Module 4 · The Periodic TablePractice · 元素周期表

Magnesium’s symbol is Mg; Mn is manganese. Both start with M, so the lowercase second letter tells them apart: Mg (magnesium) and Mn (manganese). Calcium Ca, potassium K, and copper Cu are all correct.

Chlorine’s electrons fill 3 shells (2, 8, 7), so it’s in period 3. It has 7 electrons in its outer shell, which puts it in group 17, the halogens (for groups 13–17, valence electrons = group number − 10). Period 4, group 1 is element 19, potassium.

Mercury (Hg), aluminum (Al), and iron (Fe) are metals; mercury is a metal even though it’s a liquid at room temperature. Bromine (Br) and phosphorus (P) are nonmetals: bromine is the only nonmetal element that is a liquid at room temperature.

A chlorine atom has 17 protons and 17 electrons, arranged 2, 8, 7, so the answer is C. A has 18 electrons, one more than its protons: that’s a chloride ion, $\ce{Cl-}$. B has a nuclear charge of 18, so it’s argon. D has 17 electrons, but it starts the third shell before the second is full (7 instead of 8), which is wrong.

With 3 shells, the first (2 electrons) and second (8) must be full. The outer shell holds 3 × 2 = 6. That’s 2 + 8 + 6 = 16 electrons, and in an atom the atomic number equals the number of electrons: 16, sulfur (S).

Elements in the same period have the same number of shells (3 here). Moving right, the nuclear charge and the number of outer electrons both increase (Na 1 → Si 4), the nucleus pulls the electrons in more strongly, and the atoms get smaller: Na 166 pm, Mg 141 pm, Al 121 pm, Si 111 pm.

X readily loses its 2 outer electrons to form $\ce{X^2+}$, and Y gains 2 electrons to complete an octet, forming $\ce{Y^2-}$. For the charges to balance, X and Y combine 1 : 1, giving XY (for example, MgS).

Elements in the same group (column) have the same number of outer electrons. O, S, and Se each have 6, so all three are in group 16. N is in group 15, F in group 17, and Ne in group 18: they share a period with O, not a group.

Shine, electrical conductivity, and malleability together are the marks of a metal, so it’s copper. Iodine crystals are shiny but barely conduct and are brittle; graphite conducts but is soft and crumbles rather than flattening; sulfur is a yellow solid that doesn’t conduct.

A number in front of an element symbol means that many atoms; it no longer stands for the element or a substance. 2N means 2 nitrogen atoms. Nitrogen gas is $\ce{N2}$, and 2 nitrogen molecules are $\ce{2N2}$; N on its own stands for the element nitrogen (or one nitrogen atom).