Chemistry

Element

Type of pure substance. Contains only one type of atom which cannot be broken down.

Compound

A substance containing two or more different atoms which are chemically bonded.

What makes elements different from each other?

Number of protons and atomic mass.

Mass number

Number of protons and neutrons.

Atomic number

Chemical properties and element’s identity.

3 subatomic particles and location

Protons and neutrons located inside nucleus (determines mass), and electrons located outside of nucleus.

7 diatomic molecules

H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

Atom

Small particle of an element, retains chemical properties (neutral P⁺ = e⁻).

Ion

Charged (P⁺ ≠ e⁻).

Molecule

Two or more joined atoms.

Positive charge (+)

Number of charges = number of electrons lost.

Negative charge (-)

Number of charges = number of electrons gained.

Valence

Electron(s) that get lost or gained (last electron shell). Involved in bonding.

Electrostatic attraction

Attraction force between negative electrons and positive nucleus.

Strong nuclear force

Holds protons and neutrons together in the nucleus by a strong nuclear force.

Aufbau Principle

Electrons around an atom will fill the lowest available shell first before higher energy levels.

Calculating electron number (in specific energy level)

2n² (max. electron in shell).

Energy level (n)

Shells get further away from nucleus, energy increases.

Sublevels/subshells

s, p, d, f subshells.

Maximum number of e⁻ Orbital diagram

s: 2, p: 6, d: 10, f: 14.

Period number

Number of energy levels/shells (rows).

Group number

Number of valence electrons (columns) (not transitional metals (d)).

Number of electrons

Number of arrows in electron configuration.

Isotopes

Atoms from the same element, but differ in neutron numbers in nuclei. Similar chemical properties but different physical properties.

Alkali (Group 1)

Soft metals, most reactive.

Alkali earth metals (Group 2)

Less reactive.

Halogens (Group 17)

Non-metals, diatomic molecules, react highly with alkali metals; reactivity decreases from F to At.

Noble gases (Group 18)

Least reactive.

Atomic radius

Size of an atom, measuring distance of atom’s centre to its electron's edge.

Trend of atomic radius down a group

Increases as atomic radius increases from top to bottom of a group due to the increase in the number of shells.

Atomic radius across a period

Declines from left to right within a period; same number of shells.

Reasons for atomic radius decrease across period

Due to increase in nuclear attraction; as proton number increases, so does the nuclear attraction of valence electrons.

Shielding effect

Reduces effective nuclear charge due to the difference in attraction force.

First ionisation energy

Energy required for a single electron to be removed from an atom; decreases down a group, increases across a period.

Electronegativity

Atoms bonded to another atom have the ability to attract electrons.

Octet rule

Atoms tend to lose, gain, or share electrons to achieve only 8 valence electrons.

Ionic bonding

Transfer of electrons.

Covalent bonding

Sharing of electrons.