Actually Sister your first entry at Genesis 16:7 was not an actual angel. It was the pre-incarnate Jesus Christ known in the Old Testament as "The angle of the Lord." At Genesis 16:10 he multiplies Hagar's descendants. You are right that he told Hagar that she would have a son which is what we know today as the Arabs.
Also notice at Genesis 16:13 she called the name of the Lord who spoke to her, "Thou art a God who sees. At Genesis 17:1-2 says, "Now when Abram was ninety-nine years old the Lord APPEARED to Abram and said to him, "I am God Almighty; Walk before Me, and be blameless. Vs2, And I will establish My covenant between Me and you, And I will multiply you exceedingly."
The same being that multiplied Hagar's descendants is the same being who identified Himself as God Almighty multiplied Abram's descendants. As a side note you should know that "The angel of the Lord" never appears in the New Testament as the angel of the Lord. Keep up the good work!
IN GOD THE SON,
bluto