A Level Chemistry Periodicity Questions and Answers

A Level Chemistry Periodicity Questions and Answers How is the periodic table arranged? By atomic number Define period A horizontal row of elements Define group A vertical column. Elements in a group have similar chemical properties as they have the same number of outer shell electrons Define periodicity A repeating pattern of properties shown across different periods What is the s-block? Groups 1 and 2 What is the p-block? Groups 3-8 What is the d-block? Transition elements What is the f block? Lanthanides and actinides Define first ionisation energy The energy needed to remove one mole of electrons from one mole of atoms in the gaseous state What is the trend across a period for first ionisation energy? Increases with a dip between group 2 and 3 and between 5 and 6 Why does first ionisation energy increase across a period? As you go across the period, the electron being removed is in the same shell with similar shielding. There are more protons in the nucleus, so the nucleus attraction increases What is the trend in atomic radius across a period? Decreases Why does atomic radius decrease across a period? There are the same number of shells with similar shielding and more protons in the nucleus so the nuclear attraction is greater and the electrons are held more tightly What is the size of ions compared to their atoms? Cations are smaller, anions are larger Why are cations smaller than their atoms? Same number of protons attract fewer electrons so they are held more tightly Why are anions larger than their atoms? Same number of protons attract more electrons so they held less tightly Define electronegativity The power of an atom to attract the pair of electrons in a covalent bond Why does electronegativity increase across a period? As you go across the period there are more protons with the same number of shells and similar shielding so the pair of electrons will be attracted more strongly Why does the melting point and boiling point increase from group 1 to group 3? The charge on the ion increase, the number of delocalised electrons increases and the ions get smaller so the metallic bonds get stronger Why is the melting and boiling point of silicon and carbon so high? There are very strong covalent bonds between the atoms which must be broken as it is macromolecular Why is the melting and boiling point of simple molecular compounds low? There's only weak van der Waals forces between molecules Why is the melting point of S8 P4 Cl2? The bigger the molecule, the more electrons in the molecule, the stronger the van der Waals forces between the molecules