Free TCE Physical Sciences Flashcard Review

The arrangement of an atom's electrons in distinct energy levels (shells) around the nucleus, written as the number of electrons in each shell from the innermost outwards, e.g. chlorine is 2,8,7. For elements 1 to 20 the shells fill 2, then 8, then 8, then 2, so calcium is 2,8,8,2. Electrons are held in these shells by the electrostatic force of attraction to the positive nucleus.

Draw the nucleus labelled with the atomic number (and mass number if asked), then concentric circles for each occupied shell with the correct number of electrons marked as dots or crosses: 2 in the first shell, up to 8 in the second, up to 8 in the third, then 2 in the fourth. Only elements 1 to 20 are examinable, and no s, p, d or f orbital theory is required.

The period number equals the number of occupied shells, and the group number is set by the number of outer-shell (valence) electrons: 1 outer electron is group 1 (I), 2 is group 2 (II), and for the main groups after that the group number is the outer electrons plus 10, so 7 outer electrons is group 17 (VII) and a full outer shell is group 18 (VIII). Example: 2,8,7 has 3 shells and 7 outer electrons, so period 3, group 17.

A measure of the size of an atom, taken as the distance from the nucleus to the outermost occupied electron shell. It is a physical property that shows clear trends across periods and down groups of the periodic table.

Atomic radius decreases from left to right across the period. Each successive element adds one more proton to the nucleus while the extra electron goes into the same outer shell, so the increasing nuclear charge pulls the same shell in more strongly and the atom gets smaller. Sodium is the largest atom in period 3 and chlorine the smallest of the reactive elements.

Atomic radius increases down the group. Each element down the group has one more occupied electron shell, so the outer electrons are further from the nucleus and are also shielded by the inner shells, and this outweighs the extra nuclear charge. Lithium is smaller than sodium, which is smaller than potassium.

The number of bonds an atom can form, determined by how many electrons it must gain, lose or share to reach a full outer shell. Sodium (2,8,1) has valency 1, magnesium (2,8,2) has valency 2, and chlorine (2,8,7) has valency 1 because it needs one electron.

Na 1, Mg 2, Al 3, Si 4, P 3, S 2, Cl 1, Ar 0. Valency rises to a maximum of 4 at silicon then falls back to 0 at argon, because up to silicon it equals the number of outer electrons available and after silicon it equals the number of electrons needed to fill the outer shell.

An atom loses or gains electrons to achieve a full outer shell. If the outer shell has few electrons the atom loses them, giving a positive charge equal to the number lost; if it is nearly full the atom gains electrons, giving a negative charge equal to the number gained. Magnesium (2,8,2) loses 2 electrons to form Mg2+; oxygen (2,6) gains 2 to form O2-.

Group 1 (I) forms 1+ ions, e.g. Na+. Group 2 (II) forms 2+ ions, e.g. Mg2+. Group 17 (VII) forms 1- ions, e.g. Cl-. Group 18 (VIII) atoms already have a full outer shell and do not normally form ions.

The atom is a very small, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons that occupy distinct energy levels (shells). The atom is held together by the electrostatic force of attraction between the positive nucleus and the negative electrons. The atomic number is the number of protons and the mass number is the number of protons plus neutrons.

Atoms of the same element that have the same number of protons (the same atomic number) but different numbers of neutrons, and therefore different mass numbers. Carbon-12, carbon-13 and carbon-14 are isotopes of carbon, all with 6 protons but 6, 7 and 8 neutrons respectively.

Write the element symbol with the mass number as a superscript on the left and the atomic number as a subscript on the left, e.g. carbon-14 is written with 14 above and 6 below the C. In plain text it is also written as the element name or symbol followed by a hyphen and the mass number, e.g. carbon-14 or C-14.

Neutrons = mass number minus atomic number. For uranium-235 with atomic number 92 there are 235 - 92 = 143 neutrons. Electrons in a neutral atom equal the atomic number.

Chemical behaviour is determined by the electrons, and isotopes have the same number of protons and therefore the same electron configuration, so they react identically. Physical properties such as mass, density, rate of diffusion and nuclear stability depend on the mass of the nucleus, which differs because the neutron number differs, so some isotopes are radioactive and others are not.

A mass spectrometer ionises the atoms of a sample and separates the ions using a magnetic field according to their mass. You are not required to know the mechanics of the instrument. You must read the resulting spectrum to identify which isotopes are present (from the mass numbers on the horizontal axis) and their relative abundances (from the peak heights), and use those abundances to calculate relative atomic mass.

The ratio of the weighted average mass per atom of the naturally occurring form of an element to one twelfth of the mass of an atom of carbon-12. Because it is a weighted average over the natural isotopic composition, it has no units and is usually not a whole number.

Multiply each isotope's mass number by its fractional abundance and add the results: RAM = sum of (isotope mass x percentage abundance) / 100. For chlorine, 75.0% chlorine-35 and 25.0% chlorine-37 gives (35 x 75.0 + 37 x 25.0)/100 = 35.5. Check that the abundances total 100%, and quote the answer to the significant figures of the data.

Because it is a weighted average of the masses of all the naturally occurring isotopes of the element, weighted by their abundances. Chlorine's value of 35.5 does not mean any chlorine atom has that mass; it reflects a mixture of chlorine-35 and chlorine-37 atoms in roughly a 3 to 1 ratio.

An alpha particle is a helium-4 nucleus, 2 protons and 2 neutrons, symbol He with mass number 4 and atomic number 2, charge 2+. In alpha decay the mass number falls by 4 and the atomic number falls by 2, producing a new element. Example: uranium-238 decays to thorium-234 plus an alpha particle, and both mass numbers (238 = 234 + 4) and atomic numbers (92 = 90 + 2) balance.