Atoms and Molecules: Matter's Building Blocks

• Atoms and Molecules are the basic building blocks of matter.

Atoms

• Atoms are the smallest unit of an element that retains the properties of that element.
• Atoms are simplified and usually represented by a circle.
• Different sizes of circles represent different elements.
• The atom is smaller in size than a cell.
• Cells are made of different cell structures (nucleus, cell membrane, vacuole etc.) and these cell structures are made up of different elements.

Atomic Models

• Atomic models are continuously revised when more information and evidence are revealed.
• Technology plays an important role in this aspect, along with the attitudes of scientists, like open- mindedness and creativity
• In 1803, John Dalton stated that atoms are indivisible, those of a given element are identical, and compounds are combinations of different types of atoms.
• In 1904, JJ Thomson discovered electrons, and the ‘plum pudding’ model of the atom which shows the atom as composed of electrons scattered throughout a spherical cloud of positive charge.
• In 1911, Ernest Rutherford discovered that the positive charge was located in the nucleus of an atom.
• In 1913, Niels Bohr proposed stable electron orbits, explaining the emission spectra of some elements.
• In 1926, Erwin Schrödinger stated that electrons don't move around the nucleus in orbits, but in clouds.
• Atoms are made up of sub-atomic particles: protons, neutrons, and electrons
• Protons are positively charged, neutrons are neutral, and electrons are negatively charged.
• Protons and neutrons contribute to the mass of the nucleus.
• Electrons have negligible mass and do not contribute to the mass of the nucleus
• An atom is electrically neutral; it has an equal number of protons and electrons.
• In the planetary model, negatively charged electrons move around the nucleus positively charged protons and non-charged neutrons are in the nucleus.

Atomic Number and Mass Number

• The top number in the symbol is the mass number, and the bottom number is the proton number
• Adding more protons to the nucleus, while keeping the number of neutrons and electrons constant, increases the proton number and the mass number, while changing the symbol of the particle.
• Adding more neutrons to the nucleus while keeping the number of protons and electrons constant, increases the mass number, while the proton number and symbol remain the same.
• Proton number is the number of protons in the nucleus of an atom
• Mass number is the total number of protons and neutrons in the nucleus of an atom.
• The identity (symbol) of a particle can be identified using the number of protons.
• Proton (atomic) number, mass (nucleon) number, symbol of element can be represented using nuclide notation.

Electronic Configuration

• Arrangement of electrons in an atom can be represented using electronic configuration or electronic structure

• First Shell:

  • The shell is closest to the nucleus.
  • The shell corresponds to the lowest energy level.
  • The shell can hold a maximum of 2 electrons.
  • The shell needs to be filled first.

• Second and Third Shells:

  • Have increasing energy levels (e.g. the third shell has higher energy than the second shell).
  • Usually hold up to 8 electrons each
  • They are filled in order – usually, the second shell is fully filled before the third shell.

• Valence Shell:

  • Valence shell or outer shell of an atom refers to the shell that is furthest away from the nucleus of the atom.
  • The electrons in the outer shell of an atom are known as valence electrons.

• In the same group, elements have the same number of valence electrons.

• The number of electron shells is the same.

Molecules

• Atoms can combine with one or more atoms of the same or different elements to form molecules.
• The chemical formula can represent molecules, which identifies each constituent element in a molecule by its chemical symbol and indicates the number of atoms of the element.
• A molecule is a group of two or more atoms chemically combined together.
• Molecules of elements are made up of identical atoms chemically combined.
• Molecules of compounds are made up of two different atoms chemically combined.
• A mixture is made up of two or more elements and/or compounds that are not chemically combined

Ions

• Helium, neon, and argon are examples of noble gases (Group 18 elements).
• Atoms of noble gas are generally unreactive.
• Noble gases have completely filled valence electron shells, so these atoms will not gain or lose electrons to other atoms.
• Atoms form ions to obtain a stable electronic configuration like that of a noble gas by gaining or losing electrons
• Formation of a positive ion is the loss of electrons.
• Formation of negative ion is the gain of electrons.
• Ions are not electrically neutral.
• Ions are electrically charged as the number of protons and number of electrons is different.
• Atoms of elements (except noble gases) do not have fully filled outer shells.
• Atoms can achieve the stable noble gas configuration by losing electrons to form positive ions (cations), or gaining electrons to form negative ions (anions).