The Articles of Confederation was a weak central government that gave most of the powers of government to the states. Its framers were so concerned with their rights being taken away that there was no single executive, rather an executive committee. The main powers lacked by the nation were the powers to tax or enforce laws. The result was a financially and politically unstable nation.
The Constitution was born out of the necessity for the national government to have more power. The national government has the power to tax and enforce laws under the Constitution. The framers of the Constitution set up a system of checks and balances and separation of powers to ensure that the individual liberties of its citizens were secured while giving the government the powers necessary to maintain itself.