How many electrons can fit in the first, second, and third electron shells?

First: 8; Second: 8; Third: 8 First: 2; Second: 8; Third: up to 18 (though fills 8 before the next shell starts for most common elements) First: 1; Second: 4; Third: 9 First: 2; Second: 4; Third: 6

What is the electron configuration of a sodium atom (atomic number 11)?

2, 9 3, 8 2, 8, 1 11

What are valence electrons?

The electrons closest to the nucleus in the innermost shell The electrons in the outermost occupied shell — they determine how an atom bonds and reacts with other atoms All electrons in the atom — every electron has the same value for bonding Electrons that have been removed from the atom to form ions

Why are noble gases (helium, neon, argon) so unreactive?

Noble gases are too large for other atoms to get close enough to react Their outer electron shells are completely full — they have no tendency to gain, lose, or share electrons Noble gases have no electrons at all — they are made of bare nuclei Noble gases react only at very high temperatures beyond what we experience on Earth

Chlorine has atomic number 17 and electron configuration 2, 8, 7. How does it typically form an ion?

It loses 7 electrons to empty its outer shell, forming Cl⁷⁺ It shares 7 electrons with other atoms, forming 7 covalent bonds It gains one electron to complete its outer shell (8), forming Cl⁻ It loses 1 proton from its nucleus to become the noble gas argon

Elements in the same group (column) of the periodic table have similar chemical properties. Why?

They have the same mass number They all have the same number of valence electrons — which determines how they bond and react They all have the same number of electron shells They were all discovered in the same era and share physical similarities by coincidence

Why does an electron closer to the nucleus have lower energy than one in an outer shell?

Inner electrons move faster, so their kinetic energy is lower Electrons closer to the nucleus are more strongly attracted to it — energy must be added to move them to a higher shell further away Inner electrons have more neutrons around them, which give them lower energy There is no difference — all shells have the same energy

Oxygen has atomic number 8. What is its electron configuration, and how many valence electrons does it have?

2, 4 — four valence electrons 8 — all electrons in one shell 2, 6 — two electrons in shell 1, six in shell 2; it has 6 valence electrons 2, 8 — eight valence electrons, making it a noble gas

What does the Aufbau principle state?

Electrons fill the highest energy level first before filling lower ones Electrons are distributed evenly across all available energy levels Electrons fill the lowest available energy level first before occupying higher levels Electrons pair up before occupying separate orbitals in the same sub-shell

Why do full electron shells (like in noble gases) represent a particularly stable configuration?

Full shells cause atoms to emit light, which carries away energy and makes them stable Full shells mean all protons are paired with electrons, making the charges cancel A full shell means all available bonding orbitals are occupied — the atom has no tendency to gain or lose electrons to achieve greater stability Full shells are unstable — atoms immediately eject extra electrons to become more reactive