Atoms have discrete energy levels

Energy Levels & Wave-Particle Duality — AQA A-Level Physics

Electrons in atoms can only exist at specific energyThe capacity to do work. Measured in joules (J). levels — not between them.
The lowest energy levelA discrete amount of energyThe capacity to do work. Measured in joules (J). that an electron in an atom can have. Electrons can only exist at specific energyThe capacity to do work. Measured in joules (J). levels, not between them. ($n = 1$) is the ground stateThe lowest energy level of an atom. The state in which all electrons are in their lowest possible energy levels..
Higher levels ($n = 2, 3, 4, ...$) are excited states.
Energy levels are negative because energy must be supplied to free the electron. The zero of energy is at $n = \infty$ (free electron).
The gap between levels decreases as $n$ increases. Levels converge towards the ionisation energyThe minimum energy required to remove an electron completely from an atom in its ground stateThe lowest energy level of an atom. The state in which all electrons are in their lowest possible energy levels. to infinity..

Hydrogen energy levels (from data sheet)

Ground stateThe lowest energy level of an atom. The state in which all electrons are in their lowest possible energy levels. ($n = 1$): $-13.6$ eV.
$n = 2$: $-3.40$ eV.
$n = 3$: $-1.51$ eV.
$n = 4$: $-0.85$ eV.
$n = \infty$: $0$ eV (ionisation).

Examiner Tips and Tricks

Energy levels are always negative.
A 'higher' energy levelA discrete amount of energy that an electron in an atom can have. Electrons can only exist at specific energy levels, not between them. has a less negative value (closer to zero).
Don't confuse 'higher level' with 'larger magnitude'.