We have an atom of sodium, but I really don't understand this homework. Could anyone help?
A. We are told that the atom is sodium. Sodium has the atomic number of 11, which means that it has 11 protons. As an atom (as opposed to an ion) sodium also has 11 electrons. The diagram represents a nucleus with 11 electrons around it. Since protons have a charge of +1 and electrons have a charge of -1, the same number of protons and electrons results in a net charge of zero.
B. You are told that this is an atom, which means it hasn't gained or lost electrons. The diagram shows that it has 2 electrons. The element that has 2 electrons as a neutral atom is helium, atomic number 2. Helium is unreactive because it has a full outer level of electrons so it doesn't react to gain or lose electrons. (The first energy level holds two electrons.)
C. The two atoms that have similar chemical properties are B and C. We can tell that they're both Alkali Metals (Group 1) because they both have one electron in the outer level or valence level. An elements' chemical properties are largely determined by its number of valence electrons. Elements in the same group (vertical column) on the periodic table have similar chemical properties.
The other two elements are:
Atom A - helium, in Group 18 (noble gases). It has 2 electrons therefore it's element number 2.
Atom D - nitrogen, in Group 15 (also called V-A). It has 7 electrons therefore it's element number 7.