Chem - topic 1

Question 1

What was stated in Dalton’s atomic theory? (4)

Answer 1

• atoms are tiny particles made of elements
• atoms cannot be divided
• all the atoms in an element are the same
• atoms of one element are different to those of other elements

Question 2

What did Thompson discover about electrons

Answer 2

• they have a negative charge
• the can be deflected by electromagnetic fields
• they have very small mass

Question 3

Explain the current model of the atom

Answer 3

• protons and neutrons are found in the nucleus
• electrons orbit the nucleus in shells
• the nucleus is tiny compared to the total volume of the atom
• most of the atom’s mass is in the nucleus
• most of the atom is empty space between the nucleus and electrons

Question 4

Why do different isotopes of the same element react in the same way? (2)

Answer 4

• neutrons have no impact on the chemical reactivity
• reactions involve electrons, isotopes have the same number of electrons in the same arrangement

Question 5

What two assumptions are made when calculating mass number?

Answer 5

• contribution of the electron is neglected
• mass of both proton and neutron is taken as 1.0 u

Question 6

What are the uses of mass spectrometry? (3)

Answer 6

• identify unknown compounds
• find relative abundance of each isotope of an element
• determine structural information

Question 7

What is the m/z value of the M+ ion

Answer 7

The m/z value of the M+ ions is the value of the last peak

Question 8

What does the principal quantum number indicate

Answer 8

The shell occupied by the electrons

Question 9

What is a shell

Answer 9

A group of orbitals with the same principal quantum number

Question 10

What is an orbital

Answer 10

A region around the nucleus that can hold up to two electrons with opposite spins

Question 11

What are the rules by which electrons are arranged in the shell? (5)

Answer 11

• electrons are added one at a time
• lowest available energy level is filled first
• each energy level must be filled before the next one can fill
• each orbital is filled singly before pairing
• 4s is filled before 3d

Question 12

Why does 4s orbital fill before 3d orbital?

Answer 12

4s orbital has a lower energy than 3d before it is filled

Question 13

How can the electron configuration be written in short?

Answer 13

The noble gas before the element is used to abbreviate

eg. Li (1s2 2s1 -> [He] 2s1)

Question 14

What is meant by periodicity

Answer 14

The repeating trends in chemical and physical properties

Question 15

What change happens across each period?

Answer 15

Elements change from metals to non-metals

Question 16

Define first ionisation energy

Answer 16

The energy required to remove a mole of electrons from a mole of gaseous atoms to form one mole of gaseous 1+ ions under standard conditions

Question 17

Explain the trend in first ionisation energy from Na to Ar

Answer 17

• First ionisation energy increases across period 3 because of:
  → increased nuclear charge
  → decreased atomic radius
  → same electron shielding
• this means more energy is needed to remove the first electron
• Dips at Al because: outer electron is in a 3p orbital, higher energy than 3s orbital (less energy needed to remove electron)
• Dips at S because one 3p orbital contains two electrons (less energy needed to remove one)

Question 18

Why does first ionisation energy decrease between group 2 to 3?

Answer 18

• decreases between 2 to 3 because in group 3 the outermost electrons are in p orbitals
• whereas in group 2 they are in s orbital, so the electrons are easier to be removed

Question 19

Why does first ionisation energy decrease between group 5 to 6?

Answer 19

• the decrease between 5 to 6 is due to the group 5 electrons in p orbital which are single electrons
• in group 6 the outermost electrons are spin paired, with some repulsion
• therefore the electrons are slightly easier to remove

Question 20

Does first ionisation increase or decrease between the end of one period and the start of next? Why?

Answer 20

• decrease
• there is increase in atomic radius
• increase in electron shielding

Question 21

Does first ionisation increase or decrease down a group? Why?

Answer 21

• decrease
• shielding increases → weaker attraction
• atomic radius increases → distance between the outer electrons and nucleus increases → weaker attraction
• Increase in number of protons is outweighed by increase in distance and shielding

Question 22

Describe the structure, forces and bonding in every element across Period 2

Answer 22

• Li and Be → giant metallic; strong attraction between positive ions and delocalised electrons; metallic bonding
• B and C → giant covalent; strong forces between atoms; covalent
• N2, O2, F2, Ne → simple molecular; weak intermolecular forces between molecules; covalent bonding within molecules and intermolecular forces between molecules

Question 23

Describe the structure, forces and bonding in every element across Period 3