Atoms and the Periodic Table

Chemistry on this subtest starts and ends with counting particles. Nearly every question is one of three quantities: how many protons, how many neutrons, how many electrons.

The three particles

Protons and neutrons are in the nucleus and have essentially all the mass. Electrons orbit and have almost none. An electron is about 1/1836 the mass of a proton, which is why mass number ignores them entirely.

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The three numbers

Atomic number = number of protons. This is the element's identity. Every carbon atom has 6 protons; anything with 6 protons is carbon.

Mass number = protons + neutrons.

Neutrons = mass number - atomic number. That subtraction is the most asked calculation in the topic.

Carbon-14 has an atomic number of 6. How many neutrons?

14 minus 6 is 8 neutrons.

In a neutral atom, electrons = protons. So neutral carbon has 6 electrons.

Isotopes and ions

Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 and carbon-14 are both carbon - both have 6 protons - and differ by two neutrons.

Ions are atoms that have gained or lost electrons, giving them a charge.

• Lose electrons, become positive. A cation. Metals do this.
• Gain electrons, become negative. An anion. Nonmetals do this.

Isotopes differ in neutrons; ions differ in electrons. Neither ever changes the proton count, because that would make it a different element. That sentence answers a whole family of questions.

Electron shells

Electrons occupy shells at increasing distance from the nucleus.

For the first eighteen elements: the first shell holds 2, the second 8, the third 8.

The outermost shell is the valence shell, and the electrons in it are valence electrons. They are what determine how an element reacts, because they are the ones available to be shared, given or taken.

A full valence shell means an unreactive element. That is why the noble gases in group 18 barely react at all: their outer shells are already full.

Electron configuration

The harder questions write the shells out in more detail, as sublevels labeled s, p, d and f. Each holds a fixed number of electrons:

For the first 18 elements they fill in this order: 1s, 2s, 2p, 3s, 3p. The number in front is the shell, and the small raised number counts the electrons in that sublevel. To write a configuration, fill in order until the electrons run out:

Carbon has 6 electrons: 1s2 2s2 2p2 - two, two, and the last two in 2p.

Sodium has 11: 1s2 2s2 2p6 3s1.

Add up the raised numbers to check - they must equal the atomic number. Read backward, the last shell tells you the group: an atom ending in 3s2 3p3 has 5 outer electrons and sits in group 15.

This notation is the same shell count as above written out in more detail. The first shell (1s) holds 2; the second (2s plus 2p) holds 2 + 6 = 8; which is where the 2, 8, 8 pattern comes from.

The periodic table

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Groups are columns. Elements in a group have the same number of valence electrons, which is why they behave chemically alike. That is the organizing principle of the whole table.

Periods are rows. Moving down a period number means one more electron shell.

Atomic number increases left to right across a period and down the table.

The groups worth naming

Noble gases are the ones to know, because their unreactivity follows directly from the full-shell rule rather than having to be memorized.

The trends across a row

Three properties change steadily across the table, and they are asked:

Going down a group reverses all three, because each row adds a shell farther from the nucleus. So the largest atoms are at the lower left (potassium is larger than sodium or chlorine) and the most electronegative at the upper right, which is why fluorine is the most electronegative element of all.

Metals, nonmetals and metalloids

Metals occupy the left and center. They are shiny, conduct heat and electricity, are malleable and ductile, and lose electrons in reactions.

Nonmetals occupy the upper right. They are dull, brittle as solids, poor conductors, and gain electrons.

Metalloids run in a stepped line between them - boron, silicon, germanium, arsenic, antimony, tellurium - and have properties of both. Silicon is the one the test names, because semiconductors are made from it, which is also why this lesson connects to Electronics Information.

Hydrogen sits in group 1 but is a nonmetal, which is the table's most common gotcha.

Chemical bonds

Atoms bond to reach a full valence shell, and how they get there is the type of bond. This is asked often - "ionic" alone is in 16 of our General Science questions - and it follows straight from the shell rule above.

Transfer is ionic; sharing is covalent. Sodium has one valence electron to lose and chlorine wants one to fill its shell, so the electron moves across, and the resulting positive and negative ions attract. Two oxygen atoms both need two more, so they share.

Covalent bonds can share more than one pair. A single bond shares one pair of electrons, a double bond two (as in oxygen gas, O2), a triple bond three (as in nitrogen gas, N2) - and more sharing holds the atoms tighter.

The metallic "sea" of electrons is why metals conduct electricity: the electrons are free to drift.

Polar molecules and hydrogen bonds

When atoms share unequally - because one pulls harder on the electrons - the bond is polar, with a slightly negative end and a slightly positive end. Fluorine is the most electronegative element, the hardest puller of all.

Water is polar, and most of its unusual behavior follows from that. Water molecules attract each other through hydrogen bonds - weak attractions between molecules, not bonds within one - which give water its unusually high boiling point and surface tension, and which also hold the two strands of DNA together. Polarity is also why water dissolves so much: its charged ends pull other charged particles apart.

One element, two forms

Carbon shows how structure changes everything: arranged in a rigid 3D lattice it is diamond, the hardest natural material; arranged in slippery flat sheets it is graphite, the "lead" in pencils.

Compounds and mixtures

Water is a compound; salt water is a mixture. You can boil the salt water and separate it; you cannot boil water into hydrogen and oxygen.

A compound has properties unlike its elements. Sodium is a metal that reacts violently with water and chlorine is a poisonous gas, and sodium chloride is table salt. That is a question, and it is the clearest demonstration that a chemical bond makes something new.

What you can skip

Across all 2,104 General Science questions in our bank:

• Orbital shapes, quantum numbers and the Aufbau principle by name never appear. The fill order above for the first 18 elements covers every configuration question; none goes past chlorine in the bank.
• Transition metal configurations. No question asks you to fill a d sublevel; one asks only how many electrons d holds.
• Memorizing the whole table. Questions ask about the named groups, a handful of common elements and how to read an atomic number and mass. They do not ask you to recall which element is in period 5.

Where people lose points

Saying isotopes differ in protons. They differ in neutrons.

Saying ions differ in protons. They differ in electrons.

Getting the ion sign backward.

Including electrons in mass number.

Calling hydrogen a metal because of where it sits.

Confusing a compound with a mixture.

Work one in under a minute

An atom has 17 protons, 18 neutrons and 18 electrons. What is it, what is its mass number, and what is its charge?

Identity: 17 protons means atomic number 17, which is chlorine.

Mass number: 17 plus 18 is 35.

Charge: 17 positive against 18 negative is a net negative one, so it is a chloride ion.

Three questions, three counts, and none of them needs the periodic table in front of you beyond knowing that 17 is chlorine.

Where this leads

The valence electrons introduced here are what bonds and reactions are made of, and the metalloids are the semiconductors Electronics Information is built on.