chemistry (AS lvl)

chapters 1-16

What is the nucleus?

the central part of the atom containing protons & neutrons

What is a proton?

a positively charged subatomic particle found in the nucleus

What is a neutron?

a neutral subatomic particle found in the nucleus

What is an electron?

a negatively charged subatomic particle found around the nucleus

What are the relative masses of the three subatomic particles?

protons → 1

neutrons → 1

electrons → 1/1836

What are the relative charges of the three subatomic particles?

protons → +1

neutrons → 0

electrons → -1

Why do protons in the nucleus not repel each other?

because they’re held together by the strong nuclear force, which overcomes electrostatic repulsion

How are electrons arranged in an atom?

in shells surrounding the nucleus, further shells being further from the nucleus

What is meant by the atomic number (Z)?

the number of protons in the nucleus of an atom

Why are atoms electrically neutral?

because the number of protons equals the number of electrons

What determines the chemical properties of an element?

the number of electrons in the outer shell

What is meant by the mass number (A)?

the total number of protons & neutrons in the nucleus

Which subatomic particles account for most of an atoms mass?

protons & neutrons

What are isotopes?

atoms of the same element with the same number of protons but different number of neutrons

Why do isotopes of the same element react chemically the same?

because they have the same electron configuration

Why do isotopes have different mass numbers/

because they contain different numbers of neutrons

What are the atomic numbers (Z) of the three carbon isotopes?

carbon-12 → 6

carbon-13 → 6

carbon-14 → 6

What are the mass numbers (A) of the three carbon isotopes?

carbon-12 → 12

carbon-13 → 13

carbon-14 → 14

What is carbon dating?

a method for estimating the age of once-living materials by measuring the remaining carbon-14

Why can carbon-14 be used for dating?

because it decays at a constant rate with a half-life of about 5700 years post death

What is relative atomic mass (Ar)?

the weighted average mass of an elements isotopes compared with 1/12 of a carbon-12 atom

What is relative molecular mass (Mr)?

the average mass of a molecule compared with 1/12 of a carbon-12 atom

What is a mass spectrometer?

an instrument that measures the mass-to-charge ratio of ions

What is the main use of a mass spectrometer in chemistry?

accurate determination of relative atomic masses

What is low-resolution mass spectrometry/

mass spectrometry that can’t distinguish very small differences

What are the main stages in a time-of-flight mass spectrometer?

(1) ionisation

(2) accelertion

(3) ion drift
(4) detection
(5) data analysis

Why is the appartus kept under vacuum?

to prevent ions colliding with air molecules

How does a mass spectrum show isotopes?

• different peaks represent isotopes

• peak heigh shows reative abundance

How many electrons fit the first shell?

2

How many electrons fit the second shell?

8

How many electrons fit the third shell?

18

What does quantised energy mean?

electrons can only exist in fixed energy levels, not between them

What is the octet rule?

atoms tend to achieve eight electrons in their outer shell for stability

What is an atomic orbital?

a region of space where theres a high probability of finding an electron

What shapes do s-orbitals have?

spherical

What shapes do p-orbitals have?

dumbbell-shaped, orientated along x,y, & z axes

How many electrons can one orbital hold?

a maximum of two with opposite spins

What is first ionisation energy?

the energy required to remove one mole of electrons from one mole of gaseous atoms

Why does ionisation energy increase across a period?

due to increasing nuclear charge with little extra shielding

Why does ionisation energy decrease down a group?

due to increased distance & shielding reducing nuclear attraction

Why is there a drop in ionisation energy from Mg to Al?

the outer electron in Al is in a higher-energy 3p orbital

Why is there a drop from P to S?

due to electron-electron repulsion in paired 3p electrons making removal easier

What is meant by a mole?

the amount of substance that contains exactly 6.02×10²³ elementary entities

What is Avogrados constant?

the number of articles in one mole of a substance

6.02 × 10²³ mol⁻¹

What is meant by relative atomic mass (Ar)?

the weighted average mass of an elements atoms compared with 1/12 of the mass of a carbon-12 atom

What is meant by relative formula mass (Mr)?

the sum of the relative atomic masses of all the atoms present in a chemical formula

What is meant by concentrattion of a solution?

the number of moles of solute per dm³ of solution

How can you distinguish between an acid & an alkali without tasting?

by using an indicator that changes color in acidic & alkaline solutions

What is meant by the emperical formula of a compound?

the simplest whole-number ratio of atoms of each element in a compound

What is meant by the molecular formula of a compound?

a whole-number multiple of the emperical formula & indicates the actual number of atoms of each element in a molecule

How is titration used to find concentration?

by reacting it with a solution of known concentration by using a suitable indicator & balanced equation

What is meant by atom economy?

a measurement on how efficently atoms in reactnats are used to form the desired product

What does atom economy indicate about a reaction?

how many atoms are wasted in a reaction in theory

What is meant by percentage yield?

the comparision between both the actual & theoretical yield + indicates the efficiency of a reaction

What is actual yield?

amount of product obtained experimentally from a chemical reaction

What is theoretical yield?

the maximum amount of product that could be formed based on stoichiometric calculations from a balanced equation

Why is actual yield usually less than theoretical yield?

• incomplete reactions

• side reactions

• losses during collection

• impurities

What are cations?

positively charged ions

What are anions?

negatively charged ions

What is ionic bonding?

the electrostatic force of attraction between oppositely charged ions

How are ions held together in ionic compounds?

each positive ion attracts all negative ions & vice versa by strong electrostatic forces that extend throughout the compound

What is meant by a lattice structure?

a regular, three-dimensional arrangement of cations & anions held togeteher by strong electrostatic forces

What are the properties of ionic compounds?

• solid at room temperature

• high melting points

• electricity conducters

• brittle/shatter

Why do ionic compounds have high melting points?

due to ther giant lattive structure with strong electrostatic attractions between oppositely charged ions that require a lot of energy to break

How do non-metals become stable?

by gaining/sharing electrons

What is a covalent bond?

a shared pair of electrons between two non-metal atoms, allowing each atom to achieve a stable noble gas arrangement

How does electron sharing hold atoms together?

by electrostatic attraction between the nuclei & the shared electrons, balanced by repulsive forces at a fixed distance

What is a double covalent bond?

a bond in which two pairs of electrons are shared between atoms

What is a coordinate covalent bond

covalent bond in which both electrons in the shared pair come from the same atom

What are some examples of coordinate bonding?

ammonium ion & hydronium ion

What are the properties of substances with molecular structures?

• low melting points

• poor electrical conductivity

Where are metals found in the periodic table?

on the left side of the periodic table

Why are metals conductors of electricity?

due to delocalised electrons

What is metallic bonding?

the electrostatic attraction between positive metal ions & a sea of delocalised electrons extending throughout the lattice

What determines the strength of metallic bonding?

higher ionic charge = stronger bonding

smaller ionic radius = stronger attraction

Why are metals malleable & ductile?

due to layers of ions sliding past each other without breaking bonds, due to delocalised electrons

What is the particle arrangement of solids?

tightly packed particles, vibrating about fixed positions

What is the particle arrangement of liquids?

close particles, moving by rapid jostling

What is the particle arrangement of gases?

far particles, moving rapidly & randomly

What energy changes occur during changes of state?

melting → energy weakens forces between particles

boiling → energy breaks intermoelcular forces

temperature remains constant

What is enthalapy?

the heat energy measured at constant pressure

What are crystals?

solids with a regular, three-dimensional arrangement of particles held together by strong/weak forces

Why do ionic & metallic crystals have high melting points?

due to strong electrostatic attractions extend throughout the structure, requiring large amounts of energy to overcome

What is a molecular crystal?

a crystal held togeteher by weak intermolecular forces (eg; iodine - I2)

What is the structure of a diamond?

covalent bonding, 4 bonds per carbon

What is the structure of grahite?

covalent bonding, 3 bonds per carbon & delocalised electrons

Why is graphite a conductor of electricity but diamond isn’t?

because graphite has delocalised electrons that can move along layers, while diamonds have no free eleectrons

What is the VESPR theory?

Valence Shell ELectron Pair Repulsion theory states that electron pairs repel each other & arrange to minimise repulsion

What affects molecular shape in VESPR theory?

• lone pairs repel more compared to bonding pairs

• lone pairs reduce bond angles

• shape depends on electron pair arrangement

What is electronegativity?

the ability of an atom to attract electron density in a covalent bond

How does electronegativity change in the periodic table?

increases across a period

decreases down a group

What is a polar covalent bond?

bond where electrons are shared unequally, creating partial charges

How is the polarity of a molecule determined?

by drawing the molecular shape & chcecking if the net dipole movement is ≠ to 0

What are intermolecular forces?

forces of attraction between molecules, weaker than covalent/ionic bonds

What types of intermolecular forces exist?

• van der Waals forces

• dipole-dipole forces

• hydrogen bonding

What is hydrogen bonding?

a strong intermolecular force between H bonded to N, O, or F

Why is hydrogen bonding stronger than dipole-dipole forces?

• large polarity

• partial covalent character

• very close molecular approach

Why is hydrogen bonding crucial?

• raises boiling/melting points

• stabilisies DNA & proteins

• gives water unique properties

What is meant by thermochemistry?

the branch of chemistry that studies heat changes that occur during chemical reactions & physical changes

What are exothermic reactions?

reactions that release energy to the surroundings as (heat, light, sound)