# Chem Question

1. Mar 2, 2005

How do you know whether a group of ions is isoelectronic? Also what does this notation mean:

$$Ca: [Ar]4s^{2}$$?

Thanks

2. Mar 2, 2005

### xanthym

An atom or ion is isoelectonic with another atom or ion if the two contain equal numbers of electrons. This can easily be determined from the Atomic Numbers of the 2 species under consideration:
{Number of Electrons} = {Atomic Number} - {Net Charge}

Thus, for a neutral atom, the number of electrons equals the Atomic Number. For example, the neutral atom Ar with Atomic Number (18) has 18 electrons. For the negative ion Cl-1 with charge (-1) and Atomic Number (17), the number of electrons is {(17) - (-1)}=(18). For the positive ion Ca+2 with charge (+2) and Atomic Number (20), the number of electrons equals {(20) - (+2)}=(18).

The terminology:
$$Ca: [Ar]4s^{2}$$
indicates that the neutral Ca atom has the same electron energy/orbital structure as neutral Ar PLUS 2 ADDITIONAL ELECTRONS in the "s" Sublevel of the 4th Energy Level (Principle Quantum Number = 4). From this indication, it can be determined that if Ca loses its 2 valence electrons (i.e., the 2 4s2 electrons), the resulting Ca+2 ion will be isoelectronic with the neutral Ar atom.

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Last edited: Mar 2, 2005
3. Mar 3, 2005

### The Bob

This has been said but not written so simply for clarity:

$$Ca: 1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}$$ means the same as $$Ca: [Ar]4s^{2}$$

It is just a short hand and easier to write but use the long hand in exams or you lose marks.

4. Mar 3, 2005

### dextercioby

Nope.It would be ridiculous,if one of the teachers would not accept the shorthanded notation...
But i know that there are a lotta dumb profs out there...:yuck:

Daniel.

5. Mar 3, 2005

### The Bob

Personally I do not understand why I cannot short hand but there is no mark for it in the mark scheme so it is better to be safe than sorry.