Atomic Structure: The Nuclear & Bohr Model

What is the scientific method?

A systematic, evidence-based approach that scientists use to gather, analyse and interpret data.

What is a scientific theory?

A structured explanation of a set of facts or a phenomenon, arrived at by thorough examination and tested in accordance with the scientific method. e.g. the big bang.

What is a scientific law?

A statement that has been deduced from empirical evidence.

What is the difference between a scientific law and a scientific theory?

Scientific laws do not change unless an experimental error is discovered, while scientific theories are refined and improved as new data becomes available.

What is scientific bias?

A lack of objectivity or a tendency to see or interpret evidence in ways that do not match what ca be observed or concluded scientifically.

What is reporting bias?

When certain evidence is purposefully ignored or not reported.

What is selection bias?

When a sample is chosen that doesn’t represent the target population.

What is confirmation bias?

When researchers focus only on information that supports their beliefs or expectations and ignore other information.

What is researcher bias?

When the personal view or interest of the researcher influence the outcome.

What is social/cultural bias?

When the researchers social or cultural view effects the experiment.

What is matter?

Anything that has a mass and takes up space.

What is an atom?

The smallest part of an element that still has the properties of that element.

What is a compound?

2 or more different elements chemically combined.

What is a model?

A simplified representation of reality that is used to understand, explain or predict phenomena. e.g maps

Why are models used throughout chemistry?

Models are used throughout chemistry to represent ideas, structures and processes of systems.

What are 3 examples of models?

Diagrams, equations and simulations.

Why/how is the periodic table model used?

To predict trends in chemical behaviour and the properties of elements, such as their reactivity, ionsation energy and atomic radius, based on their position on the table.

What are the limitations of models?

1. Oversimplification: models often simplify complex systems by leaving out important details.

2. Assumptions: models can make assumptions which can lead to errors.

3. Static vs. Dynamic: Diagrams and physical models cannot accurately capture the dynamic, 3-D nature of molecules.

What experiment did Crookes carry out?

Crookes carried out an experiment to show what happens when an electric current is passed through a glass tube containing air at low pressure.

What happened during Crookes experiemant when the gas inside was at a very low pressure? What did he call the rays?

When the gas inside was at a very low pressure - almost a vacuum - passing electric current through the tube resulted in invisible rays travelling in straight lines from the cathode to the anode. He called the rays cathode rays.

What type of electrode is the cathode?

The negative electrode.

What type of electrode is the anode?

The positive electrode.

What did Crookes experiment reveal?

Crookes experiment showed that cathode rays travel in a straight line, cathode rays are invisible but when they strike the glass they cause it to glow or fluoresce.

What did J.J Thomson discover about cathode rays?

J.J. thomson had a more advanced vaccuum tube to show that cathode rays were attracted to a postive charge (i.e. they had a negative charge).

What were Thomsons discoveries?

1. Identified cathode rays as sub-atomic particles - electron.

2. Measured the mass to charge ratio for an electron but never determined the value.

What was Thomson’s model of the atom called? what did it assume?

The plum pudding model. The plum pudding model the plum pudding model assumed there were electrons embedded within a sea of positive charge.

Who is associated with the nuclear model?

Ernest Rutherford.

What was Rutherford’s experiment?

Rutherford bombarded thin sheets of gold foil with alpha particles (positively charge particles).

What was Rutherford’s hypothesis based on the plum

Rutherford expected that the alpha particles should pass through with little to no deflection.

What were the 3 observations of Rutherford’s experiment?

1. Most alpha particles passed straight through the gold foil.

2. Some alpha particles were defelected at larger angles.

3. A few particles were reflected back along their own paths.

What were the 3 conclusions of Rutherford’s experiment?

1. Atom is mostly made up of empty space.

2. The alpha particles are repelled as they pass near the positive nucleus

3. Only a small number of particles collided head-on with the nucleus, proving it is small and dense.

What else did Rutherford find?

Rutherford discovered the proton as he continued to bombard different light elements.

What does the nuclear model of the atom proposed by Rutherford look like?

The nuclear model shows a small, positive nucleus containing protons in the centre and an electron cloud contianing electrons around it.

What are the 4 assumptions of the nuclear model?

1. Atoms contain a small dense positive core called the nucleus. This contains most of the atoms mass.

2. Positive particles called protons are located in the nucleus.

3. the atom is mostly empty space.

4. Electrons are scatted around in the space surrounding the nucleus which is called an electron cloud.

What are the 2 limitations of the nuclear model?

1. Lacks detail about electron behaviour and doesn’t explain electronic arangement.

2. Since like charges repel, why does the nucleus not fall apart due to the repulsion of protons in the nucleus.

What did Chadwick dicover?

Chadwick bombarded a sample of beryllium with alpha particles which knocked neutral particles out of the nuclei of beryllium. These particles had the same mass as protons, so he called neutrons.

What are the updated assumptions of the nuclear model?

1. Atoms contain a small dense positive core called the nucleus. This contains most of the atoms mass.

2. Positive particles called protons and neutrons are located in the nucleus.

3. The atom is mostly empty space.

4. Electrons are scattered around in the space surrounding the nucleus which is called the electron cloud.

Limitations of the updated nuclear model?

1. Electrons and thought to move around the electron cloud. Since opposite charges attract, the electrons should lose energy and spiral into the nucleus, causing the atom to collapse.

2. Therefore, doesn’t explain how electrons stay in motion around the nucleus without crashing into it.

What were Bohr’s 3 proposals?

1. Electrons orbit the nucleus in fixed energy levels (similar to how planets orbit the sun).

2. Electrons in an atom have a specific, fixed amount of energy.

3. Electrons can move around the nucleus only in certain specific orbits with a fixed radius depending on the fixed amount of energy they have.

What is an energy level?

An energy level is the fixed energy value that and electron in an atom may have.

What are the 3 assumptions of Bohr’s model?

1. Bohr assumed that electrons revolve around the nucleus in fixed paths called orbits.

2. Electrons in any one orbit have a fixed amount of energy. Orbits are also called energy levels.

3. Energy levels are represented by the letter n = 1 (lowest energy level/orbit), n = 2 (higher in energy than the first) n = 3, etc.

what are the 4 limitations of Bohr’s model?

1. Bohr’s model worked well for hydrogen, but it didn’t work well for atoms containing more than one electron.

2. Bohr’s model didn’t take into account wave-particle duality.

3. Bohr mistakenly believed that the exact position of electrons and velocity could be known at all times.

4. Bohr’s model didn’t account for sublevels.

What is the relative mass of protons, electrons and neutrons?

protons=1, electrons=1/1838, neutrons=1.

What is the relative charge of protons, electrons and neutrons?

protons=+1, electrons=-1, neutrons=0.

What is the location of protons, electrons and neutrons?

protons=nucleus, electrons= outside the nucleus in shells, neutrons=nucleus.

What is the atomic number?

The atomic number of an element is the number of protons in the nucleus of an atom of that element (the smaller number).

What is the mass number?

The mass number of an element is the number of protons and neutrons in the nucleus of an atom of that element (the bigger number).

How do you calculate the number of neutrons?

Number of neutrons = mass number - atomic number.

What is an isotopse?

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

What is a molecule?

A molecule is two or more atoms chemically combined.

What is an ion?

An ion is an atom that has lost or gained an electron. i.e. a charged atom.

What are positive ions called?

Positive ions are called cations.

What are negative ions called?

Negative ions are called anions.

What is relative atomic mass (Ar)?

Relative atomic mass is defined as the average mass of all the isotopes of an element, as they occur naturally, taking their abundances into account, relative to one twelfthof the mass of a carbon-12 atoms.

Why are the Ar rarely a whole number? (relative atomic mass)

1. Aerage masses of isotopes.

2. A mixture of isoptops.

How do we calculate Ar? (relative atomic mass)

1. multiply abundances (%) by the mass number for each isoptope.

2. Add the answers above for each isotope (100 atoms)

3. Divide the answer by 100 (for 1 atom)

What should you do when carrying out calculations with large numbers?

Round to 4 significant figures. e.g. 1821g or 0.01821g

What did Aston detect?

Aston detected the existence of isotopes using the first mass spectrometer.