Atomic Structure

Question 1

Molecules of heavy water contain two atoms of hydrogen-2 instead of two atoms of hydrogen-1

Explain why a molecule of heavy water has more mass than a normal water molecule.

You should refer to the particles in the nucleus of the two different hydrogen atoms in your answer.

Answer 1

• In hydrogen-2, there are 2 particles in the nucleus, a proton and neutron, which have a relative mass of 1, leaving hydrogen-2 with a relative mass of 2, whilst hydrogen-1 only has one particle in the nucleus, a proton, leaving it with a relative mass of 1.
  (Therefore if a molecule of heavy water has 2 hydrogen-2 it has a mass of 4 whilst normal water with 2 hydrogen-1 has a mass of 2, giving heavy water a greater mass)

Question 2

Which particle has the lowest mass?

Answer 2

Electron- negligible mass

Question 3

What is an atom?

Answer 3

An atom is the smallest part of an element that can exist

Question 4

What are the relative electrical charges of the particles in atoms?

Answer 4

protons = +1
neutrons = 0
electrons = -1

Question 5

What are the relative masses of the particles in atoms?

Answer 5

proton = 1
neutron = 1
electron = very small

Question 6

What is the mass number?

Answer 6

The sum of the protons and neutrons in an atom

Question 7

What is an isotope?

Answer 7

Isotopes are atoms of the same element that have the same number of protons and electrons, but different numbers of neutrons.

Question 8

What is the relative atomic mass?

Answer 8

The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element

Question 9

How many electrons can the first three shells hold?

Answer 9

2, 8, 8

Question 10

What is the plum pudding model?

Answer 10

The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it

Question 11

Development of the nuclear model

Answer 11

• The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged
• This nuclear model replaced the plum pudding model

Question 12

How is the periodic table arranged?

Answer 12

• The elements in the periodic table are arranged in order of atomic (proton) number
• elements with similar properties are in columns, known as groups
• The table is called a periodic table because similar properties occur at regular intervals
• Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties

Question 13

History of the periodic table

Answer 13

• Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights
• The early periodic tables were incomplete and some elements were placed in inappropriate groups if the strict order of atomic weights was followed
• Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights
• Elements with properties predicted by Mendeleev were discovered and filled the gaps
• Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.