LTE FDD Radio Frame structure: In time domain;

10 ms long with 10 subframes each of 1 ms.

Each subframe has two slots of 0.5 ms each & each slot has 7 orthogonal symbols.
In freq domain:
 72 subcarriers each of 15khz bandwidth & grouped into 6 RBs (PRBs).
Collectively:

An entity consisting of a subframe (1ms) of time domain & 12 subcarriers is known as Scheduling Block (SB).

An entity consisting of a slot (0.5ms) of time domain & 12 subcarriers is known as Resource Block (RB).

An entity consisting of an orthogonal symbol (0.071428 ms) & one subcarrier (15khz) is known as Resource Element (RE).
So one LTE radio frame consists of 10 subframes (i.e. total 1 ms) in time domain & 72 subcarriers (i.e. total 1080 Khz) in freq domain.
Why 15kHz? It is because UMTS and LTE have the same clock timing!
I’ll explain it with an example.
In LTE for BW=5MHz, there are 300 subcarriers. (with 10% guard band, 4.5MHz/15KHz=300)
But we know that in IFFT/FFT transformation, Nfft should be a power of 2 (to speedup the FFT operation). 300 is not a power of 2 and the next power of two is 512.
Fs = Nfft x Δf (because Fs=1/Ts , Ts=Tsym/Nfft and Δf=1/Tsym=15kHz)
For BW=5MHz, Fs=51215kHz=7.68MHz => Fs=23.84MHz
(3.84MHz is chip rate in UMTS).
We could reach to our timing goals by subcarrier spacing equal to 7.5KHz or 30KHz also, but 15KHz is an agreement base on multicarrier transmission challenges (ISI , Doppler effect, …).
Why 12 subcarriers is there in a RB? I think if we notice to total subcarrier in different LTE Bandwidth we can guess the answer. 20MHz>1200subcarrier, 15MHz>900, 10MHZ>600, 5MHz>300, 3MHz>180, 1.4MHZ>84 (with 10% guard band for all), 12 is the greatest common divisor of them.
Why 7 Time slots for a RB?
We know that RB is the smallest block that is allocated to an UE in LTE. I think RB Time slots number’s value is an agreement base on latency (.5mSec for one RB) and traffic efficiency (84symbol/.5msec). (84 is equal to 12 subcarrier*7 Recourse Elements).