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Unit 2: Atomic Structure and the Periodic Table

How a handful of scientists figured out what's inside an atom, and how the periodic table turns that structure into a predictive map.

14 topics · 47 questions

Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.

2.1 Dalton's Atomic Theory

  1. How it worksHow does Dalton's theory explain why water is always exactly two parts hydrogen to one part oxygen?
  2. Common mistakeWhy isn't Dalton's billiard-ball atom the same model chemists use today?
  3. Going furtherWhat part of Dalton's theory had to be thrown out once scientists discovered isotopes?
Your turn1 of 2
Chemists observed that hydrogen and oxygen always combine in the same fixed mass ratio to form water, never in some other arbitrary ratio. Why did chemists need to imagine an unbreakable particle just to explain why elements always combine in fixed ratios?

2.2 Thomson's Discovery of the Electron

  1. How it worksHow did Thomson's cathode ray experiment reveal a negatively charged particle inside the atom?
  2. How it worksHow does the plum pudding model spread positive charge and electrons through the whole atom?
  3. Common mistakeWhy isn't the plum pudding model able to explain where an atom's mass is concentrated?
Your turn1 of 2
Thomson noticed that a beam traveling through a vacuum tube bent measurably toward a positively charged plate placed nearby. Why did a beam bending inside a vacuum tube convince Thomson that atoms aren't solid balls?

2.3 Rutherford's Gold Foil Experiment

  1. How it worksHow does most of the beam passing straight through the gold foil prove that atoms are mostly empty space?
  2. How it worksHow do the rare, sharp deflections point to a tiny, dense, positively charged nucleus?
  3. Common mistakeWhy isn't the plum pudding model able to explain the particles that bounced almost straight back?
  4. Going furtherWhat would change about the gold foil results if the nucleus carried a negative charge instead?
Your turn1 of 2
Rutherford expected nearly every alpha particle fired at a thin sheet of gold foil to pass straight through, based on the plum pudding model, yet a small number bounced back almost the way they came. Why was Rutherford stunned when a few alpha particles bounced straight back off a thin sheet of gold?

2.4 The Bohr Model of the Atom

  1. How it worksHow does an electron jumping down to a lower energy level release a photon of a specific energy?
  2. Common mistakeWhy doesn't an electron actually orbit the nucleus the way a planet orbits the sun?
  3. Going furtherWhat happens to the light an atom emits when an electron jumps up two energy levels instead of one?
Your turn1 of 2
An excited hydrogen gas glows with only a few sharp, distinct colors of light rather than a smooth, continuous rainbow spanning every wavelength. Why do excited atoms give off light at only a few exact colors instead of every color at once?

2.5 The Quantum Mechanical Model and Electron Clouds

  1. How it worksHow does an electron cloud show the regions where an electron is most likely to be found?
  2. Common mistakeWhy isn't an electron cloud just a blurry picture of an electron moving in a circle?
  3. Going furtherWhat changes about the shape of an electron's probability cloud between an s orbital and a p orbital?
Your turn1 of 2
A chemist describing an electron's location talks about regions of high or low probability rather than pinpointing one exact spot the electron currently occupies. Why can chemists only describe where an electron probably is instead of exactly where it is?

2.6 Protons, Neutrons, and Electrons

  1. How it worksHow do the numbers of protons, neutrons, and electrons determine an atom's identity, mass, and charge?
  2. Worked exampleFind the number of protons, neutrons, and electrons in a neutral atom of chlorine-35.
  3. Common mistakeWhy doesn't losing an electron change which element an atom is?
  4. Going furtherWhat happens to an atom's charge when it gains two extra electrons?
Your turn1 of 2
A neutral atom has 64 protons and 95 neutrons in its nucleus. What is its mass number?

2.7 Isotopes and Mass Number

  1. How it worksHow does changing the number of neutrons in a nucleus create a different isotope of the same element?
  2. Worked exampleFind the number of neutrons in an atom of uranium-238.
  3. Common mistakeWhy doesn't changing an atom's neutron count change what element it is?
Your turn1 of 2
An atom has a mass number of 181 and 21 protons in its nucleus. How many neutrons does it have?

2.8 Average Atomic Mass from Isotope Abundance

  1. How it worksHow does a weighted average of each isotope's mass and abundance produce the number on the periodic table?
  2. Worked exampleFind the average atomic mass of an element that is 75 percent an isotope of mass 35 and 25 percent an isotope of mass 37.
  3. Common mistakeWhy isn't average atomic mass just the plain average of an element's isotope masses?
Your turn1 of 2
An element has two isotopes: one of mass 29 amu with abundance 85 percent, and one of mass 73 amu making up the rest. Find the element's average atomic mass, in amu.

2.9 Electron Configuration and the Aufbau Principle

  1. How it worksHow does the aufbau principle fill an atom's shells and subshells one electron at a time?
  2. Worked exampleFind the electron configuration of a neutral phosphorus atom, atomic number 15.
  3. Common mistakeWhy doesn't every atom's outermost shell fill up before starting a new subshell?
  4. Going furtherWhat happens to a transition metal's electron configuration when it loses its outer s electrons before its d electrons to form a positive ion?
Your turn1 of 2
A neutral atom's electron configuration accounts for 21 electrons filling its inner, lower-energy subshells and 4 electrons in its outermost, highest-energy subshell being filled. What is this atom's total electron count, equal to its atomic number?

2.10 Valence Electrons and Lewis Dot Symbols

  1. How it worksHow do you count an atom's valence electrons straight from its group number on the periodic table?
  2. Worked exampleDraw the Lewis dot symbol for a neutral oxygen atom.
  3. Common mistakeWhy doesn't an atom's total number of electrons determine how many bonds it forms?
Your turn1 of 2
A main-group element sits in periodic table group 16, using the 1 through 18 group numbering system, where 16 is between 13 and 18. How many valence electrons does an atom of this element have?

2.11 Organization of the Periodic Table

  1. How it worksHow does arranging elements by increasing atomic number line up columns of matching valence electrons?
  2. Common mistakeWhy isn't the periodic table organized by atomic mass the way Mendeleev first tried it?
  3. Going furtherWhat happens to an element's chemical behavior as you move across a period from left to right?
Your turn1 of 2
Sodium and potassium sit in the same column of the periodic table, and both react vigorously and in strikingly similar ways when dropped into water. Why do elements in the same column of the periodic table behave so much alike?

2.12 Metals, Nonmetals, and Metalloids

  1. How it worksHow does the staircase line on the periodic table separate metals from nonmetals?
  2. Common mistakeWhy isn't a metalloid just a weaker version of a metal?
  3. Going furtherWhat happens to an element's properties as it sits closer to the metalloid staircase?
Your turn1 of 2
A chunk of copper wire readily carries an electric current, while a chunk of solid sulfur does not conduct electricity at all under the same conditions. Why does a chunk of copper conduct electricity while a chunk of sulfur does not?