Atoms, Elements, and Isotopes

Materials and Atoms

• All materials consist of atoms.
• Atoms consist of electrons orbiting a nucleus containing protons and neutrons.

Electrons

• Emission and absorption spectra of elements show that electrons are arranged in distinct energy levels.
• Spectra can identify some elements in matter.

Atomic Number and Mass Number

• The atomic number and mass number give information about the numbers of subatomic particles in an atom.
• The atomic number (Z) indicates the number of protons in an atom.
• The mass number (A) indicates the number of protons and neutrons combined.
• You can find the number of neutrons (N) by subtracting the atomic number from the mass number (N = A - Z).

Isotopes

• Many elements consist of different isotopes.
• Isotopes have different physical properties but the same chemical properties.
• Isotopes are atoms of the same element with different mass numbers due to different numbers of neutrons.
• Isotopes of an element have similar chemical properties because they have the same number of electrons.
• Isotopes are identified by writing the name of the element followed by the mass number, e.g., Hydrogen-3 for Tritium.

Representing Isotopes

• Isotopes can be represented using appropriate notation.

Neutral Atoms

• Atoms have an overall neutral charge.
• Atoms contain equal numbers of protons and electrons.

Atomic Components

• Atoms are the smallest unit of matter, with a nucleus containing protons and neutrons, surrounded by orbiting electrons.
• Elements consist of only one type of atom.
• Compounds consist of two or more different chemical elements combined in a fixed ratio.

Subatomic Particles

• Protons have a charge of +1, a relative mass of 1, and are located in the nucleus.
• Neutrons have a charge of 0, a relative mass of 1, and are located in the nucleus.
• Electrons have a charge of -1, a relative mass of nearly 0, and surround the nucleus.

Electron Shell Configuration

• Electron configuration shows the arrangement of electrons within the atom.
• The n represents shell number.
• Electrons orbit the nucleus within shells.
• Each shell can hold a maximum number of electrons, given by the formula 2n², where n is the shell number.
  • Shell 1 (K) can hold 2 electrons.
  • Shell 2 (L) can hold 8 electrons.
  • Shell 3 (M) can hold 18 electrons.
  • Shell 4 (N) can hold 32 electrons.
• Electrons fill the lower shells before filling the next.
• For example, Sodium has 11 electrons, so its electron configuration is 2, 8, 1.
• The general rule assumes lower shells are filled before electrons are placed in higher shells, but there are some exceptions.

Emission Spectra

• Emission and absorption spectra can identify which elements make up a material.
• Spectra provide evidence that electrons are arranged in distinct energy levels.
• Chemical elements emit a specific colour when exposed to a flame.
• Kirchhoff made a spectroscope using a prism to split the emitted light into a spectrum.
• Each element produces a unique series of sharp colored lines, called line emission spectra.

Bohr's Theory

• Bohr proposed the theory that electrons move in specific orbits around the nucleus.
• Each orbit has a specific energy level.
• Electrons could move between energy levels by absorbing or emitting energy in the form of electromagnetic radiation (light).
• The amount of energy released or absorbed equals the energy difference between the two energy levels in the atom.
• The bright lines in emission spectra correspond to wavelengths of light emitted when electrons move from a higher to a lower energy level.

Absorption Spectra

• A line absorption spectrum is produced when a continuous spectrum of visible light passes through a gaseous or vaporized sample of an element or compound.
• The atoms in the sample absorb specific wavelengths of light, resulting in a series of dark lines in the spectrum.
• Each line corresponds to a specific wavelength of light absorbed by the electrons in the atoms as they move from lower to higher energy levels.
• Each element has its own unique spectrum because of its specific energy levels.

Subshells

• The arrangement of electrons in atoms and monoatomic ions can be described in terms of shells and subshells.
• There are subshells within the shells of an atom.
• Subshells can hold a maximum number of electrons.

Subshell Capacity

• 1s can hold 2 electrons
• 2s can hold 2 electrons
• 2p can hold 6 electrons
• 3s can hold 2 electrons
• 3p can hold 6 electrons
• 3d can hold 10 electrons
• 4s can hold 2 electrons
• 4p can hold 6 electrons
• 4d can hold 10 electrons
• 4f can hold 14 electrons

Aufbau Principle

• Electrons fill lower energy subshells before filling higher energy subshells.

Subshell Notation

• Subshell notation shows the shells and subshells in the atom, along with the number of electrons occupying them.

Exceptions to the Aufbau Principle

• In some atoms, electrons are arranged in an unexpected manner.
• Copper and Chromium each have only one electron in 4s:
  • Chromium (24Cr): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵
  • Copper (29Cu): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d¹⁰.