what is the element with the electron arrangement 2 8 7
I would look up atomic number 17 (assuming it's a neutral atom with the 2,8,7 configuration).
To find the element with the electron arrangement 2 8 7, we need to understand the electron configuration of elements.
The electron configuration of an element shows how its electrons are distributed in different energy levels or shells around the nucleus.
The first number in the electron arrangement represents the number of electrons in the 1st energy level (K shell), the second number represents the number of electrons in the 2nd energy level (L shell), and so on.
In this case, the electron arrangement 2 8 7 indicates that the element has 2 electrons in the 1st energy level, 8 electrons in the 2nd energy level, and 7 electrons in the 3rd energy level.
When we add these numbers up, we get a total of 2 + 8 + 7 = 17 electrons.
Now, to identify the element with this electron arrangement, we can refer to the periodic table.
By counting from left to right on the period (horizontal row) that corresponds to the number of electrons, we can find the element. In this case, since we have 17 electrons, we would count until we reach the 17th element.
The 17th element in the periodic table is chlorine (Cl). Therefore, the element with the electron arrangement 2 8 7 is chlorine (Cl).
Remember that the electron arrangement can change based on the ionization state of the element, but in this case, we assume the element is neutral (no charge).