Re: GAMSAT General Chemistry Atoms and Molecules
Atoms are the basic building blocks of all matter. Each atom is composed of a nucleus. The nucleus contains protons and neutrons and is surrounded by one or more electrons.
Protons have a positive charge.
Electrons have a negative charge.
Neutrons are electrically neutral.
Electrons and protons have opposite charges of equal magnitude. The size of a proton and neutron are approximately equal, however an electron is much smaller.
An element is a chemical substance containing atoms that have the same number of protons in their atomic nuclei. Elements are the building blocks of all compounds. Elements are displayed as follows:
A is the mass number of the element. This is the number of protons plus neutrons in the nucleus.
Z is the atomic number. This is the number of protons in the nucleus.
An element is characterized by the number of protons it has. Any element can only have one number of protons. However, the number of neutrons or electrons can change and the element will still retain its identity.
If there are two atoms of the same element that have a different mass number (different number of neutrons) these are said to be isotopes.
Three isotopes of carbon include:
12C, 13C and 14C.
Each isotope of carbon contains 6 protons (giving the element its identity as carbon). 12C contains 6 neutrons, 13C contains 7 neutrons, and 14C contains 8 neutrons.
The atomic weight or molar mass (M) of an atom is given in atomic mass units (amu). The amu is a ratio and is defined by carbon-12. One atom of 12C has an atomic weight or molar mass of 12 amu. All other atomic weights are measured against this standard.
12C is also used to define a mole. A mole is the number of carbon atoms in 12 grams of 12C.
Avogadro’s number also defines the number of carbon atoms in 12 grams of carbon as 6.022 x 1023.
So there are 6.022 x 1023 carbon atoms in 12 grams of 12C.
The formula that surrounds these concepts is as follows:
n = moles
m = mass in grams
M = molar mass (found on periodic table for individual atoms)
The Periodic Table
All elements are listed on the periodic table. Elements are listed based on their atomic number from left to right. The vertical columns of the periodic table are called groups and the horizontal rows are called periods.
Groups are numbered from 1 to 18 and periods are numbered from 1 to 7.
The periodic table above divides the elements into three sections:
– Metals are on the left (pink)
– Nonmetals are on the right (light green)
– Metalloids are along the darker green area that separates the metals and non-metals.
Nonmetals generally have lower melting points than metals and form negative ions. Molecular substances are usually made up from only nonmetals. E.g. H2O.
Metals tend to lose electrons to form positive ions. All metals exist as solids at room temperature with the exception of mercury, which is a liquid.
The metals that range from group 3 to group 12 are known as transition metals.
Group 18 is known as the noble gases; group 17 is known as halogens; group 1 is known as alkali metals; group 2 is known as alkaline earth metals.
Group 1 elements form 1+ cations; group 2 elements form 2+ cations; group 14 elements can all form 4 covalent bonds with nonmetals; group 15 elements can form 3 covalent bonds; group 16 contains oxygen (important); group 17 elements (halogens) are very reactive, forming only 1 bond with other elements; group 18 elements (noble gases) are nonreactive.
An element becomes an ion when it has more or fewer electrons than protons. Negative ions are known as anions and positive ions are known as cations.
General rule: Metals form cations and non-metals form anions.
General predictions about elements can be made based on their position in the periodic table.
Atomic size: Atomic size (radius) increases as you move from right to left and from top to bottom. This is because when you move from left to right the effective nuclear charge increases (more protons), and each additional electron is pulled more strongly toward the nucleus. Atomic radius increases from the top to bottom because with each added shell the atom becomes larger. Note: As you move down the periodic table, each underlying period will have elements with an extra shell of electrons. E.g. helium has one shell and Lithium has two shells.
Ionisation energy: Ionisation energy is defined as the energy required to remove an electron from an atom. If an electron is more strongly attached to the nucleus it will require more energy to be removed. Ionisation energy increases from left to right and from bottom to top. When you move from left to right the elements have a greater nuclear charge (more protons). There are more protons that create a stronger attraction with the surrounding electrons and as a result more energy is required to remove the electron from the atom – greater ionization energy.
As you move down the periodic table the distance of the electron from the nucleus increases (more electron shells as you move down the periodic table). This increased distance creates a decrease in electric field strength and as a result less energy is required to remove an electron. This explains why the ionization energy increases as you move up the periodic table.
Electronegativity is the tendency of an atom to attract an electron in a bond that it shares with another atom. Electronegativity tends to increase as you move from left to right and from bottom to top.
Atoms can be held together by bonds.
A covalent bond occurs when two electrons are shared by two nuclei. The electrons (negatively charged) are pulled toward both positively charged nuclei via electrostatic forces.
A compound is formed when a substance contains one or more elements in a definite ratio.
The empirical formula of a pure compound is the simplest whole number ratio between the number of atoms of the different elements in the compound. For example the empirical formula for glucose is CH2O.
The molecular formula of a molecule states the exact number of the different atoms that make up the molecule. The molecule formula for glucose is C6H12O6.
Naming inorganic compounds
In the GAMSAT it is uncommon that ACER will directly test the ability to name inorganic compounds. However it is important that candidates are able to identify compounds that are being referred to in the respective questions.
Ionic compounds are named after their cation and anion. When naming an ionic compound the cation name is placed in front of the anion name. For example NaCl is called sodium chloride.
Monoatomic anions and polyatomic anions are given the suffix –ide. For example H– is a hydride ion and OH– is a hydroxide ion.
Chemical Reactions and Equations
Compounds that initially react in a chemical reaction are termed reactants. Reactants are always written on the left hand side of the chemical equation. The compounds that are produced in the reaction are termed the products of the chemical reaction. Products are always written on the right hand side of the chemical equation.
2 HCl + 2 Na → 2 NaCl + H2
The coefficients in the above equation represent the relative number of moles of reactants that combine to form the relative number of moles of products.
The law of conservation of mass states that the number of atoms of a given element remains constant during the process of a chemical reaction.
Types of Chemical Reactions
In terms of the GAMSAT students should be familiar with the four fundamental types of reactions:
Combination: C + D –> E
Decomposition: E –> C + D
Single Displacement: C + DE –> D + CE
Double Displacement: CD + EF –> CF + DE
The letters C, D, E and F represent hypothetical elements or molecules.
Oxidizing vs Reducing agents
The following must be memorized for the GAMSAT.
An oxidizing agent will cause oxidation to occur, whilst the agent itself will be reduced.
A reducing agent will cause reduction to occur, whilst the agent itself will be oxidized.
If something is reduced it gains electrons. If something is oxidized it loses electrons.
OIL RIG – Oxidation Is Loss; Reduction Is Gain