Chemistry Practice Test: Atomic Structure and the Periodic Table

The atomic number gives 17 protons, and a neutral atom has the same number of electrons, 17. Neutrons = mass number − protons = 37 − 17 = 20. The mass number 37 counts protons and neutrons together, so it is not the neutron count.

A neutral magnesium atom has 12 electrons. The 2+ charge means it has lost 2 electrons, leaving 10. The answer 14 comes from adding 2 electrons, which would give a negative ion.

Isotopes of an element have the same number of protons (6 for carbon) but different numbers of neutrons: 6 in carbon-12 and 8 in carbon-14. Because the electron arrangement is the same, their chemical properties are nearly identical.

Average atomic mass = 0.601 × 69 + 0.399 × 71 = 41.469 + 28.329 = 69.798, or 69.8 amu. It is closer to 69 because that isotope is more common. A plain average gives 70.0, and swapping the percentages gives 70.2.

Most particles passing through means most of the atom is empty space; the few that bounced back must have hit a tiny, dense, positively charged center, the nucleus. The idea of positive charge spread evenly is Thomson's earlier model, which the experiment disproved.

Eight electrons fill 1s (2), then 2s (2), then the remaining 4 go into 2p. An s sublevel holds at most 2 electrons, so 2s⁴ is impossible, and 1s² 2s² 2p⁶ describes neon, with 10 electrons.

Valence electrons are those in the outermost energy level, here level 3: 3s² and 3p³, for a total of 5. Counting only the 3p electrons gives 3, and 15 is the total number of electrons.

Across a period, protons are added to the nucleus while electrons enter the same energy level, so the stronger pull draws the electrons closer and the radius shrinks. Sodium, at the left of period 3, is therefore the largest and chlorine the smallest of these.

Going down a group, the outer electron is in a higher energy level, farther from the nucleus and more shielded, so it is easier to remove. Lithium's outer electron is closest to the nucleus, so lithium needs the most energy to lose it.

Electronegativity increases toward the upper right of the periodic table (not counting the noble gases), and fluorine has the highest value of all elements, about 4.0 on the Pauling scale. Oxygen comes second.

Chemical behavior depends mainly on valence electrons, and every Group 1 element has one. Elements in the same period, not the same group, share the number of occupied energy levels.

Luster, malleability, good conductivity and the tendency to lose electrons to form positive ions are all properties of metals. Nonmetals, including halogens, are usually dull, brittle as solids and poor conductors, and they tend to gain electrons.

Noble gases have a full valence shell (8 electrons, or 2 for helium), which is a very stable arrangement, so they have little tendency to gain, lose or share electrons. They do have electrons and neutrons, and their radii are small within each period.

Electrons can only have certain energies. When an excited electron drops to a lower level, it emits a photon whose energy equals the difference between the levels, giving a line of one color. Absorbing energy moves electrons up, not down.

Sulfur has 6 valence electrons and reaches a stable octet most easily by gaining 2 more, forming S²⁻ (sulfide). Losing 6 electrons to form S⁶⁺ would take far more energy.