How can I use a ngModel on an ion-radio element?

Syntax has been changed rewritten now, ngModel should be place with ion-list & radio-group only once. No need to have them there on each ion-radio element.

<ion-list radio-group [(ngModel)]="unit">
    <ion-list-header>
        Unit
    </ion-list-header>

    <ion-item>
        <ion-label>Metric (kg)</ion-label>
        <ion-radio value="1"></ion-radio>
    </ion-item>

    <ion-item>
        <ion-label>Imperial (lbs)</ion-label>
        <ion-radio value="2" ></ion-radio>
    </ion-item>
</ion-list>

For more information you could visit ionic2 framework forum link

As of Ionic 4+, the correct sintax is:

<ion-list>
<ion-radio-group [(ngModel)]="unit"> 
  <ion-list-header>
    Unit
  </ion-list-header>
    <ion-item>
      <ion-label>Metric (kg)</ion-label>
      <ion-radio value="1"></ion-radio>
    </ion-item>

    <ion-item>
      <ion-label>Imperial (lbs)</ion-label>
      <ion-radio value="2"></ion-radio>
    </ion-item>
  </ion-radio-group>
</ion-list>

It's working with Ionic 5.

  <ion-radio-group value="answer">
       <ion-item *ngFor="let item of question?.answers">
           <ion-label> {{item.answer}} {{answer}}</ion-label>
           <ion-radio slot="start" color="tertiary" value="{{item.id}} 
                 (ionBlur)="saveAnswer(item.id)">
             </ion-radio>
        </ion-item>
   </ion-radio-group>