Chemistry unit 1

Atom made up of …

… mostly empty space, dense nucleus (with protons and neutrons), electrons are found in the shells around nucleus.

Relative masses and relative charges of proton, neutron, electron

Proton: 1 , +1

Neutron: 1 , 0

Electron: 1/1836, -1

Difference between atomic number (proton number) and atomic mass (nucleon number)

atomic number: number of proton in the nucleus

atomic mass: number of proton and neutron

Why atomic mass is equal to mass of the nucleus

Most mass of an atom are located in the nucleus where the neutron and proton are in. Electrons have such small mass that it is negligible.

Why atom are neutral

Because number of proton = number of electrons

What happen when beams of protons, neutrons and electrons moving through an electric field at constant velocity?

… Because mass of electrons are lower than proton, electrons are deflected more strongly

What is atomic radius / radii

Half the distance between two atoms nuclei of the same element

Where to use metallic radii, covalent radii, Van de Waals radii

Metallic : metal

Covalent: Non-metal that covalently bonded

Van de Waals: Noble gas

The atomic radius …. as we go down the group because ….

The atomic radius …. as we go across the period because ….

The atomic radius increase as we go down the group because additional electron shells

The atomic radius decrease as we go across the period because increase in nuclear number (stronger attraction), same shielding.

The ionic radius …. as we go down the group because ….

The ionic radius …. as we go across the period because ….

The ionic radius increase as we go down the group because additional electron shells

The ionic radius decrease as we go across the period because increase in nuclear number (stronger attraction), same shielding.

[However, cation radii are smaller than anion radii because cation loss their outer shell electrons]

Radius of metal atom compare to cation

Smaller, because:

• Loss of outer shell (electrons)

• Less electrons, more protons=> stronger nuclear attraction

• less shielding

Radius of non-metal atom to anion

Bigger, because…

• Additional electrons => more repulsion

• Expand the radius

Define isotope

Atoms that have the same number of protons but different number of neutrons

Why ¹²C, ¹³C, and ¹⁴C have the same chemical properties

They all have the same number and arrangement of electrons

Why ³⁵Cl and ³⁷Cl have different physical properties? [2 marks]

Mass: Cl-35 will be greater because it has two additional neutrons

Density: Cl-35 will be denser because it has more neutrons and protons in the same volume of space

Aufbau principle [filling orbitals]

Electrons fill in orbitals that have lower energy/ closer to the nucleus first

Hund’s rule

When orbitals are at the same energy level, electrons fill the orbitals singly first before pair up with another electron to avoid electron spin pair repulsion

Order of filling electrons [1s to 4p from left to right]

1s², 2s², 2p⁶ , 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶

Orbital

A region of space around the nucleus that has high chance (95%) of spotting an electron

How many electrons can each orbital contain

2

How many orbitals does S, P and D sub-shell has

S: 1 orbital

P: 3 orbitals

D: 5 orbitals

Describe S-orbitals

Sphere

Describe P-orbitals

dumb-bell shaped

Why group 1 and 2 elements are called S-block elements

Because their outer orbital is a S-orbital

Why group 13 to 18 elements are called P-block elements

Because their outer orbital is P-orbital

Two exceptions (elements) of Aufbau’s principle

Chromium (Cr) - 1s²2s²2p⁶3s²3p⁶3d⁵4s¹ (avoid repulsion)

Copper (Cu) - 1s²2s²2p⁶3s²3p⁶3d¹⁰4s¹

How to write electronic configuration for ions

1. Write electronic configuration for the atom

2. Add/ remove electrons

What to be consider when writing electron configuration of ions for D-block elements (transition metal)

The electrons in the 4S orbital always being removed first

Shell

A region that has high chance to find all electrons in a certain energy level

Sub-shells

• Different regions that cover slightly different energy within a shell

• Types of orbitals

Principle Quantum Number (n)

The energy level that an electron had/ located in

Ground State

The state where electron is at the lowest energy level possible

Species

Any entity (such as atom, molecule, compound, ion…)

Free radical

a species with one or more unpaired electrons

First Ionisation Energy (IE)

Energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to produce 1 mole of (1+) ions

Equation for first ionisation energy

X_(g) ——> X⁺_(g) + e^-

What to consider when finding ionisation energy [4 marks]

• Nuclear charge (attracting electrons)

• Shielding

• Atomic/ ionic radius

• Spin-pair repulsion

Describe the trend in the first Ionisation energy of period 2

• Increase across the period due to increase in nuclear charge

• Drop at Boron due to 2p orbital is further away from the nucleus than 2s orbital

• Drop at Oxygen due to spin-pair repulsion

Describe the trend in First Ionisation energy when going down group

• First IE decrease due to increase in atomic radius and inner electrons shielding

Define Second Ionisation Energy and its equation

• Energy needed to remove one mole of electron from 1 mole of 1+ ions to produce 1 mole of 2+ ions

• X⁺_(g)——→ X^2+_(g) + e^-

Trends in successive ionisation energy of any element

• Electrons closer to the nucleus need higher Ionisation energy

• There’s a big jump in IE between electron from one energy level (shell) to another