Topic 1 - Key concepts in chemistry

Question 1

What was the Dalton model of an atom?

Answer 1

John Dalton described atoms as solid spheres

Question 2

What atomic model came after Dalton’s?

Answer 2

JJ Thomson’s measurements of charge and mass caused him to replace the ‘solid sphere’ idea with his ‘plum pudding model’ - one large proton ‘pudding’, topped with electron ‘cherries’.

Question 3

What atomic model came after the plum pudding model and how was it found?

Answer 3

• Ernest Rutherford and his students conducted the famous gold foil experiment where they fired positively charged alpha particles at an extremely thin sheet of gold.
• The plum pudding model would expect the particles to pass straight through the sheet or be slightly deflected at most as the positive charge of each atom was thought to be spread out through the ‘pudding’ of the atom.
• But some atoms were deflected more than expected and a small number were deflected backwards.
• So Rutherford replaced the plum pudding model with the nuclear atom theory - a tiny, positively charged nuclear atom surround by a ‘cloud’ of negative electrons (most of the atom is empty space).

Question 4

What atomic model do we trust now?

Answer 4

• Niels Bohr realised Rutherford’s ‘cloud’ of electrons would be attracted to the nucleus causing the atom to collapse.
• So his Bohr model suggests that the negative electrons exist in fixed orbits, or shells, circling the tiny, positively charged nucleus.
• Bohr’s theory is close to the currently accepted version of the atom.

Question 5

Which subatomic particles are contained within the nucleus?

Answer 5

Protons and neutrons

Question 6

How is the mass of an atom distributed?

Answer 6

Almost the entire mass of an atom is concentrated in the nucleus

Question 7

What is the relative mass of a proton?

Answer 7

1

Question 8

What is the relative mass of a neutron?

Answer 8

1

Question 9

What is the relative mass of an electron?

Answer 9

0.0005 - negligible

Question 10

What is the relative charge of a proton?

Answer 10

+1

Question 11

What is the relative charge of a neutron?

Answer 11

0

Question 12

What is the relative charge of an electron?

Answer 12

-1

Question 13

Why do atoms contain equal numbers of protons and electrons?

Answer 13

In order to have no charge overall

Question 14

How large is the nucleus in relation to the overall size of an atom?

Answer 14

Very small

Question 15

What does the ‘mass number’ of an atom tell you?

Answer 15

The total number of protons and neutrons in the atom

Question 16

What identifies the element of an atom?

Answer 16

The number of protons

Question 17

What are isotopes?

Answer 17

Different atoms of the same element (with the same number of protons) containing different numbers of neutrons in their nuclei

Question 18

What does the ‘mass number’ of an atom tell you?

Answer 18

How many protons the atom has

Question 19

Using the mass number and the atomic number of an atom, how can you calculate the number of protons it has?

Answer 19

atomic number = no. of protons

Question 20

Using the mass number and the atomic number of an atom, how can you calculate the number of neutrons it has?

Answer 20

mass number - atomic number = no. of neutrons

Question 21

Using the mass number and the atomic number of an atom, how can you calculate the number of electrons?

Answer 21

atomic number = no. of electrons

Question 22

How does the existence of isotopes result in relative atomic masses of some elements not being whole numbers?

Answer 22

If an element has more than one isotope, its relative atomic mass (Ar) is the average of the mass numbers of all the different isotopes, taking into account how much there is of each one. Therefore that average might not be a whole number.