Electron arrangement

• Why do most atoms have an incomplete outer shell?

  They need a full outer shell to be stable
• What happens when an atom has an incomplete outer shell?

  It reacts with another atom to gain or lose electrons
• What is the atomic number of sodium?

  11
• How many protons does sodium have?

  11 protons
• How many electrons does sodium have?

  11 electrons
• How are electrons arranged around the nucleus of an atom?

  In shells around the nucleus
• How many electrons can the first shell hold?

  Two electrons
• How many electrons can the second shell hold?

  Eight electrons
• What is the best way to add electrons to the shells?

  Place one electron in each position first
• What do you do when you run out of electrons while filling shells?

  Place remaining electrons in the next shell
• Why is sodium considered unstable?

  It has one electron in its outer shell
• What do most atoms want to do to achieve stability?

  React to form molecules or compounds
• What do noble gases have in common?

  Completely full outer shells
• What is the atomic number of argon?

  18
• How many electrons does argon have?

  18 electrons
• How do you represent the electron structure of argon?

  2, 8, 8
• How many electrons does calcium have?

  20 electrons
• What is the electron structure of calcium?

  2, 8, 8, 2
• Why is calcium considered unstable?

  It has two electrons in its outer shell
• What happens when calcium loses two outer electrons?

  It becomes a calcium ion with a +2 charge
• What is the atomic number of fluorine?

  9
• How many electrons does fluorine have?

  9 electrons
• What is the electron structure of fluorine?

  2, 7
• How does fluorine achieve stability?

  By gaining one more electron
• What charge does a fluoride ion have?

  -1 charge
• How is the electron structure of a fluoride ion represented?

  2, 8
• What are the key points about electron arrangements in atoms?

  • Atoms need a full outer shell to be stable.
  • Electrons are arranged in shells around the nucleus.
  • First shell holds 2 electrons, second holds 8, third holds 8.
  • Atoms react to achieve a full outer shell.
  • Noble gases have full outer shells and are stable.
• How do you determine the electron arrangement for the first 20 elements?

  1. Identify the atomic number.
  2. Draw the nucleus.
  3. Fill the first shell with 2 electrons.
  4. Fill the second shell with 8 electrons.
  5. Fill the third shell with 8 electrons.
  6. Write the structure as numbers (e.g., 2, 8, 8).
• What is the difference between an atom and an ion?

  • Atoms have equal protons and electrons.
  • Ions have a charge due to lost or gained electrons.
  • Example: Calcium atom vs. Calcium ion (+2 charge).