Array, hexadecimal and serial write
You need to manually split your values into bytes. You can either do that by storing them as individual bytes (as shown by Ignacio), or by using bit shifting:

Serial.write(mem_freq[mem_index] &gt;&gt; 16);
Serial.write(mem_freq[mem_index] &gt;&gt; 8);
Serial.write(mem_freq[mem_index]);


That sends the bits 16-23 (0x12) then bits 8-15 (0x10) followed by bits 0-7 (0x50).