Atom Recap: Atomic Number, Shells and Electron Configuration
The modern periodic table is arranged by atomic number (അറ്റോമിക നമ്പർ). An element's place in it, its group and period, can be read from its electron configuration (ഇലക്ട്രോൺ വിന്യാസം). So before the chapter starts you need to be sure of a few things from Chapter 1. These are the three particles in an atom, atomic number and mass number, the shells K, L, M, N, and the rules for writing configurations such as 2, 8, 1. Above all, you need to know the outermost electrons (ബാഹ്യതമ ഇലക്ട്രോണുകൾ), the electrons that decide how an element behaves.
- Elements and their symbols (page 1).
- Nothing else: this page re-builds Chapter 1's ideas from the start.
1. Inside an atom
An atom has a tiny, heavy centre called the nucleus (ന്യൂക്ലിയസ്), with electrons moving around it.
| Particle | Where it is | Charge | Mass (approx.) |
|---|---|---|---|
| Proton പ്രോട്ടോൺ (p) | in the nucleus | +1 | 1 u |
| Neutron ന്യൂട്രോൺ (n) | in the nucleus | 0 (no charge) | 1 u |
| Electron ഇലക്ട്രോൺ (e) | in shells around the nucleus | −1 | about 1/1837 u, almost nothing |
An atom has equal numbers of protons and electrons, so the + and − charges cancel and the atom is neutral. Almost all its mass is in the nucleus.
2. Atomic number and mass number
- Atomic number (അറ്റോമിക നമ്പർ), Z = the number of protons. In a neutral atom it is also the number of electrons. The atomic number decides the element: every atom with 6 protons is carbon, and every atom with 11 protons is sodium.
- Mass number (മാസ് നമ്പർ), A = protons + neutrons. So neutrons = A − Z.
- We write them beside the symbol: mass number on top, atomic number below. 2311Na has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Change the protons and the element itself changes. Change the neutrons and only the mass number changes: you get another isotope of the same element. Change the electrons and you get a charged atom (an ion, more about it in the next unit). The drawing is not to scale: a real nucleus is about 100 000 times smaller than the atom.
Try it. (a) An atom has 17 protons and 18 neutrons. What are its atomic number and mass number? (b) How many neutrons are there in 4020Ca?
Atoms of the same element with different numbers of neutrons are isotopes. They have the same atomic number but different mass numbers. Hydrogen has three: 11H (no neutron), 21H (1 neutron) and 31H (2 neutrons). Their atomic masses differ, which caused trouble for Mendeleev's table, as you will see.
3. Shells: where the electrons live
Bohr showed that electrons move around the nucleus only in fixed paths called shells (ഷെല്ലുകൾ). Counting outwards from the nucleus they are numbered n = 1, 2, 3, 4 … and named K, L, M, N … Each shell can hold only a limited number of electrons:
| Shell | n | Maximum electrons = 2n² |
|---|---|---|
| K | 1 | 2 × 1² = 2 |
| L | 2 | 2 × 2² = 8 |
| M | 3 | 2 × 3² = 18 |
| N | 4 | 2 × 4² = 32 |
- Electrons fill the shells from the inside out: K first, then L, then M, then N.
- A shell can hold at most 2n² electrons.
- The outermost shell (ബാഹ്യതമ ഷെൽ) can hold at most 8 electrons (K, when it is the only shell, holds 2).
Rule 3 is why potassium (19) is 2, 8, 8, 1 and not 2, 8, 9. The M shell could hold 18, but as the outermost shell it can take only 8. So the 19th electron starts a new shell, N.
4. Electron configuration
The electron configuration (ഇലക്ട്രോൺ വിന്യാസം) lists the electrons in each shell from the inside out, separated by commas. Sodium (11) is 2, 8, 1: 2 in K, 8 in L and 1 in M. Here are the first 20 elements:
Up to Z = 20 the three rules above give every configuration. From Z = 21 onwards your textbook's table (Fig 2.1) also shows longer configurations: scandium is 2, 8, 9, 2 and iron 2, 8, 14, 2. There the shell just inside the outermost one keeps filling up towards 18. The reason belongs to higher classes; on page 7 you will see what it means for the transition elements.
5. Outermost electrons and the number of shells
Two numbers read from a configuration are used all through this chapter:
- Number of shells (ഷെല്ലുകളുടെ എണ്ണം) = how many numbers there are in the configuration. Sodium, 2, 8, 1, has 3 shells.
- Outermost electrons (ബാഹ്യതമ ഇലക്ട്രോണുകൾ) = the last number, the electrons in the outermost shell (ബാഹ്യതമ ഷെൽ). Sodium has 1 outermost electron.
Type the numbers with commas or spaces. Then say how many shells it has and how many outermost electrons. Elements come from Z = 1 to 20.
6. Why the outermost electrons matter
When atoms meet in a chemical reaction (രാസപ്രവർത്തനങ്ങൾ), it is their outer parts that touch. The nucleus stays deep inside, so it is the outermost electrons that take part. That leads to one of the big ideas of this chapter:
Elements with the same number of outermost electrons show similar chemical properties (രാസഗുണങ്ങൾ). Lithium (2, 1), sodium (2, 8, 1) and potassium (2, 8, 8, 1) each have 1 outermost electron, and all three are soft metals that react quickly with water. In the periodic table they stand in one column, group 1.
Try lithium with sodium, fluorine with chlorine, neon with argon, then magnesium with chlorine. Helium is a special case. It has only 2 electrons, but its single shell, K, is already full, so it behaves like neon and argon (full outer shells), not like beryllium or magnesium.
- Protons (+1) and neutrons (0) are in the nucleus; electrons (−1) move in shells around it.
- Atomic number Z = protons = electrons (neutral atom); mass number A = protons + neutrons. Isotopes have the same Z but different A.
- Shells K, L, M, N hold at most 2, 8, 18, 32 electrons (2n²). They fill from the inside, and the outermost shell holds at most 8.
- The number of shells and the number of outermost electrons come straight from the configuration. Same outermost electrons → similar chemical properties.