Atomic structure: flashcards, quiz and summary

Flashcards

1. 1What determines which element an atom is?

   The number of protons in the nucleus, which is the atomic number (Z). Every carbon atom has 6 protons, so Z = 6 for carbon.

2. 2How do isotopes of the same element differ, and why does their average mass matter?

   Isotopes have the same number of protons but different numbers of neutrons, so their mass numbers differ. The average atomic mass on the periodic table is a weighted average of these isotope masses, based on their natural abundance.

3. 3Why did Rutherford's gold foil experiment show that the atom has a small, dense, positive nucleus?

   Most alpha particles passed straight through the foil, which showed that most of the atom is empty space. A few were deflected at large angles, which meant they hit a tiny, dense, positive center.

4. 4How did Bohr's model explain hydrogen's line spectrum?

   Bohr proposed that electrons orbit at fixed energy levels. An electron absorbs energy to jump to a higher level and releases light when it drops back down, and only certain energies produce the observed lines.

5. 5How does the quantum mechanical model change the idea of an electron's path?

   It treats electrons as waves, so an orbital is a region where an electron is likely to be found, not a fixed path around the nucleus.

6. 6How many electrons can energy level n hold, and why?

   Level n holds up to 2n^2 electrons. This gives 2 for n = 1, 8 for n = 2, 18 for n = 3 and 32 for n = 4, because each level has n^2 orbitals and each orbital holds 2 electrons.

7. 7What are the Pauli exclusion principle and Hund's rule, and how do they apply to filling orbitals?

   The Pauli exclusion principle says each orbital holds at most 2 electrons with opposite spins. Hund's rule says electrons fill empty orbitals in a sublevel one at a time before any pairing occurs.

8. 8Why do chromium and copper break the normal filling order?

   Chromium is [Ar] 4s1 3d5 and copper is [Ar] 4s1 3d10 because a half-filled or completely filled d sublevel is more stable, so one 4s electron moves into 3d.

9. 9Why do valence electrons matter for chemical bonding?

   Valence electrons are in the highest occupied energy level, and they are the electrons that atoms gain, lose or share when forming bonds. Their count explains how an element reacts.

10. 10What does the octet rule say, and how does it explain ion formation?

    The octet rule says atoms tend to have 8 valence electrons like a noble gas. Metals lose electrons to reach this state, forming positive cations, while nonmetals gain electrons to reach it, forming negative anions.

11. 11How many protons and electrons does Mg2+ have, and why?

    Mg2+ has 12 protons and 10 electrons. Magnesium loses 2 electrons to form the 2+ ion, but its protons do not change.

12. 12Why does the Fe2+ ion have the configuration [Ar] 3d6?

    Iron's neutral configuration is [Ar] 4s2 3d6. Transition metals lose their 4s electrons before 3d electrons when they form ions, so Fe2+ loses both 4s electrons and keeps 3d6.

Practice quiz

Pick an answer to check it.

1. 1An atom has 17 protons, 17 electrons and 20 neutrons. What is its mass number?

   1. 20
   2. 17
   3. 37
   4. 34
   Show the answer

   C. 37 Mass number equals protons plus neutrons, so 17 + 20 = 37. The atomic number 17 identifies chlorine, and 34 is only the sum of protons and electrons, not the mass number.

2. 2Chlorine is about 75.8 percent Cl-35 (34.969 amu) and 24.2 percent Cl-37 (36.966 amu). What is its average atomic mass?

   1. 35.97 amu
   2. 36.47 amu
   3. 35.00 amu
   4. 35.45 amu
   Show the answer

   D. 35.45 amu Multiply each mass by its abundance: 34.969 × 0.758 = 26.507 and 36.966 × 0.242 = 8.946. Adding them gives 35.453, or about 35.45 amu. The average is closer to 35 because Cl-35 is more common.

3. 3Which experiment and result led Rutherford to propose a small, dense, positive nucleus?

   1. Oil drop experiment, which measured the charge of a single electron
   2. Hydrogen line spectrum, where light was emitted at specific wavelengths
   3. Cathode ray tube experiments, which showed that electrons carry negative charge
   4. Gold foil experiment, where a few alpha particles bounced back at large angles
   Show the answer

   D. Gold foil experiment, where a few alpha particles bounced back at large angles In Rutherford's gold foil experiment, most alpha particles passed through, but a few were deflected at large angles. That result pointed to a tiny dense nucleus. The cathode ray work led to the electron, and the spectrum work led to Bohr's model.

4. 4What is the electron configuration of oxygen, atomic number 8?

   1. 1s2 2s1 2p5
   2. 1s2 2s2 2p4
   3. 1s2 2s2 2p6
   4. 1s2 2s2 2p3 3s1
   Show the answer

   B. 1s2 2s2 2p4 Oxygen has 8 electrons. Two fill 1s, two fill 2s, and four fill 2p, giving 1s2 2s2 2p4. The option with 2p6 has too many electrons for oxygen.

5. 5Which electron configuration is correct for chromium, which is an exception to the normal filling order?

   1. [Ar] 4s2 3d4
   2. [Ar] 4s1 3d5
   3. [Ar] 4s2 3d3 4p1
   4. [Ar] 3d6
   Show the answer

   B. [Ar] 4s1 3d5 Chromium is [Ar] 4s1 3d5 because the half-filled 3d sublevel is more stable. The normal Aufbau order would predict [Ar] 4s2 3d4, which is the common mistake.

6. 6Which statement about the Fe2+ ion is correct?

   1. Fe2+ has the configuration [Ar] 3d6 because it loses both 4s electrons first
   2. Fe2+ has the configuration [Ar] 4s2 3d4 because it loses two 3d electrons first
   3. Fe2+ has 28 electrons because it gains two protons
   4. Fe2+ has the configuration [Ar] 4s1 3d5 because one 4s electron moves into 3d
   Show the answer

   A. Fe2+ has the configuration [Ar] 3d6 because it loses both 4s electrons first Iron's neutral configuration is [Ar] 4s2 3d6. Transition metals lose 4s electrons before 3d electrons when forming ions, so Fe2+ is [Ar] 3d6. Ions do not gain protons, so the proton count stays at 26.

Test traps

What teachers like to test on this, and the mistakes students make.

• Confusing mass number with average atomic mass. Mass number is a whole number for one isotope, while average atomic mass is a decimal for the mix of isotopes.
• Changing the wrong particle when an atom becomes an ion. A changed electron count makes an ion, a changed neutron count makes an isotope, and only a changed proton count makes a new element.
• Filling 3d before 4s when writing a neutral configuration for transition metals, or writing the wrong order for ions. Iron's neutral configuration is [Ar] 4s2 3d6, but Fe2+ is [Ar] 3d6.
• Forgetting the exceptions to the Aufbau order. Chromium is [Ar] 4s1 3d5 and copper is [Ar] 4s1 3d10, not the orders the basic rule predicts.

Summary

• Protons and neutrons are in the nucleus, electrons are outside it, and the nucleus holds almost all of the atom's mass.
• Atomic number (Z) equals the number of protons and identifies the element, while mass number (A) equals protons plus neutrons.
• Isotopes of an element have the same number of protons but different numbers of neutrons, so average atomic mass is a weighted average of isotope masses.
• The atomic model changed from Dalton's solid spheres to Thomson's electron, Rutherford's dense nucleus, Bohr's fixed energy levels, and the quantum mechanical model with orbitals.
• Energy level n holds up to 2n^2 electrons, and the s, p, d and f sublevels hold 2, 6, 10 and 14 electrons, with at most 2 electrons per orbital.
• Electron configurations are built by the Aufbau principle and Hund's rule, and chromium and copper are common exceptions.
• Valence electrons in the highest occupied level drive bonding, and atoms gain, lose or share electrons to reach 8 valence electrons.
• Ions form when atoms gain or lose electrons, and the number of protons never changes in a chemical reaction.

Key terms

Atomic number (Z)

The number of protons in an atom's nucleus. It defines the element, so every carbon atom has 6.

Mass number (A)

The total number of protons and neutrons in an atom's nucleus. Neutrons can be found by subtracting Z from A.

Isotope

Atoms of the same element that have different numbers of neutrons, such as carbon-12, carbon-13 and carbon-14.

Average atomic mass

The sum of each isotope's mass multiplied by its fractional abundance. This is the decimal value shown on the periodic table.

Principal energy level (n)

The main energy level of an electron. A larger n means the level is farther from the nucleus and has higher energy.

Aufbau principle

The rule that electrons fill the lowest available energy levels first when building an electron configuration.

Hund's rule

Within a sublevel, electrons occupy empty orbitals one at a time before any orbital holds a pair of electrons.

Octet rule

The tendency of atoms to gain, lose or share electrons so they have 8 valence electrons, like a noble gas.

How to study it

The notes this pack came from

Questions