) State two observations you would make when potassium reacts with water. [2]

Worked Solution:

(a)

1. Group I elements have one outer shell electron. $\boxed{1}$ [Group number equals number of outer shell electrons]

How to earn full marks:

• State the answer as "1" (or "one").

(b)

1. Write the correct chemical formulas for reactants and products. $2K(s) + 2H_2O(l) \rightarrow 2KOH(aq) + H_2(g)$ [Correct formulas for all substances]

2. Balance the equation. [Correct balancing of the equation]

How to earn full marks:

• 1 mark for correct formulas of all reactants and products.
• 1 mark for correct balancing.

(c)

1. Identify an observable effect of the reaction. "Potassium melts" [Melting is observed]

2. Identify another observable effect of the reaction. "Effervescence/bubbles/fizzing" [Gas production is observed]

How to earn full marks:

• 1 mark for each correct observation.
• Acceptable observations include: potassium melts, potassium moves around the surface, effervescence/bubbles/fizzing, potassium disappears, a lilac flame.
• Do not accept "heat is produced" or "it gets hot".

Common Pitfall: When describing observations, be specific. Saying "heat is produced" is not an observation; you need to describe what you would see or hear. Also, make sure your equation is properly balanced, showing the correct ratios of reactants and products.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

The table below shows information about three subatomic particles.

Particle	Relative Mass	Relative Charge
Proton	1	+1
Neutron	1	0
Electron

(a) Complete the table by stating the relative mass and relative charge of an electron. [2]

(b) Define the term "proton number". [2]

(c) An atom of element X has a mass number of 37 and contains 20 neutrons. State the proton number of element X. [2]

Worked Solution:

(a)

1. State the relative mass of an electron. $\boxed{negligible}$ [Electrons have very little mass]

2. State the relative charge of an electron. $\boxed{-1}$ [Electrons are negatively charged]

How to earn full marks:

• 1 mark for correct relative mass. Acceptable answers include "negligible", "approximately 0", or "1/1840".
• 1 mark for correct relative charge.

(b)

1. Relate proton number to the nucleus of an atom. "The number of protons in the nucleus of an atom" [Focus on the nucleus]

How to earn full marks:

• Must mention "number of protons" AND "nucleus of an atom".

(c)

1. Calculate the proton number. $Proton\ number = Mass\ number - Number\ of\ neutrons$ $Proton\ number = 37 - 20 = 17$ $\boxed{17}$ [Apply the definition of mass number]

How to earn full marks:

• Correct calculation of the proton number.
• State the final answer as "17".

Common Pitfall: Remember that the proton number (atomic number) defines the element. It's the number of protons in the nucleus, not the number of electrons. Also, be sure to include both parts of the definition of proton number: the number of protons and that they are in the nucleus.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

Chlorine exists as two isotopes: chlorine-35 ($^{35}Cl$) and chlorine-37 ($^{37}Cl$).

(a) Define the term "isotopes". [2]

(b) An analysis of a sample of chlorine shows that 75% of the atoms are chlorine-35 and 25% are chlorine-37. Calculate the relative atomic mass ($A_r$) of chlorine in this sample. [3]

(c) Chlorine reacts with hydrogen to form hydrogen chloride, HCl. Draw a 'dot-and-cross' diagram to show the bonding in a molecule of hydrogen chloride. Show only the outer shell electrons. [3]

Worked Solution:

(a)

1. Isotopes have the same proton number. "Atoms of the same element with the same number of protons" [Focus on same element/protons]

2. Isotopes have different mass number. "...but different numbers of neutrons/different mass numbers." [Focus on different neutrons/mass]

How to earn full marks:

• Must mention "same element/protons" AND "different neutrons/mass number".

(b)

1. Calculate the contribution from chlorine-35. $(75/100) \times 35 = 26.25$ [Multiply percentage abundance by mass number]

2. Calculate the contribution from chlorine-37. $(25/100) \times 37 = 9.25$ [Multiply percentage abundance by mass number]

3. Calculate the relative atomic mass. $A_r = 26.25 + 9.25 = 35.5$ $\boxed{35.5}$ [Sum the contributions and state the final answer]

How to earn full marks:

• 1 mark for calculating the correct contribution from chlorine-35.
• 1 mark for calculating the correct contribution from chlorine-37.
• 1 mark for the correct final answer.

(c)

1. Show the correct number of outer shell electrons for hydrogen. [Hydrogen has 1 electron]

2. Show the correct number of outer shell electrons for chlorine. [Chlorine has 7 electrons]

3. Show the shared pair of electrons. [One shared pair shown]

How to earn full marks:

• 1 mark for correct number of outer shell electrons for hydrogen.
• 1 mark for correct number of outer shell electrons for chlorine.
• 1 mark for showing the shared pair of electrons.

Common Pitfall: When defining isotopes, remember to mention both the similarities (same number of protons) and the differences (different number of neutrons). For dot-and-cross diagrams, make sure you only show the outer shell electrons and that you clearly distinguish between the electrons from each atom (using dots and crosses).

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Element Z has the electronic configuration 2,8,6.

(a) State the group number of element Z in the Periodic Table. [1]

(b) State the period number of element Z in the Periodic Table. [1]

(c) Predict whether element Z is a metal or a non-metal. Explain your answer. [2]

(d) Element Z reacts with hydrogen to form a compound with the formula $H_2Z$.

(i) Draw a dot-and-cross diagram to show the bonding in the molecule $H_2Z$. Show outer shell electrons only. [3]

(ii) Predict the type of bonding in $H_2Z$. Explain your answer. [2]

Worked Solution:

(a)

1. Determine the group number from the number of outer shell electrons. $\boxed{6}$ [Group number equals number of outer shell electrons]

How to earn full marks:

• State the answer as "6".

(b)

1. Determine the period number from the number of electron shells. $\boxed{3}$ [Period number equals number of occupied electron shells]

How to earn full marks:

• State the answer as "3".

(c)

1. Predict whether Z is a metal or non-metal. "Non-metal" [Based on electronic configuration]

2. Explain the reasoning. "Elements with 5, 6, or 7 outer shell electrons are typically non-metals. Non-metals tend to gain electrons to achieve a full outer shell." [Relate electronic configuration to metallic/non-metallic character]

How to earn full marks:

• 1 mark for correct prediction.
• 1 mark for correct explanation relating electronic configuration to non-metallic character.

(d)(i)

1. Show the correct number of outer shell electrons for each hydrogen atom. [Each hydrogen has 1 electron]

2. Show the correct number of outer shell electrons for element Z. [Element Z has 6 electrons]

3. Show the shared pairs of electrons. [Two shared pairs shown]

How to earn full marks:

• 1 mark for correct number of outer shell electrons for each hydrogen atom.
• 1 mark for correct number of outer shell electrons for element Z.
• 1 mark for showing the two shared pairs of electrons.

(d)(ii)

1. Predict the type of bonding. "Covalent" [Sharing of electrons]

2. Explain the reasoning. "Hydrogen and element Z are both non-metals, so they will share electrons to form covalent bonds." [Relate non-metal bonding to covalent bonding]

How to earn full marks:

• 1 mark for correct prediction.
• 1 mark for correct explanation relating non-metal bonding to covalent bonding.

Common Pitfall: Remember that the group number corresponds to the number of outer shell electrons, and the period number corresponds to the number of electron shells. When predicting bonding type, consider whether the elements involved are metals or non-metals. Non-metals typically form covalent bonds by sharing electrons.