Question

the bohr models of the first 20 elements are shown in the diagram.
which of the following statement(s) accurately identifies trends in electron arrangement for the first 20 elements?
once the lower energy level(s) is filled, additional electrons are only added to the outermost shell or energy level.
the number of valence electrons goes up as you move across a period.
the number of valence electrons goes up as you move down a group.
the second energy level or electron shell can hold up to 8 electrons.
the number of electron shells or energy levels goes up as you move down a group.
the number of electron shells or energy levels goes up as you move across a period.
the first energy level or electron shell can hold up to 2

Explanation:

• First statement: Electrons fill lower - energy levels first. Once filled, electrons go to the outermost shell. This is in line with the Aufbau principle.
• Second statement: In a period (horizontal row in the periodic table), as we move from left to right, the number of valence electrons increases. For example, in period 2 (Li to Ne), valence electrons go from 1 (Li) to 8 (Ne).
• Third statement: In a group (vertical column in the periodic table), the number of valence electrons is the same. For example, group 1 (Li, Na, K) all have 1 valence electron.
• Fourth statement: The second energy level (\(n = 2\)) has \(s\) and \(p\) sub - levels. The \(2s\) can hold 2 electrons and the \(2p\) can hold 6 electrons. So in total, \(2 + 6=8\) electrons.
• Fifth statement: As we move down a group, the principal quantum number (\(n\)) of the outermost electron increases. So the number of electron shells increases. For example, H (\(n = 1\)), Li (\(n = 2\)), Na (\(n = 3\)) in group 1.
• Sixth statement: In a period, all elements have the same number of electron shells. For example, all elements in period 2 have 2 electron shells.

Answer:

The first, second, fourth, and fifth statements are correct.