APCE Atomic Structure & Periodicity 1 — Questions and Answers
Question 1: What is the atomic number of an element?
- The number of electrons.
- The number of protons in the nucleus. (Correct answer)
- The number of neutrons.
- The total number of subatomic particles in an atom.
Correct answer: The number of protons in the nucleus.
The atomic number of an element is defined by the number of protons found in the nucleus of an atom of that element. This number is unique to each element and determines its identity and position on the periodic table. For example, all carbon atoms have 6 protons, so carbon's atomic number is 6.
Question 2: What does the periodic table’s periodicity refer to?
- The arrangement of elements in order of increasing atomic mass.
- The repetition of chemical properties in groups of elements. (Correct answer)
- The random arrangement of elements.
- The number of protons in an atom.
Correct answer: The repetition of chemical properties in groups of elements.
The periodic table's periodicity refers to the recurring trends and patterns in the chemical and physical properties of elements. As elements are arranged by increasing atomic number, elements with similar properties appear at regular intervals, forming groups or families. This repetition allows scientists to predict the properties of elements based on their position in the table.
Question 3: What is the electron configuration of oxygen (O)?
- 1s² 2s² 2p⁶.
- 1s² 2s² 2p⁴. (Correct answer)
- 1s² 2s² 3p⁶.
- 1s² 2p⁶.
Correct answer: 1s² 2s² 2p⁴.
Oxygen (O) has an atomic number of 8, meaning a neutral oxygen atom has 8 electrons. Following the Aufbau principle, these electrons fill the lowest energy orbitals first: 2 electrons in the 1s orbital, 2 in the 2s orbital, and the remaining 4 electrons in the 2p orbital. Therefore, its electron configuration is 1s² 2s² 2p⁴.
Question 4: How does atomic radius change across a period in the periodic table?
- It increases across a period.
- It decreases across a period. (Correct answer)
- It stays the same across a period.
- It fluctuates without a clear trend.
Correct answer: It decreases across a period.
Across a period, the number of protons in the nucleus increases, leading to a stronger positive charge. This increased nuclear charge pulls the valence electrons closer to the nucleus, effectively reducing the atomic radius. Although electrons are added to the same principal energy level, the increasing nuclear pull dominates, causing the atoms to shrink.
Question 5: What is ionization energy?
- The energy required to add an electron.
- The energy required to remove an electron from an atom or ion. (Correct answer)
- The energy required to break a chemical bond.
- The energy released when an electron is added.
Correct answer: The energy required to remove an electron from an atom or ion.
Ionization energy is a fundamental property that quantifies the energy needed to overcome the attraction between the nucleus and an electron. Specifically, it's the minimum energy required to remove the most loosely held electron from a gaseous atom or ion in its ground state. This process results in the formation of a cation.
Question 6: What does electronegativity refer to?
- The number of protons in an atom.
- An atom's ability to attract electrons in a bond. (Correct answer)
- The number of neutrons in an atom.
- The energy required to remove an electron.
Correct answer: An atom's ability to attract electrons in a bond.
Electronegativity is a measure of an atom's inherent ability to attract shared electrons towards itself within a chemical bond. This property is crucial for determining the polarity of a bond and, consequently, the overall polarity of a molecule. Elements with high electronegativity, like fluorine, strongly pull bonding electrons towards themselves.
Question 7: What is the periodic trend for electron affinity?
- Electron affinity decreases across a period.
- Electron affinity increases across a period. (Correct answer)
- Electron affinity stays the same across a period.
- Electron affinity decreases down a group.
Correct answer: Electron affinity increases across a period.
Electron affinity generally increases across a period because the effective nuclear charge increases, pulling incoming electrons more strongly. Atoms with a stronger nuclear attraction for electrons will release more energy when an electron is added, indicating a higher (more negative) electron affinity. Halogens, for instance, have high electron affinities as they readily accept an electron to achieve a stable octet.
Question 8: What is the shielding effect in atomic structure?
- The attraction of the nucleus for the electrons.
- The blocking of nuclear attraction by inner electrons. (Correct answer)
- The repulsion of electrons in the same energy level.
- The energy required to remove an electron.
Correct answer: The blocking of nuclear attraction by inner electrons.
The shielding effect describes how inner-shell electrons reduce the effective nuclear charge experienced by outer-shell (valence) electrons. These inner electrons repel the valence electrons and block some of the positive nuclear charge, making it easier to remove valence electrons. This effect is a key factor in explaining periodic trends like atomic radius and ionization energy down a group.
Question 9: What is the role of quantum numbers in describing atomic structure?
- To determine the atomic number.
- To describe the properties of atomic orbitals and electrons. (Correct answer)
- To determine the number of neutrons.
- To calculate the molecular weight.
Correct answer: To describe the properties of atomic orbitals and electrons.
Quantum numbers are a set of four numbers that uniquely describe the state of an electron in an atom. They specify the electron's energy level (principal quantum number), the shape of its orbital (azimuthal), its orientation in space (magnetic), and its spin (spin quantum number). Together, they provide a comprehensive description of an electron's probable location and energy within an atom.
APCE Atomic Structure & Periodicity 1
What is the atomic number of an element?