Chemistry OCR AS - Unit 1, Module 1

Atomic Structure

·         Each atom is made from protons, electrons and neutrons.

·         Protons and neutrons make up the nucleus.

·         Electrons orbit the nucleus.

·         Protons have a positive charge and a mass of 1.

·         Neutrons have no charge and have a mass of 1.

·         Electrons have a negative charge and a mass of 1/2000 (negligible).

·         Isotopes are atoms of the same element with different numbers of neutrons.

·         The atomic number is the number of protons in the nucleus of an atom.

·         The mass number is the total number of particles (protons and neutrons) in the nucleus of an atom.

·         Different isotopes of the same element react in the same way.

o   Chemical reactions involve electrons

o   Neutrons make no difference to chemical reactivity.

·         An ion is a positively or negatively charged atom or group of atoms.

Atomic Masses

·         Relative isotopic mass is the mass of an atom of an isotope relative to 1/12th the mass of an atom of carbon-12.

·         Relative atomic mass is the weighted mean mass of an atom of the isotopes of an element relative to 1/12th the mass of an atom of carbon-12.

·         Relative molecular mass is the weighted mean mass of a molecule relative to 1/12^th the mass of an atom of carbon-12.

·         Relative formula mass is the weighted mean mass of a formula unit compared to 1/12^th the mass of an atom of carbon-12.

Amount of substance and the mole

·         Avogadro Constant = 6.02 x 10²³

·         The number of moles = mass ÷ molar mass.

Types of formula

·         Empirical formula shows the smallest ratio of atoms in a molecule.

o   It is used for compounds with giant structures like NaCl and SiO₂.

·         Molecular formulae show the actual number of atoms in a molecule.

o   It is used to show simple molecules like C₃H₈

Moles and gas volumes

·         At room temperature and pressure (RTP) one mole of a gas takes up approximately 24.0 dm³

·         Therefore, the volume of gas per mole of gas molecules is 24.0…