Science Exam: Matter, Atoms, Elements & Compounds

Topics for Science Exam 30th March

• Topics to be covered in the science exam include atomic structure, elements, compounds and mixtures, states of matter, and chemical reactions.

Key Words

• Mass signifies the amount of "stuff" inside an object; a rock has more mass than a feather because it contains more "stuff."
• Matter refers to anything that occupies space and possesses mass.
• Volume denotes the amount of space that an object or substance occupies.
• A molecule consists of two or more atoms bonded together, which can be the same or different.
• An element is a pure substance composed of only one kind of atom.
• A compound is a substance formed by two or more different atoms chemically bonded together through strong forces.
• Density is the measure of how much mass is contained within a given volume; it is calculated as Density = Mass/Volume.

Atomic Structure

• An atom is the smallest unit of matter, made of protons, neutrons, and electrons, and is the building block of everything.
• A molecule is formed when two or more atoms are chemically bonded together, whether they are the same or different, for example, Oxygen gas (O2).
• A compound must be made of different atoms, unlike a molecule, where the atoms may be the same.
• The nucleus is the central part of an atom, which is very dense and contains almost all of the atom's mass.
• Nucleus is composed of protons, which are positively charged, and neutrons, which have no charge.
• Electrons reside in shells or orbitals around the nucleus, where they are negatively charged.
• Atoms have 4 shells, each shell has multiple electrons.
  • Shell 1 can hold up to 2 electrons.
  • Shell 2 can hold up to 8 electrons.
  • Shell 3 can hold up to 8 electrons.
  • Shell 4 can hold whatever electrons are left.
• Protons have a positive charge (+1) and determine the atomic number of an element.
• Protons have a mass of 1 atomic mass unit (amu), which is a unit to measure mass at a tiny scale.
• Neutrons have no charge (neutral) and a mass of about 1 amu.
• Neutrons help hold the nucleus together and contribute to the atom's mass.
• Electrons have a negative charge (-1) and balance out the positive charge of protons to keep the atom neutral.
• Electrons are also responsible for chemical reactions.
• Electrons' mass is tiny, almost 0 amu, so its mass is usually ignored in relation to the mass of protons and neutrons.

Compounds and Mixtures

• An element is a pure substance consisting of only one type of atom, such as Oxygen.
• A compound is a substance made of two or more different elements chemically bonded together, such as water(H₂O), which is made of 2 Hydrogen atoms and 1 oxygen.
• A mixture is a combination of two or more substances that are not chemically bonded, like sand and salt mixed together.
• Sand and Salt is a combination of 2 substances that are physically mixed but not chemically bonded.

States of Matter

• Solid particles are tightly packed and only vibrate in place, such as ice.
• Liquid particles are less tightly packed and can move around, such as water.
• Gas particles are very far apart and move quickly, like air.
• Melting occurs when a solid turns into a liquid as particles gain energy and move around, like chocolate melting.
• Boiling occurs when a liquid turns into a gas as particles are heated and gain enough energy to escape into the air.
• Evaporation occurs when a liquid turns to gas at the surface when particles gain enough energy to escape into the air.
• Freezing occurs when a liquid turns to solid as particles lose energy and move closer together.
• Condensation occurs when gas turns to liquid as particles lose energy and move closer together.

Kinetic Particle Theory

• The Kinetic Particle Theory states that all matter is made up of particles that are always moving.
• When a substance is heated, particles gain energy and move faster.
• When a substance is cooled, particles lose energy and move slower.
• When particles gain energy, they move faster and spread apart (melting and boiling).
• When particles lose energy, they move slower and come closer together (freezing and condensation).

Density

• Density is a measure of how much mass (stuff) is packed into a certain volume (space), essentially indicating how heavy something is for its size.
• Density is calculated using formula: Density = Mass / Volume, with mass always considered first.
• "g / cm 3" is the unit for density, meaning grams per cubic centimeter and illustrating how many grams of mass fit into one cubic centimeter of space.
• Example problem: the density of a 6 cm³ cork that has a mass of 3 g would be 0.5 g/cm³.
• Every cubic centimeter of the cork has a mass of 0.5 grams.
• Some materials have particles that are packed more tightly, while others are more spread out which is why some materials feel denser because it has more matter packed in.
• Materials with heavier atoms will have a more dense.
• Materials expand (get bigger) when hot and contract (get smaller) when cooled.
• When heated, particles move faster and spread out, while when cooled, particles move slower and get closer together.

Chemical Reactions

• A chemical reaction occurs when atoms in a substance rearrange to form a new substance, and generally these processes are irreversible.
• Chemical reactions form new substances.
• Chemical reactions transfer energy (e.g., heat or light) to or from the surroundings.
• A physical change involves a change in the appearance or state of a substance, but no new substance is formed.
• Atoms remain the same during physical changes.
• Chemical reactions can go both ways.
  • For instance, hydrated copper sulfate can turn into anhydrous copper sulfate and water, and this process can be reversed under certain conditions.
• The change cannot be undone in Chemical Non-Reversible Reactions.
  • For instance, combining sodium metal and chlorine gas to form table salt, as the new substances cannot return to original forms.
• Melting ice is reversible as the original substance can be restored.
• Breaking glass cannot be undone.
• Some chemical reactions are reversible, meaning they can go forward or backward, depending on the conditions, and are usually denoted with the symbol =.

Effect of Heat on Hydrated Compounds

• Hydrated compounds contain water molecules as part of their structure.
• Heating a hydrated compound causes it to lose its water content, turning it into an anhydrous compound.
• Hydrated Copper(II) sulfate is blue because it contains water in its structure; when heated, it loses the water and becomes anhydrous copper(II) sulfate, which is white.
• Heating a hydrated compound removes water, and adding water back rehydrates it.
• Adding water to an anhydrous compound makes it hydrated again.

Atomic Number & Atomic Mass

• Atomic Number (AN) = Protons
• Atomic Mass (AM) = Neutrons + Protons
• number of protons = number of e-
• The number of shells in the atom correlates with the group in the periodic table.
• Amount of Protons = Amount of e-
• The number of e- on the outermost shell is shown under the group (Columns).