Question 1

Why was hydrogen used to fill the Hindenburg airship, and what property made it dangerous?

Accepted Answer

Hydrogen is less dense than air and highly reactive, making it flammable and prone to explosion.

Question 2

Modern blimps use helium instead of hydrogen. What makes helium inert?

Accepted Answer

Helium has a completely filled valence shell with two electrons, making it chemically stable.

Question 3

According to the Bohr model, what happens when an electron in a hydrogen atom jumps from a higher energy orbit ($$n=3) to a $$lower energy orbit ($$n=2$$)?

Accepted Answer

A photon is emitted with energy equal to the difference between the two orbits.

Question 4

Which statement best describes the quantum-mechanical model of the atom?

Accepted Answer

Electrons exist in orbitals, which are probability maps showing where an electron is likely to be found.

Question 5

What is the principal quantum number ($$n) $$and what does it specify?

Accepted Answer

It specifies the principal shell of the orbital and its energy level.

Question 6

Which orbital has the highest probability of finding an electron close to the nucleus?

Accepted Answer

The 1s orbital

Question 7

What is Hund’s rule regarding electron configuration?

Accepted Answer

Electrons fill orbitals of equal energy singly first, with parallel spins.

Question 8

Which of the following electron configurations represents a ground-state carbon atom?

Accepted Answer

$$1s^2$$\ $$2s^2$$\ $$2p^2$$

Question 9

How many valence electrons does silicon (Si) have, and in which principal shell are they found?

Accepted Answer

4 valence electrons in the $$n=3$$ shell

Question 10

Which element has the electron configuration $$[Ne]\ 3s^2$\ 3p^5$?

Accepted Answer

Chlorine (Cl)

Question 11

What is the general trend in atomic size as you move down a group in the periodic table?

Accepted Answer

Atomic size increases

Question 12

Why are noble gases generally inert?

Accepted Answer

They have filled s and p orbitals with 8 valence electrons, making them stable.

Question 13

Which of the following best explains why alkali metals are highly reactive?

Accepted Answer

They have one valence electron and can easily lose it to achieve a noble gas configuration.

Question 14

What is the electron configuration for sodium (Na) using noble gas core notation?

Accepted Answer

$$[Ne]\ 3s^1$

Question 15

Which subshell is higher in energy in a multi-electron atom: 4s or 3d?

Accepted Answer

3d is higher in energy than 4s

Question 16

A transition metal in the first row of the d block has its outermost d electrons in which principal quantum shell?

Accepted Answer

$$n = 3$$

Question 17

Which of the following is true about the filling order of electron orbitals?

Accepted Answer

Lower-energy orbitals fill before higher-energy orbitals.

Question 18

Which periodic trend increases as you move from left to right across a period?

Accepted Answer

Ionization energy

Question 19

If an atom absorbs energy and an electron moves from the 1s orbital to the 2p orbital, what is this state called?

Accepted Answer

Excited state

Question 20

Which of the following elements is most likely to form a $$+2$$ cation to achieve a noble gas configuration?

Accepted Answer

Magnesium (Mg)

Electrons in Atoms and the Periodic Table: Models, Configurations, and Periodic Trends

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