1) Where does most of the mass of an atom come from? 2) What do you predict the

Question

1) Where does most of the mass of an atom come from?

2) What do you predict the trend in atomic mass will be going down a group?

3) Across a period?

4) Will this trend be the same as the one observed for atomic number? Why/why not?

5) Use the periodic table in the back of your course pack to find the trend in atomic mass.

6) One notable exception to this trend is that argon has a mass greater than potassium. Why could the mass of argon be greater than the mass of potassium even though it has a lower atomic number?

7) Which shell will determine the atomic radius of carbon? of aluminum?

8) As one goes down each column on the periodic table, an additional shell level is added, thus what is the trend in atomic radius down a group?

9) Figure 2 shows the atomic radii of atoms in the s- and p-blocks. Does this data support your prediction?

10) What is the trend across a period?

Explanation / Answer

(1) In an atom most of the mass is equal to the sum of the masses of protons and neutrons which are quite similar. Mass of electron is not considered because it is 1/1837 times mass of proton.Hence mass of an atom comes from the sum of masses of protons and neutrons which are present in the nucleus of the atom.

(2) As we move down in a group the number of protons and neutrons increases hence the atomic mass will increase but not necessarily by the same amount.

(3) As we move from left to right in a period the atomic mass increases due to the arrangement of elements in the periodic table.

(4) Atomic number will also increase on moving down a group or moving from left to right in a period because the number of protons increases in both cases.

(5) Atomic mass increases as we move from left to right in a group but there is an exception in which Argon has atomic mass greater than Potassium and when we move down in a group the atomic mass also increases but not by same amount i.e when moving from H to Na the atomic mass increases by 2, but it's not the same going down from Na to Li.

(6) Argon has a greater proportion of heavier isotopes with more neutrons than Potassium hence atomic mass of Argon is grater than Potassium even though it has lower atomic number.

(7) The outermost shell will determine atomic radius hence in Carbon it will be 2nd shell( L shell) and in ALuminium it will be 3rd shell( M shell)

(8) As one goes down each column on the periodic table, an additional shell level is added, hence the atomic radius increases because the valence electrons occupy higher levels due to increasing quantum number.

(9) Yes this data supports our prediction the trends in atomic size result from differences in the effective nuclear charges (Zeff) experienced by electrons in the outermost orbitals of the elements. As we proceed across the row, the increasing nuclear charge is not effectively neutralized by the electrons being added to the 2s and 2p orbitals. Hence there is steady decrease in atomic size.

(10) As we move from left to right in a period the atomic radius decrease because electrons try to stay far apart within the shell and an additional electron has a less dramatic effect compared to the increased attraction that the electrons feel toward the increased positive charge of the nucleus.

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