Atomic Structure 3.2-3.3 (Isotopes & Electronic Config)

Which number is unique to an element?

(no two elements have this same number)

• Group number in Periodic Table

• nucleon (mass) number

• neutron number

• proton number

Answer explanation

Look the Periodic Table - the number of protons in an atom determines what element it is.

For example, an atom with 8 protons is oxygen.

The nucleus of an atom has neutrons and protons.

Electrons are in different shells around the nucleus.

Which statement is correct?

• Electrons and protons have the same relative mass.

• The nucleus contains most of the mass of the atom.

• The nucleus and electrons repel each other.

• The nucleus has a negative charge.

Answer explanation

The protons and neutrons in the nucleus make up most of the mass of the atom, since electrons are so light that their mass is negligible.

The nucleus has positive charge since protons have positive charge but neutrons have no charge.

Two atoms of the same element have different nucleon numbers.

(Recall: nucleon no. = no. of p + no. of n)

Which row is correct for these atoms?

• A

• B

• C

• D

Answer explanation

Isotopes are atoms of the same element with different number of neutrons.

Which statement explains why isotopes of the same element have similar chemical properties?

• They have the same number of protons.

• They have the same number of electrons.

• They have different numbers of neutrons.

• They have different relative atomic mass.

Answer explanation

Isotopes undergo the same type of chemical reactions as they have the same number of electrons.

How many of each sub-atomic particle are present in an atom of the isotope nitrogen-15?

(The '15' represents the nucleon number.)

• A

• B

• C

• D

Answer explanation

Nitrogen has 7 protons (check Periodic Table).

No. of neutrons = Nucleon no. - no. of p = 15 - 7 = 8

Which nuclide notation represents this atom?

• 
• 
• 
• 

Answer explanation

Nucleon no. = no. of p + no. of n = 4 + 5 = 9

What is the correct electronic configuration of each atom?

• X: 2, 2, 2

  Y: 2, 2, 3

• X: 2, 4

  Y: 2, 5

• X: 2, 8, 4

  Y: 2, 8, 4

• X: 6

  Y: 7

Answer explanation

No. of p = no. of e

X has 6 electrons and Y has 7 electrons.

1st electron shell can hold a max. of 2 electrons.

X: 2, 4 and Y: 2, 5

Which statement about these two atoms X and Y is correct?

• They are isotopes of the same element.

• They have the same number of neutrons.

• They belong to the same group in the Periodic Table.

• They belong to the same period in the Periodic Table.

Answer explanation

Electronic configuration of X: 2, 4

Electronic configuration of Y: 2, 5

Both atoms have 2 electron shells hence both belong to Period 2.

Which statement explains why Group 18 elements are stable and unreactive?

• They have 1 valence electron.

• They have 2 valence electrons.

• They have 2 electrons in the first shell.

• They have full valence shells of electrons.

Answer explanation

Since the atoms of Group 18 elements have full valence shells of electrons, they are stable.

Positive ions are formed when atoms …….1…….. electrons.

Negative ions are formed when atoms …….2…….. electrons.

• A

• B

• C

• D

Answer explanation

Recall: Electrons are negatively charged.

When an atom gains electron(s), it becomes a negatively charged ion.

When an atom loses electron(s), it becomes a positively charged ion.