Atoms

Question 1

Atomic Structure

Answer 1

• The structure of an atom is thought to be a central nucleus (containing protons and neutrons) surrounded by electrons, which occupy various energy levels (shells) around the nucleus. It’s a dense, positively charged nucleus consisting of protons and neutrons, around which move negatively charged electrons.
• We differentiate between atoms by looking at the atomic mass of the different particles. We have also grouped these different atoms, based on the numbers of protons, neutrons, and electrons, and on their properties.

Question 2

Atomic Notation

Answer 2

• You will often see 3 symbols in the configuration:
• X is the symbol of the atom
• Z is the atomic number (or number of protons)
• A is the mass number (the number of protons and neutrons combined)

Question 3

Ions

Answer 3

• Ion - species that has either gained or lost electrons.
• Nearly all atoms will readily form ions of some sort.
• The term polyatomic ion refers to a charged species that is made up of more than one atom. e.g. NH4+, OH-

Question 4

Electrons

Answer 4

• Electrons are not located in the nucleus which means they can move between atoms.
• Electrons have a favoured location around the nucleus that we call energy levels (or shells)
• The max. number of electrons that can occupy a specific energy level can be found using the following formula: 2n2
• After the 3rd shell reaches 8e- we put the next 2e- in the 4th shell. Then the following 10e- go back in the 3rd shell before the 4th shell continues to fill.
• That is why atoms such as Calcium will have an electron configuration of 2, 8, 8, 2 rather than 2, 8, 10

Question 5

Valence Electrons

Answer 5

• Are the outermost electrons of an atom. It is also an indicator of the likely valency (or charge) that an atom is going to have.

Question 6

Chemical Properties

Answer 6

• Elements in same group have same number of valence electrons and have similar chemical properties.
• The most reactive kind of metallic element are those from group 1 (e.g. Na or K).
• A metal from group 2 (e.g., Mg) is less reactive.
• Within each group of metals, reactivity increases with each lower row of the table.
• The most reactive kind of non-metal is in group 17 (e.g., F or Cl).
• Within each group of non-metals, reactivity decreases with each lower rows of the table.

Question 7

Metals, Non-Metals, Metalloids

Answer 7

• A metal is a chemical element that is a good conductor of both electricity and heat. It forms cations and ionic bonds with non-metals.
• The definition of a non-metal is far less specific, but they are poor conductors and form anions.
• The elements in the middle (edge of the step, shaded green) are metalloids; they have a mixture of metallic and non-metallic properties.

Question 8

Isotopes

Answer 8

• Isotopes are atoms of the same element with the same number of protons (same number of protons and same number of electrons), but with a different number of neutrons.
• When you see Atomic Weights (also known as Relative Molar Masses) listed on the Periodic Table, some are not whole numbers. This is due to isotopes.
• Elements can consist of a variety of atoms
• Whilst atoms of a given element always have the same number of protons, they may have different numbers of neutrons.
• Atoms that differ in this way are called isotopes.
• Isotopes are virtually identical in their chemical reactions. (There may be slight differences in speeds of reaction).
• This is because they have the same number of protons and the same number of electrons.
• The uncharged neutrons make no difference to chemical properties but do affect physical properties such as melting point and density.
• Examples: Hydrogen, Carbon, Cobalt. Similar Chemical Properties to original element but different physical properties.

Question 9

Isotopes and Relative Atomic Mass:

Answer 9

• Many natural elements are a mixture of isotopes.
• This means that when we react atoms of an element, we are using a mixture of atoms with different mass numbers.
• The relative atomic mass given in the periodic table takes account of this.
• E.g For 100 atoms of bromine:
• Mass of 50.5 atoms of bromine 79: 50.5 x 79 = 3989.5
• Mass of 49.5 atoms of bromine 81: 49.5 x 81 = 4009.5
• Total = 7999
• Average (divide by 100) = 79.99

Question 10

Significant Figures

Answer 10

• All non-zero digits are significant
  7.92 (3 SF) 23.56 (4 SF)
• Zeros between two significant digits are significant
  60.3 (3 SF) 9.0002 (5 SF)
• Zeros before the first non-zero digit are not significant
  0.35 (2 SF) 0.009217 (4 SF)
• Zeros to the right of a decimal point at the end of a number are significant
  3.500 (4 SF) 0.0710 (3 SF)
• Zeros at the end of a number and before the decimal point are not significant (unless otherwise indicated)
  3500 (2 SF) 3.500 x 104 (4 SF) 3.50 x 104 (3 SF)

Question 11

Periodic Table Trends

Answer 11

3 main factors affect how strongly electrons are attracted to the nucleus:
* Nuclear Charge: the greater the nuclear charge, the more strongly electrons are attracted
* Distance between the nucleus & outermost electron: as the distance between the nucleus & the outer electron increases, the strength of attraction decreases
* Shielding by inner shell electrons: Electrons in the inner shell repel other electrons between the nucleus and the outer electron

Question 12

Mass Spectrometry

Answer 12

• Mass Spectrometry is based on the differences in masses of atoms in a sample. It can tell you about content of atoms or isotopes. (We will look at isotopes)
• It is used to determine the relative molecular mass and molecular structure of organic compounds and the relative atomic mass of isotopes.
• The instrument used is call a mass spectrometer.
• The operation of a mass spectrometer involves several key steps:
• Vaporisation of the sample being analysed.
• Ionisation of the vaporised sample.
• Acceleration and separation of the resulting ions based on their mass to charge ratio (m/z).
• Detection or counting the number of each of the ions of different mass to charge ratio.
• A vaporised (gaseous) sample of the chosen element is bombarded with a high energy electron beam. (or ultraviolet light)
• This knocks out or removes electrons, leaving the atom with an overall positive charge. (+1 ions and a few +2 ions)
• A sample will show two or more positive ions, each with a different mass. (+1 and +2)
• The positive ions are accelerated by an electric field, so they all move at high speeds.
• They then pass through a magnetic field.
• Lighter ions are deflected more by the magnetic field.
• Heavier ions are deflected less by the magnetic field.
• A detector measures the intensity and radius of the ions that strike it.
• This detector count is a measure of the isotope abundance while the degree of beam deflection is related to the isotopic mass (actually m/z ratio).
• This information is transformed into a graphical form called a mass spectrum.

Question 13

Emission Spectroscopy

Answer 13

• When a sample is heated it will absorb energy. Electrons move to the next energy level, then fall back to ground state, emitting light. The process of analysing this light is called SPECTROSCOPY.
• A spectroscope is a device used to take the light emitted from an element & separate it into component wavelengths.
• This produces a line emission spectrum.
• The light is composed of multiple wavelengths which are dispersed through a prism & shone onto a film.
• Each element has a unique set of energy levels & hence a unique line emission spectrum.