Calculate the effective nuclear charge of the last electron in an atom. The electronic configuration is `1s^(2), 2s^(2)2p^(6), 3s^(2)3p^(5)`.

Explanation

To calculate the effective nuclear charge (Z_eff) of the last electron in the atom with the electronic configuration \(1s^2, 2s^2, 2p^6, 3s^2, 3p^5\), we will follow these steps:

### Step 1: Identify the Atomic Number (Z)
The atomic number (Z) is equal to the total number of electrons in the atom.
- From the given electronic configuration:
- \(1s^2\) contributes 2 electrons
- \(2s^2\) contributes 2 electrons
- \(2p^6\) contributes 6 electrons
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