Periodicity (AQA A Level Chemistry): Flashcards
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What is meant by periodicity in the context of the periodic table?
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What is meant by periodicity in the context of the periodic table?
Periodicity refers to the repeating trends in chemical and physical properties of elements as atomic number increases. These trends repeat in each period of the periodic table.
True or False?
All elements in the d-block have at least one d-electron and no p-electrons in their outer shell.
True.
D-block elements have at least one d-electron and at least one s-electron, but no f- or p-electrons in the outer shell (up to 5d).
An element in period 3 of the periodic table has its outermost electrons in the .......... principal quantum shell, and all period 3 elements have .......... electron shells.
An element in period 3 of the periodic table has its outermost electrons in the third principal quantum shell, and all period 3 elements have three electron shells.
What is the product when sodium reacts with cold water, and what is the approximate pH of the solution formed?
The reaction produces sodium hydroxide (NaOH) and hydrogen gas (H2).
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
The solution is strongly alkaline with a pH of approximately 14.
When magnesium is heated in steam, the products are .......... and .......... . The equation is: Mg (s) + H2O (g) → .......... (s) + .......... (g).
When magnesium is heated in steam, the products are magnesium oxide and hydrogen gas. The equation is: Mg (s) + H2O (g) → MgO (s) + H2 (g).
True or False?
Magnesium reacts vigorously with cold water to produce a strongly alkaline solution.
False.
Magnesium reacts extremely slowly with cold water. The solution formed is only weakly alkaline (pH 9–10) because magnesium hydroxide is only slightly soluble.
What are valence electrons?
Valence electrons are the electrons in the outermost shell of an atom. Elements in the same group of the periodic table share the same outer-shell electronic configuration and therefore similar chemical properties.
How is atomic radius defined and measured for an element?
Atomic radius is the distance between the nucleus and the outermost electron of an atom. It is measured by taking two atoms of the same element, measuring the distance between their nuclei, and halving it.
True or False?
Atomic radius increases across Period 3 from Na to Cl.
False.
Atomic radius decreases across Period 3. The number of protons (nuclear charge) increases while shielding remains approximately constant, so the nucleus pulls electrons closer, reducing the atomic radius.
Across Period 3, atomic radius decreases because the .......... increases while the .......... effect remains approximately the same, so outer electrons are pulled .......... to the nucleus.
Across Period 3, atomic radius decreases because the nuclear charge increases while the shielding effect remains approximately the same, so outer electrons are pulled closer to the nucleus.
Why are cations smaller than their parent atoms?
When a metal atom loses its valence electrons to form a cation, there are fewer electrons remaining. This reduces electron-electron repulsion and shielding, so the nuclear charge attracts the remaining electrons more strongly, producing a smaller ionic radius.
Why are anions larger than their parent atoms?
When a non-metal atom gains electrons to form an anion, the additional electrons increase electron-electron repulsion within the outer shell.
The nuclear charge does not change, so the electron cloud expands, giving a larger ionic radius.
Going across the cations Na+ to Si4+ in Period 3, ionic radius .......... because the nuclear charge .......... , pulling the electrons in the second principal quantum shell .......... to the nucleus.
Going across the cations Na+ to Si4+ in Period 3, ionic radius decreases because the nuclear charge increases, pulling the electrons in the second principal quantum shell closer to the nucleus.
True or False?
All elements in the same period have the same number of electron shells.
True.
All elements in Period 3, for example, have three electron shells. This is why shielding is approximately constant across a period, and nuclear charge is the dominant factor determining atomic radius.
Shielding effect
The reduction in the attractive force felt by outer electrons due to repulsion from inner electrons between them and the nucleus. Shielding is approximately constant across a period, making nuclear charge the dominant factor in atomic radius.
What is the first ionisation energy of an element?
A lattice enthalpy is the enthalpy change when one mole of an ionic compound is formed from its gaseous ions, bringing them together to form the ionic lattice under standard conditions. It is always exothermic (negative).
True or False?
First ionisation energy increases uniformly from Na to Ar across Period 3 with no exceptions.
False.
There is a general increase, but two dips occur: between Mg and Al, and between P and S. These reflect the removal of a 3p electron (Al) and paired electron repulsion (S).
The first ionisation energy of Al is lower than that of Mg because the outer electron in Al is removed from a .......... subshell, which is at a .......... energy than the 3s subshell of Mg.
The first ionisation energy of Al is lower than that of Mg because the outer electron in Al is removed from a 3p subshell, which is at a higher energy than the 3s subshell of Mg.
Why is the first ionisation energy of sulfur lower than that of phosphorus?
Sulfur's electron configuration is [Ne]3s23p4, giving it a paired electron in one 3p orbital.
Electron-electron repulsion makes this paired electron easier to remove than any of the unpaired 3p electrons in phosphorus ([Ne]3s23p3), so sulfur has a lower first ionisation energy.
Across Period 3, first ionisation energy generally increases because the .......... charge increases while .......... remains approximately constant, making it .......... to remove the outer electron.
Across Period 3, first ionisation energy generally increases because the nuclear charge increases while shielding remains approximately constant, making it harder to remove the outer electron.
True or False?
First ionisation energies must be measured using gaseous atoms.
True.
The definition specifically requires gaseous atoms. This eliminates the energy contributions from lattice energy or intermolecular forces, isolating the nuclear attraction on the outer electron.
What is meant by effective nuclear charge in the context of ionisation energy?
An effective nuclear charge is the net positive charge experienced by an electron after accounting for shielding by inner-shell electrons. It determines the attraction between the nucleus and the outer electrons.
Why does silicon have the highest melting point of all Period 3 elements?
Silicon has a giant molecular (covalent) structure in which each Si atom is bonded to four neighbours by strong covalent bonds. A large amount of energy is required to break this extensive network of bonds.
True or False?
The melting points of Na, Mg, and Al increase in that order because more electrons are donated to the delocalised 'sea', strengthening metallic bonding.
True.
Na donates 1 electron per atom, Mg donates 2, and Al donates 3. More delocalised electrons and higher ionic charge (1+, 2+, 3+) mean stronger electrostatic attraction in the metallic lattice, raising the melting point.
P, S, Cl, and Ar exist as simple .......... with only weak .......... forces between molecules, so they have .......... melting points compared to the metallic and giant covalent elements.
P, S, Cl, and Ar exist as simple molecules with only weak instantaneous dipole-induced dipole forces between molecules, so they have low melting points compared to the metallic and giant covalent elements.
Among the simple molecular elements P4, S8, Cl2, and Ar, which has the highest melting point and why?
S8 has the highest melting point among the simple molecular elements.
Larger molecules have more electrons and a greater surface area for contact, leading to stronger instantaneous dipole-induced dipole forces that require more energy to overcome.
To explain a melting point trend in Period 3, an answer must state the .......... , the type of .......... or forces being broken, and their relative .......... .
To explain a melting point trend in Period 3, an answer must state the structure, the type of bonding or forces being broken, and their relative strength.
True or False?
The covalent bonds within simple molecular elements such as Cl2 are broken when the element melts.
False.
When simple molecular elements melt, only the weak intermolecular forces (instantaneous dipole-induced dipole forces) between molecules are overcome. The strong covalent bonds within each molecule remain intact.
What type of structure and bonding does aluminium have, and how does this explain its melting point relative to sodium?
Aluminium has a giant metallic structure. Each Al atom donates three electrons to the delocalised electron sea, creating stronger electrostatic attraction between Al3+ ions and three times as many delocalised electrons compared to Na+, giving Al a significantly higher melting point than sodium.
Giant molecular structure
A structure in which atoms are held together by a continuous network of strong covalent bonds extending throughout the whole solid. Silicon is an example — its giant molecular structure gives it the highest melting point of all Period 3 elements.
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