2A- The Atom and Redox

mass number definition

number of protons and neutrons

relative mass of an electron

1/1840

atomic number definition

number of protons in the nucleus of the atom of an element

charge on an ion is

number of protons subtract the number of electrons

isotope definition

atoms with the same number of protons but with different number of neutrons therefore different masses

relative isotopic mass definition

average mass of 1 atom of an isotope

measured relative to 1/12th mass of a carbon-12 atom

why is carbon-12 chosen as international standard

can be obtained in an isotopically pure state and is unreactive

relative atomic mass (Ar) definition

average mass of 1 atom of an element

compared to 1/12th mass of a carbon-12 atom

how can percentage abundance of an isotope be found experimentally

using a mass spectrometer

how does a mass spectrometer work

Atoms can be deflected by a magnetic field provided they are first converted into a positive ion. Electrically charged particles are affected by a magnetic field although electrically neutral ones are not. The deflected ions are detected and displayed on a mass spectrum as a mass-to-charge ratio, m/z.

example of mass spectrum labelled

y axis- relative abundance- can be percentage or not, check!

x axis- mass to charge ratio (m/z), as the charge on all the ions is 1+, this is the same as their masses

relative molecular mass (Mr) definition

the weighted mean mass of a molecule compared with 1/12th of the mass of an atom of carbon-12

relative formula mass definition

the weighted mean mass of a formula unit compared with 1/12th of the mass of an atom of carbon-12

1st ionisation energy definition

the amount of energy required to remove 1 mole of electrons from 1 mole of gaseous atoms of an element to make 1 mole of gaseous 1+ ions

chlorine first ionisation energy equation

Cl_(g) → Cl⁺_(g) + e^-

how to compare first ionisation energies

Nuclear charge (no. of protons)

Atomic radius (size of atoms)

Shielding (no. of inner shells)

Attraction (nuclear attraction)

atomic radius trend for nuclear attraction

as atomic radius increases, nuclear attraction decreases

shielding effect trend for nuclear attraction

as the number of inner shells increase, shielding increases, so nuclear attraction decreases

nuclear charge trend for nuclear attraction

as the nuclear charge increases, nuclear attraction increases

first ionisation energy trends across a period

across a period first ionisation energy is most likely affected by nuclear charge and atomic radius; shielding remain almost the same. FIE increases across a period

first ionisation energy trends down a group

down a group there is an increase in atomic radius and shielding; FIE therefore more affected by atomic radius and shielding, despite the increase in nuclear charge, FIE decreases down a group

order of filling subshells

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 4d, 4f

calcium electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s²

shape of an s-orbital

shape of a p-orbital

order which electrons are lost

always from the orbital furthest from the nucleus regardless of energy level (4s before 3d this time)

2 exceptions in filling order for orbitals

chromium and copper

chromium reason for electron configuration (has 5 electrons in 3d subshell but only 1 in 4s subshell)

one electron in each 3d orbital is symmetrical and therefore more stable than expected. This extra stability more than compensates for the energy needed to promote and electron from the 4s subshell

copper reason for electron configuration (has 10 electrons in 3d subshell but only 1 is 4s subshell)

pairs in each 3d orbital makes it symmetrical and so is more stable than expected. This more than compensates for the energy needed to promote an electron from 4s.

oxidation definition (electrons)

the loss of electrons

reduction definition (electrons)

the gain of electrons

oxidation half equation magnesium

Mg → Mg ²⁺ + 2e^-

reduction half equation oxygen

O₂ + 4e^- → 2O^2-

redox reaction definition

both reduction and oxidation occurring in a chemical reaction

oxidation number rules

1. every atom in a species has its own oxidation no.- written with +/- before the number

2. uncombined elements have oxidation no. of 0

3. in neutral compounds sum of oxidation numbers=0

4. in ions sum of oxidation numbers=charge of ion

5. oxidation number in fluorine is always -1

6. oxidation number of a combined group 1 metal is always +1

7. oxidation number of a combined group 2 metal is always +2

8. oxidation number of a combined oxygen is +2

oxidation number exceptions

• in metal hydrides, H has an oxidation number of -1 (LiH)

• in hydrogen peroxide (H₂O₂) O has an oxidation number of -1

disproportionation definition

same element is oxidised and reduced

oxidising agent definition

a reagent that oxidises (takes electrons from another species)

reducing agent definition

a reagent that reduces (adds electrons to another species)

oxidation definition (oxidation no.)

increase in oxidation no.

reduction definition (oxidation no.)

decrease in oxidation no.