The verb secede (split from, disaffiliate) forms adjectives with its present or past participle. Depending on the usage, it would be "seceding" or "seceded" (e.g. seceded states that formed the Confederacy).
They all did. But the Western counties of Virginia formed a separate state and seceded from Virginia - a new Union state called West Virginia. So Virginia itself was a smaller version of what it had been.
All the states that formed the original confederacy seceded before the civil war, 11 in total: South Carolina Mississippi Alabama Florida Texas Arkansas Georgia North Carolina Tennessee Virginia Louisiana
Washington D. C. would have been surrounded by the Confederacy if Maryland had seceded. Washington D.C. is on the northern side of the Potomac River within the state of Maryland and just across the river from Virginia.